Chapter Vii., Is Termed the Endoskeleton. the Latter Consists (i.) of an

axial part, including the vertebral column and the skull; and (ii.) of an appendicular portion, consisting of the skeleton of the limbs and their supporting pectoral and pelvic girdles.

THE VERTEBRAL COLUMN.—The individual segments or vertebrae which, arranged in a linear series, collectively form the vertebral column, are highly complex structures, each being formed by a number of vertebral elements, the sum total of which constitutes a vertebra. Perhaps the best conception of the nature of vertebral elements is to be gleaned from the study of such primitive Fishes as the Elasmobranchs, in which not only are all the vertebral components present, but they are less modified by suppression and fusion than in most other Fishes, and on this account they afford a convenient introduction to the study of the puzzling eccentricities of vertebral structure in other groups. Selecting any common Dog-Fish, such as Scyllium canicula, and starting with an early embryonic stage, it may be stated that the first indication of a vertebral column is the formation of the notochord, which, invested by its chordal sheath, extends from the tip of the tail to a point on the under surface of the brain just behind the hypophysis or pituitary body.

{194}[Illustration: FIG. 111.—A, side view of precaudal vertebrae of Scyllium canicula; B, similar view of caudal vertebrae. b.d, Basi-dorsal; c, centrum; h, basi-ventral; h.s, haemal spine; i.d, inter-dorsal; p, parapophysis; r, rib; s.d, supra-dorsals. The vertical dotted lines indicate the limits of neuromeres and myotomes. The small circles represent the exits of the dorsal and ventral roots of spinal nerves. (After Ridewood.)]

Subsequently, a number of cartilaginous pieces are developed in connexion with the dorsal and ventral surfaces of the notochord, which, as they form portions of a system of dorsal and ventral arches, are termed "arcualia" (Fig. 111). On the dorsal side there are: (i.) a series of paired basi-dorsal cartilages (neurapophyses or neural arches), the two elements of each pair contributing to form the side walls of the neural canal in which the spinal cord is lodged (Fig. 112, A); (ii.) a series of inter-dorsal cartilages (intercalary neural arches), regularly alternating with the preceding, and completing the walls of the neural canal by filling up the intervals between the basi-dorsals; and (iii.) a series of supra-dorsal elements, typically also in pairs, but in the Dog-Fish fused to form single median cartilages. Of the latter there are two sets—one the supra-basi-dorsals, or neural spines, are situated over the basi-dorsals; and the other, supra-inter-dorsals, alternating with the former, lie over the inter-dorsals, the two series forming the keystones of the dorsal arches, and thus completing the roof of the neural canal. On the ventral side of the notochord this arrangement is substantially repeated by a series of ventral arcualia, which, however, are somewhat differently arranged in the trunk and tail. Thus, in the trunk there are: (i.) a series of basi-ventral or haemal cartilages, corresponding with the basi-dorsals above, which grow out laterally into short processes, the parapophyses or transverse processes, and terminate in (ii.) short, slender cartilages—the costal elements or ribs—which may perhaps be regarded as the ventral equivalents of supra-basi-dorsals. The ribs project outwards into the dorsal wall of the coelom and end in the myocommata separating the myotomes of the body-wall. In the tail the basi-ventrals lose their ribs and, growing downwards into ventral prolongations, they unite in pairs beneath the caudal artery and vein, and so form a series of inverted arches {195}(haemal arches) enclosing a haemal canal (Fig. 112, B). The apex of each arch is prolonged into a median process or haemal spine. Although not recognisable in the Dog-Fish, paired inter-ventral cartilages, corresponding with the inter-dorsals above, are present in some Elasmobranchs and alternate with the basi-ventrals. In the caudal region of others (e.g. Skates and Rays) ventral counterparts of the supra-interdorsals are present, and are termed infra-ventral cartilages. Much in the same way that their dorsal equivalents enclose a neural canal, so the ventral arcualia partially surround the viscera-containing coelom in the trunk; and in the tail, but more completely, the vestigial coelom of that region or the haemal canal.

The different vertebral components are by no means of equal morphological value. The basi-dorsals and basi-ventrals, and the inter-dorsals and inter-ventrals, are the primary elements and the most important. The supra-dorsals are merely cartilages segmented off from the basi-dorsals and inter-dorsals, while the ribs and the infra-ventrals are similarly derived from the basi-ventrals and inter-ventrals respectively. As to the vertebral elements which collectively form a vertebra in the Dog-Fish, it would seem from evidence afforded by the neuromeres, and more especially by the facts of development, that each complete skeletal segment or vertebra consists of a pair of basi-dorsals with the preceding pair of inter-dorsals, and of a pair of basi-ventrals with the next succeeding pair of inter-ventrals. It must be emphasised, however, that, considered as a joint or segment in a flexible back-bone, a vertebra is a physiological unit, the morphological value of which may differ widely in different Fishes. Hence, in other Fishes, the grouping of vertebral components to {196}form individual vertebrae may be quite different to that which takes place in the Dog-Fish, and may even be accompanied by their more or less complete fusion.

In the more primitive types of vertebral column, such as are characteristic of many fossil and not a few existing Fishes, arcualia alone are present, and remain associated with a persistent notochord which has grown with the growth of the animal. In the more specialised Fishes, on the contrary, the need of an axial support for the body, which, while retaining the necessary flexibility, must possess greater strength, has resulted in the development of a series of solid cartilaginous, calcified or bony, discoidal joints or segments, the centra, which surround and more or less completely replace the notochord, and, while supporting, form also a bond of connexion between the dorsal and ventral arches. Notwithstanding their superficial resemblance, an important developmental distinction is to be noted in the mode of formation of centra in different Fishes, which enables one kind to be distinguished as "chorda-centra," and another as "arch-centra." Chorda-centra are centra formed by the conversion of the chordal sheath into a series of ring-like cartilaginous segments, which subsequently, by a process of inward thickening, become biconcave, disc-like structures, and more or less completely replace the notochord, except in the spaces between them. Arch-centra, on the other hand, owe their formation to the growth of the bases of the primary arcualia round the notochord, external to the chordal sheath, and their subsequent fusion to form annular segments, which, later, become biconcave centra. Of Fishes which possess vertebral centra the Elasmobranchs alone have chorda-centra; the Holostei and the Teleostei, and very probably the Crossopterygii also, having arch-centra. The Dipnoi and the Holocephali, and the Chondrostean Teleostomi are acentrous—that is, they are devoid of vertebral centra and possess a persistent notochord. Neither in their embryonic development nor in their evolution in time are the different vertebral components synchronous in their appearance. Developmentally, the arcualia are the first to be formed, and of these those on the dorsal aspect of the notochord appear earlier than their representatives on the ventral side, while the centra are the last of all; and in a general way the palaeontological sequence agrees with the embryological.

{197}The independent evolution of a more specialised vertebral column from a more primitive one may often be traced within the limits of the same group of Fishes when the more ancient genera are compared with the more recent. In the Elasmobranchs and the Crossopterygii, for example, the oldest known types were acentrous, while the more recent have acquired calcified or bony centra, and altogether they have reached a more advanced stage of vertebral evolution. Some Fishes, like the Chondrostei and the Dipnoi, seem, however, to exhibit comparatively little advance in vertebral structure, since both the Palaeozoic and the living representatives of these groups agree in being acentrous.

Some of the more notable features in the structure of the vertebral column in the Cyclostomata and Fishes will now be briefly considered.

In the Cyclostomata the acentrous vertebral column is more primitive than in any other Craniates, and in the Lamprey it consists of a persistent notochord, supporting a series of isolated cartilages on each side of the spinal cord. As two pairs of these cartilages are included in each neuromere it is possible that they represent alternating basi-dorsals and inter-dorsals. There are no ventral arcualia in the trunk and no ribs. In the Hag-Fish (Myxine) the dorsal cartilages are restricted to the tail.

The description of the vertebral column of the Dog-Fish may be taken as fairly applicable to Elasmobranchs in general, and hence only certain notable features in some other members of the group need be referred to here. The most primitive Elasmobranchs, the Palaeozoic genera Cladoselache and Pleuracanthus were acentrous, although calcified rings have been observed in a Permian species of the latter genus and scattered calcifications in others. Some of the earlier Mesozoic genera (e.g. Hybodus) were also devoid of centra, at least in the trunk-region. The first indication of complete centra occurs in the Lower Lias Cestraciont, Palaeospinax. All the later extinct, as well as all existing forms, have more or less well-developed centra, hardened by the deposit of lime salts in their primitively cartilaginous substance, but never in the form of true bone.

{198}[Illustration: FIG. 113.—Schematic transverse section through the middle of a Cyclospondylic (A), a Tectospondylic (B), and an Asterospondylic vertebra (C). d, Middle portion of the calcified double cone; d′, additional concentric calcified layers; d″, double cone with radiating calcified layers; ex.m, external elastic membrane; h.a, haemal arch; n.a, neural arch; n.c, notochordal cavity. (From Zittel, after Hasse.)]

The mode in which the lime is deposited is marked by certain peculiarities which are characteristic of particular families (Fig. 113). In some genera, as in the extinct Palaeospinax and the living Acanthias and Scymnus, the calcified portion of each centrum takes the form of a cylinder constricted across the middle, like two cones joined apex to apex (cyclospondylic). This condition is probably the most primitive, but it may be modified in other genera by the further addition of calcic salts in two different ways. Thus, the deposit may take place by the simple addition of concentric layers to the original constricted cylinder (tectospondylic), as in the Skates and Rays; or it may take the form of a series of longitudinal plates radiating outwards from the cylinder, and giving rise to a star-like pattern in cross-section (asterospondylic), as in Scyllium and Lamna. In most living Elasmobranchs (e.g. Scyllium), but not in such genera as Notidanus, Heterodontus, and Squatina, the bases of the dorsal and ventral arches grow round the centra and meet, or even fuse, so that the latter become surrounded by rings of cartilage which, after a fashion, suggest incipient arch-centra (Fig. 112, A). The caudal portion of the vertebral column is often described as "diplospondylic," that is, there are two centra, two pairs of basi-dorsals, two pairs of inter-dorsals, and two pairs of basi-ventrals, {199}or in other words, two vertebrae to each neuromere (Fig. 111, B).

The Holocephali have a vertebral column essentially similar to that of other Elasmobranchs, but of a more primitive type (Fig. 114). The notochord is persistent and there are no centra; but ring-like calcifications, four or five to each neuromere, occur in the chordal sheath in Chimaera, although not in Callorhynchus. Ribs are absent. In the whip-like terminal portion of the tail the arcualia and the notochord become replaced by a slender continuous filament of cartilage.

In the more obvious features of vertebral structure the Dipnoi have much in common with the Elasmobranchs, especially with certain of the acentrous Palaeozoic representatives of that group. The notochord is persistent, centra are wanting, and the different vertebral components continue to retain their primitive distinctness. On the other hand, the basi-dorsals are much better developed than the inter-dorsals, which are either vestigial or absent. The basi-dorsals unite in pairs over the spinal cord to form complete neural arches, and each arch supports dorsally the legs of a Λ-shaped, gable-like element or neural spine, which probably represents a pair of fused supra-basidorsals. Ventrally, there are basi-ventral cartilages, fused in pairs beneath the notochord, and supporting well-developed, bone-ensheathed ribs. {200}Inter-ventrals appear to be absent. Each neuromere corresponds with a pair of basi-ventrals, of basi-dorsals and of inter-dorsals. The haemal arches and spines are formed partly by the basi-ventrals, but mainly by the ventral union of the successive pairs of ribs. As in the Holocephali, the terminal arcualia of the tail become fused into a straight axial cartilaginous filament, transversely divided into segments, which replaces the notochord. Each segment supports a variable number of dorsal and ventral gable-pieces, or neural and haemal spines. Certain of the vertebral components, such as the ribs, and the neural and haemal spines, are ensheathed by membrane bone.

With certain modifications in details the preceding description will also apply to the vertebral column of the Chondrostei (Fig. 115). It will be noted, however, that the inter-dorsals are much better developed than in the Dipnoi, although when {201}compared with the basi-dorsals they take but a small share in forming the walls of the neural canal. Well-developed but somewhat fragmentary inter-ventrals are present. The haemal arches and spines are formed by the downgrowth and ventral union of the basi-ventrals as in the Dog-Fish, and apparently without the aid of costal elements. In Polyodon the ribs are vestigial, but in Acipenser they are well developed. The neural arches and spines, and their haemal representatives in the tail, and also the ribs, are partially ossified, or ensheathed by bone.

In the existing Crossopterygii, Holostei, and Teleostei, popularly known as the "bony Fishes," the vertebral column assumes a more familiar character, and at the same time we meet with interesting illustrations of the different methods by which the separate component vertebral elements of the more primitive types of "backbone" are concentrated together in groups, and fused to form that complex physiological product, the complete bony vertebra. In most of these Fishes each vertebra is formed by the aggregation and fusion of a pair of basi-dorsals and a pair of basi-ventrals, and includes, in addition, a pair of inter-dorsals, which may either be the pair in front of the basi-dorsals or the pair behind, and also a pair of inter-ventrals, which, similarly, may be the {202}pair in front or behind the basi-ventrals (Fig. 116). The product of this fusion is a series of bony vertebrae, each consisting of a biconcave arch-centrum, which includes the fused basal portions of a pair of basi-dorsals and a pair of basi-ventrals. The distal portions of the basi-dorsals form the neural arch, while the rib-bearing parapophyses are lateral outgrowths from the basi-ventrals which otherwise have become merged in the centrum. Finally, the centrum is completed by its fusion with a pair of inter-dorsals and a pair of inter-ventrals. Supra-dorsal elements may also be included as minor contributory factors. The supra-basi-dorsals co-ossify with their basi-dorsals and then unite to form the ordinary unpaired neural spine of most bony Fishes, or, as in Amia, they remain distinct from each other, and are obvious as a double spine. In Lepidosteus these elements co-ossify with the neural arches and form the post-zygapophyses. Supra-inter-dorsals have been identified in the embryo as distinct elements, but their eventual fate is not always known. In Lepidosteus they persist as distinct cartilages in the adult (Fig. 118, A). Well-developed bony ribs are usually present. The haemal arches of the tail are formed by the downgrowth of the parapophyses and their ribs, or by the latter alone, and by their ventral union to form haemal spines; consequently, each arch always includes a pair of costal elements. With such general features in common there are certain notable variations in some of these Fishes, to which brief reference may be made.

Little is at present known of the development of the vertebral column in either of the only two existing genera of Crossopterygii, Polypterus and Calamichthys, and hence the precise mode of grouping of their vertebral components to form vertebrae is unknown. The condition of the vertebral column in the fossil forms varies greatly in different families, but in none is it so specialised as in the surviving members of the group. In the Devonian Holoptychidae, and even in genera so comparatively recent as the Upper Cretaceous Coelacanth Macropoma, the persistence of the notochord and the absence of centra indicate a very primitive grade of vertebral evolution. The Devonian and Carboniferous Rhizodontidae (e.g. Eusthenopteron and Rhizodus), on the contrary, seem to have had well-ossified ring-like vertebrae.

{203}In the caudal region of Amia the basi-dorsals and basi-ventrals, and the inter-dorsals and inter-ventrals, form separate arch-centra which remain distinct; hence each vertebra is double, and there is a regular alternation of arch-bearing "pre-centra" and arch-less "post-centra" (Fig. 117, D). In the trunk-region the pre- and post-centra have fused, and in this region the vertebrae are single.

A very primitive type of vertebral column occurs in some of the Jurassic allies of Amia, in which certain of the vertebral components, confluent in the adult Amia, retain some measure of their primitive distinctness. Thus, in the precaudal region of Eurycormus (Fig. 117, B) there is a series of alternating dorsal and ventral half-rings of bone, which between them completely invest the persistent notochord. Each ventral half-ring or "hypocentrum" represents a pair of fused and ossified basi-ventrals, and possibly also a pair of included inter-ventrals, and supports dorsally a pair of basi-dorsals, forming a neural arch, and laterally a pair of ribs. The dorsal semi-rings, or "pleuro-centra," similarly represent fused and ossified pairs of inter-dorsals. In the tail, modifications approximating to what is seen in the caudal region of Amia are to be noticed (C). By the upgrowth of the ventral arch-bearing semi-rings, and their conversion into complete rings encircling the notochord, incipient pre-centra are formed, and by a similar modification of the {204}down-growing, archless, dorsal half-rings, structures comparable to post-centra are produced. In brief, Eurycormus, as well as such other extinct Amioid genera as Caturus (Fig. 117, A), Callopterus, and Euthynotus, retain in the adult a stage of vertebral evolution which is closely paralleled by transitory stages in the embryonic and young forms of Amia.

Lepidosteus is unique amongst existing Fishes in having opisthocoelous vertebrae; that is, the centra are convex in front and concave behind, and therefore articulate with one another by ball-and-socket joints (Fig. 118). This condition is due to the development of a series of intervertebral rings of cartilage round the notochord. The subsequent inward growth of each of these rings leads to the constriction, and ultimately to the complete obliteration, of the notochord, much in the same way as by the growth of ordinary centra. Later, this solid mass of cartilage becomes transversely divided by a cleft which is convex anteriorly and concave behind (Fig. 116, C), and of the two portions one fuses and co-ossifies with the centrum of the vertebra in front, and the other with the one pertaining to the vertebra behind. Reference to Fig. 116 will show that the grouping of the vertebral elements to form the individual vertebrae is not the same as in Amia.

{205}In the dominant group of existing Fishes, the Teleostei, the centra are almost invariably biconcave, although in the Eels they may be flat or even slightly convex in front. Ribs are absent in the Syngnathidae and in the Plectognathi. In addition to the usual articulation between the centra, the vertebrae often articulate with one another by means of paired processes arising from the anterior margin of each neural arch, or from the centrum at the base of the arch (pre-zygapophyses), and meeting similar processes which project either from the hinder margin of the arch of the vertebra in front, or from the adjacent portion of its centrum (post-zygapophyses). The haemal arches may have similar processes (Fig. 119). One, two, or in some Teleosts, three pairs of slender intermuscular bones radiate outwards from the centra into the myocommata (epicentrals), or from the neural arch (epineurals), or from the ribs (epipleurals).

THE RIBS.—It is doubtful if the structures termed "ribs" are homologous in the different groups of Fishes. There appear to be two kinds, distinguishable as dorsal and ventral ribs (Fig. 156). Dorsal ribs are situated in the fibrous tissue separating the epiaxial from the hypaxial muscles of the body wall, and they take no part in forming the haemal arches of the caudal region. Ventral ribs, on the other hand, always lie internal to the hypaxial muscles, and directly external to the peritoneal lining of the coelom, and they usually contribute to the formation of {206}the haemal arches. To the former belong the ribs of the Elasmobranchs, and to the latter the ribs of the Teleostomi and Dipnoi. Polypterus alone has both kinds of ribs.

THE SKULL.

The skull is a highly complex structure, the various components of which are as different physiologically as they are morphologically. It consists (i.) of the cranium, for the enclosure and protection of the brain; (ii.) of sense capsules, which fulfil a like function for the auditory, visual, and olfactory organs; (iii.) of certain vertebrae or vertebral elements fused with the hinder part of the cranium; (iv.) of a series of visceral arches; and (v.) of a series of paired or median cartilages developed in relation with the mouth and nostrils, which may be collectively spoken of as "labial" cartilages.

The cranium is formed in the embryo from two pairs of cartilaginous rods or plates, developed in the mesoblast of the head. Of these the posterior pair, or parachordals, underlie the hinder part of the brain, and are situated one on each side of the cranial portion of the notochord. The anterior pair or trabeculae are pre-notochordal, and lie beneath the anterior portion of the brain. Between their hinder extremities, and in front of the anterior termination of the notochord, is the pituitary body. As development proceeds the parachordals blend with each other and with the trabeculae, while the latter fuse in front to form a median plate—the mesethmoid cartilage. The hinder portions of the two trabeculae remain distinct for some time, and enclose between them the pituitary fontanelle, but later they fuse beneath the pituitary body, leaving, however, a pit for its reception—the pituitary fossa. Cartilaginous capsules are formed round the cranial sense organs. The auditory or periotic capsules fuse on each side with the parachordals. The optic capsules, either fibrous or cartilaginous, remain free, and do not fuse with the adjacent trabecular region. The olfactory capsules alone are not developed independently, but are formed as lateral {207}outgrowths from the mesethmoid plate. Later, the parachordals and trabeculae grow upwards on each side round the brain, and to a greater or less extent they meet and fuse on its dorsal surface, thus enclosing the latter organ in a cranial cavity, leaving, nevertheless, a large foramen behind (foramen magnum) through which the brain is continuous with the spinal cord. In this condition the primitive cartilaginous cranium, with its included sense-capsules, has reached a stage which is permanently retained in such Fishes as the Elasmobranchs.

The visceral arches consist of a number of pairs of curved rods of cartilage, at first simple, but subsequently segmented, and developed in the splanchnic mesoblastic walls of the oral cavity and pharynx. Each rod is connected with its fellow by a median cartilage in the floor of the pharynx, so that the whole form a series of dorsally incomplete hoops encircling the anterior portion of the alimentary canal. No doubt all the visceral arches were originally branchial arches, and were so disposed between the successive gill-clefts as to support their walls and the vascular folds or gill-lamellae to which they gave rise. In Fishes most of the arches still retain their primitive gill-supporting function, but the first or mandibular arch has become modified to form upper and lower jaws, although in the Sharks and Dog-Fishes it may lie in front of a gill-cleft and still be associated with vestigial gills. The second or hyoid arch is less removed from the condition of a branchial arch, and generally supports either a functional or a vestigial gill, but in most Fishes it has acquired the secondary function of forming a suspensorium for the attachment of the jaws to the cranium.

The skull of the common Dog-Fish, Scyllium canicula (Fig. 120), may be studied as a type which in the adult remains cartilaginous, and has no secondary addition of cartilage- or membrane-bones. In this Fish the chondrocranium, or primary cartilaginous cranium, presents the appearance of a somewhat depressed oblong box, which has a complete roof, side-walls, and floor, but is open in front (anterior cranial fontanelle) and also behind (foramen magnum). The hinder, or parachordal portion of the cranium surrounds the foramen magnum, and there forms the occipital region. At the ventral margin of the foramen there are two prominences, or occipital condyles, for articulation with the first vertebra, and between them the remains of the notochord are traceable into the cranial floor.

{208}[Illustration: FIG. 120.—Side view of the skull of the common Dog-Fish (Scyllium canicula). aud.cp, Auditory capsule; br.a 1, 5, branchial arches; br.r, br.r′, cartilaginous rays attached to the hyoid arch and the first four branchial arches; Cr, cranium; ex.br, extra-branchial cartilages; hy.cn, cerato-hyal; hy.m, hyomandibular; lb, labial cartilages; lg, ligaments passing from the jaws to the cranium and to the distal end of the hyomandibular; lg′, ethmo-palatine ligament; l.j, lower jaw or Meckel's cartilage; Nv. 2, optic foramen; Nv. 5, foramen for the Vth and part of the VIIth cranial nerves; olf.cp, olfactory capsule; or, orbit; up.j, upper jaw or palato-quadrate cartilage. (From Wiedersheim, after W. K. Parker.)]

In front of the occipital region two lateral bulgings indicate the periotic capsules, and more anteriorly still, in the trabecular region, the sides of the cranium are modified to form two spacious lateral recesses, the orbits, each of which is bounded above and below by supra-orbital and infra-orbital ridges respectively, behind by an outgrowth from the periotic capsule (post-orbital process), and in front by a similar projection from the hinder wall of the olfactory capsule (lateral ethmoidal process). In front of the cranial cavity and the orbits may be seen the laterally-placed dome-like olfactory capsules, which are open below, where the nasal sacs communicate with the exterior. Between the two capsules an anterior extension of the cranial floor forms a flattened mesethmoidal plate, behind which is the large, membrane-closed, anterior cranial fontanelle. The lateral walls of the cranium are perforated by numerous apertures, some of which serve for the entrance or exit of blood-vessels, and others, mostly pertaining to the inner walls of the orbits, for the transmission of the different cranial nerves from the brain to {209}various parts of the head. In many Elasmobranchs the roots of certain of the anterior spinal nerves perforate the side-walls of the occipital region, and indicate the fusion of vertebral components with the cranium. In the cranial roof between the two periotic capsules there are two small apertures at the bottom of a common median depression: through each aperture the ductus endolymphaticus (aqueductus vestibuli) passes from the vestibular part of the auditory organ to the exterior of the skull.

Three cartilaginous rods, one from the roof of each olfactory capsule, and one, the prenasal or rostral process, from the ethmoid cartilage, converge and meet, or nearly meet, in front to form the rostrum or support for the preoral or "cut-water" portion of the head.

The visceral arches are seven in number. The first or mandibular arch consists on each side of an upper portion, the palato-pterygo-quadrate or palato-quadrate cartilage, which passes forwards in the side-wall of the oral cavity, along the upper margin of the mouth, its anterior or palatine part curving inwards to a ligamentous connexion with its fellow beneath the cranial floor. Each cartilage has an upwardly directed process (ethmo-palatine process) which is connected by a suspensory ethmo-palatine ligament with the lateral wall of the cranium behind the lateral ethmoid process. The lower or ventral half of the mandibular arch (Meckel's cartilage) is similar in shape to the upper; it articulates behind with the quadrate portion of the latter by a movable joint, and is thence prolonged forwards and downwards in relation with the lower margin of the mouth to a median ligamentous union with its fellow of the opposite side. The palato-pterygo-quadrate and Meckel's cartilages together form the primitive upper and lower jaws, and support the teeth. The hyoid arch also consists of a dorsal and a ventral half on each side. The dorsal half or hyomandibular element articulates above with the periotic capsule. The ventral portion, or cerato-hyal, passes downwards and is connected with its fellow by a median copula or basi-hyal cartilage situated in the floor of the oral cavity. A series of simple cartilaginous rays (branchial rays) are attached to the hinder margins of the hyomandibular and cerato-hyal elements. The distal end of the hyomandibular is connected by strong ligaments with the hinder portions of both the palato-pterygo-quadrate cartilage and Meckel's cartilage; in fact, {210}the hyomandibular is the effective suspensorium by which the upper and lower jaws are connected with the skull, and all Fishes in which this arrangement exists are said to be hyostylic. Behind the hyoid arch follow five branchial arches. Each of these is segmented into a dorsal or pharyngo-branchial element, followed by an epi-, a cerato-, and a hypo-branchial piece, but the later element is absent in the fifth arch. The lateral halves of the last three arches are connected ventrally by a large median basi-branchial cartilage, but in the first and second arches by the median apposition of their respective hypo-branchial elements. Like the hyomandibular and cerato-hyal segments of the hyoid arch, the epi- and cerato-branchial elements of all the branchial arches except the fifth are fringed along their outer convex margins by a series of branchial rays, and, in addition, there are three pairs of slender, curved, cartilaginous rods, or extra-branchials, in relation with the distal extremities of the branchial rays of the second, third, and fourth branchial arches. The function of the branchial arches, and their branchial rays, and extra-branchial cartilages, is to support the inter-branchial septa which separate the gill-clefts and carry the vascular gill lamellae. All the arches lie near the inner margins of the septa, close to the hypoblastic epithelium of the pharynx, while the outer portions of the septa are supported by the branchial rays and the extra-branchials, the latter lying directly beneath the external skin. The segments of the arches are movably connected with one another by ligaments; and by the contraction of the branchial muscles the arches may be separated or approximated so as to enlarge or diminish the size of the intervening clefts.

The labial cartilages are represented by a pair of slender rods in relation with the outer surfaces of the palato-pterygo-quadrate cartilages, and a similar pair in connexion with the Meckelian cartilages. There is also a pair of small cartilages in relation with the nostrils. It is probable that the rods which constitute the lateral elements of the rostrum belong to the same category.

In the Cyclostomes and the Elasmobranchs the skull is entirely cartilaginous, although it may often be superficially calcified in Elasmobranchs, and although there may even be definitely and symmetrically arranged calcified plates in Pleuracanthus, true bone is never present. In many Fishes, and notably in the Teleostomi, {211}the embryonic cartilaginous cranium becomes complicated by the addition of an extensive series of investing membrane bones, formed by the ossification of the connective tissue external to the cartilage, so that a secondary bony cranium is formed external to the primary cranium much in the same way that a secondary pectoral girdle is formed in connexion with the primary girdle. Such bones probably owe their primary origin to the fusion and insinking of exoskeletal structures (scales or dermal spines). To these investing bones there may also be added a series of bones formed by the actual conversion of the cranial cartilage into osseous tissue (cartilage bones), which to a greater or less extent in different Fishes replaces the original cartilage. The bones of the skull may conveniently be classified as follows:—(i.) Dermal or membrane bones. Under this head are included—(a) the ordinary investing bones of the skull. (b) Tooth-bones, that is, bones formed by the fusion of the bases of teeth and developed in relation with the walls of the oral cavity. Probably all tooth-bearing bones are of this nature. (c) Sensory canal bones, that is, tubular bones developed round the sensory canals of the head. Certain of these bones may secondarily acquire the shape and character of investing bones while still retaining protective relations to their sensory canals. (ii.) Cartilage bones.

As an easily obtainable example of a skull which has acquired a fairly complete series of both cartilage- and membrane-bones, while retaining a well-developed primary cranium, the skull of the Salmon (Salmo salar) may be described. At an early stage of development, even so late as the second week of hatching, the primary cranium is still entirely cartilaginous, and in this condition the Salmon's skull is comparable with that of an adult Dog-Fish. As development proceeds the primary cranium becomes supplemented by the addition of numerous investing dermal bones which form the secondary cranium, and later cartilage bones appear and, to a considerable extent, replace the original cartilage. The Salmon's skull is interesting in this respect, that the primary cranium grows with the growth of the Fish, so that in the adult the nasal, ethmoidal, and prenasal regions are entirely cartilaginous, and in the hinder part of the cranium cartilage is largely persistent between the cartilage bones.

Dealing first with the cartilage bones of the primary cranium, {212}it may be stated that there are formed in that part of the parachordal cartilage surrounding the foramen magnum a median basioccipital below, which is concave behind where it articulates with the centrum of the first vertebra, a supraoccipital above, and two laterally-placed exoccipital bones (Figs. 121, 122). Each periotic capsule is ossified by the formation of five bones in the primitively cartilaginous mass, the prootic, sphenotic, opisthotic, epiotic, and the pterotic. The inner walls of the capsules have atrophied in the adult, and hence the cavities which contain the auditory organs appear as open lateral recesses of the cranial cavity. In front of the periotic capsules there are various bones which are formed in the cartilage of the trabecular part of the cranium. Thus, in front of the basi-occipital, and developed in the cartilage of the cranial floor, there is a median Y-shaped basisphenoid, and, at some distance above it on each side, an alisphenoid helps to form the lateral wall of the cranial cavity. Between the eyes the side walls of the cranium fuse to form a vertical inter-orbital septum, and, in consequence, two orbito-sphenoid bones, which normally form the lateral cranial walls in this region, become partially confluent in the median line and close the cranial cavity in front. The only cartilage bones found in the massive persistent portion of the primary cranium which forms the pre-orbital region are the projecting lateral ethmoids, forming the posterior boundaries of the recesses for the olfactory organs, and separating the latter from the orbits.

The roof and floor of the primary cranium is completed by {213}certain investing dermal bones (Fig. 123, A). A pair of large frontal bones form the cranial roof, and also help to roof in the orbital cavities. Behind the frontals, and separated from each other by the supraoccipital, there is a pair of small parietals, and anterior to the frontals a median dermal mesethmoid. A small nasal bone overlies each olfactory recess. Ventrally, the base of the cranium, from the basi-occipital to the prenasal region, is strengthened by a large parasphenoid behind, and a much smaller vomer in front, both of which lie in the roof of the mouth. The vomer is a tooth-bone, and probably the parasphenoid also.

The mandibular arch (Fig. 123, B) is more modified than that of the Dog-Fish. The palato-pterygo-quadrate bars, or primitive upper jaw, no longer meet in front beneath the cranial floor, but each separately articulates in front with the lateral ethmoid of its side. Although still partly cartilaginous each bar is largely replaced either by cartilage bones, or by bones which begin as membrane bones or as tooth-bones and complete their growth by invading the cartilage and becoming in part cartilage bones. Its anterior portion is formed by a palatine bone which articulates with the lateral ethmoid, and the middle portion by a pterygoid and a mesopterygoid bone, while the hinder part is ossified above as a metapterygoid and below as a quadrate. The latter articulates with the lower jaw. Functionally, however, the primitive upper jaw is now replaced by a secondary upper jaw, formed on each side by a series of tooth-bones, situated external to the former, and meeting in front of the prenasal region of the primary cranium (Fig. 123, A). The series includes a dentigerous premaxilla and maxilla, and a small toothless, scale-like jugal bone. Each half of the lower jaw (Fig. 123, A, B) consists of a rod-like Meckel's cartilage or primary lower jaw.

{214}[Illustration: FIG. 123.—A, view of the left side of the skull of a Salmon; B, the left half of the primary upper and lower jaws, and the hyoid arch. The cartilage is dotted. an, Angular; ar, articular; b.hy, basi-hyal; br.r, branchiostegal rays; c, cranium; c.h, cerato-hyal; c.or, circum-orbital bones; d, dentary; d.eth, dermal mesethmoid; ep.h, epihyal; ep.o, epiotic; eth.p, ethmo-palatine process; f, frontal; h.hy, hypo-hyal; hym, hyomandibular; i.op, inter-operculum; j, jugal; mks, Meckel's cartilage; mpg, mesopterygoid; mt.pg, metapterygoid; mx, maxilla; n, nasal; op, operculum; op′, condyle on the hyomandibular for the operculum; orb, orbit; p, parietal; pa, palatine; p.mx, premaxilla; p.op, pre-operculum; pt, pterygoid; pt.o, pterotic; q, quadrate; so, supra-occipital; s.op, suboperculum; sp.o, sphenotic; s.t, supra-temporal (or squamosal); st.hy, stylo-hyal; sy, symplectic; u.l.c, u.l.c′, upper labial cartilages; u.l.c^2, second upper labial. (From W. K. Parker.)]

The hinder part of this is ossified to form the articular, which has a deeply concave surface for articulation with the quadrate; and below this there is a small membrane bone, the angular. The rest of the cartilage is partially {215}ensheathed on its outer side by a large tooth-bone, the dentigerous dentary. The hyoid arch is similar to that of the Dog-Fish, except that its primitively cartilaginous segments are almost completely ossified (Fig. 123, B). The large upper segment or hyomandibular bone articulates mainly with the pterotic, but partly also with the sphenotic element of the periotic capsule; below it is connected with a slender symplectic bone, and from the cartilage connecting the two depends the rest of the hyoid arch, consisting in succession of stylo-hyal, epi-hyal, cerato-hyal, and hypo-hyal bones, with a median teeth-bearing basi-hyal. The palato-pterygo-quadrate bar has no direct connexion with the skull, except anteriorly where its palatine element articulates with the lateral ethmoid. The real suspensorium is formed by the hyomandibular and symplectic bones, to which the hinder margins of the quadrate and metapterygoid bones are rigidly attached by suture, hence, as in the Dog-Fish, the skull is hyostylic. Behind the hyoid arch there are five branchial arches, which generally resemble those of the Dog-Fish, except that their component segments are ossified as cartilage bones.

Connected with the hyomandibular and cerato-hyal elements of the hyoid arch there is, on each side, a series of membrane bones for the support of the movable operculum or gill-cover. These consist of an operculum above, which articulates with a backwardly projecting process from the hyomandibular, followed in succession below by a sub-operculum and an inter-operculum, the latter being connected by ligament with the angle of the lower jaw. The series is completed by ten sabre-shaped branchio-stegal rays, which are attached to the cerato-hyal and support the lower margin of the gill-cover.

Sensory canal bones are represented in the Salmon by a ring of small bony plates which encircle the orbit (Fig. 123, A), and by one or two small bones situated above and on the outer side of each periotic capsule (squamosals). To these may be added the pre-operculum situated external to the hinder margins of the hyomandibular and quadrate bones, firmly clamping these bones together, and also the post-temporals, by which the secondary pectoral girdle is attached to the skull. The nasal bones may also be regarded as pertaining to the same series.

In other Fishes with a more or less complete bony skull there are certain additional cartilage- and membrane-bones which are not {216}present in the Salmon. There is usually a median ossification of the ethmoid cartilage, the mesethmoid. An entopterygoid is sometimes added to the palato-pterygo-quadrate series of bones. An ossification of the anterior extremity of each Meckelian cartilage may form a mento-Meckelian bone. Certain additional membrane bones are sometimes developed in relation with the lower jaw, such as splenial and coronary bones on the inner side, and a supra-angular bone at the angle of the jaw, above the angular element. To these there may be added the singular series of infra-dentaries, which in some fossil Crossopterygii (e.g. Rhizodopsis) fringe the outer margin of the jaw beneath the true dentary (Fig. 274, A). A system of jugular plates may also form a characteristic armature for the throat between the lateral halves of the lower jaw (Fig. 274, C). Besides those already mentioned, additional sensory canal bones are present in some Fishes. A transverse row of plates (supra-temporals) sometimes crosses the occipital region behind the parietals. There are also other canal-ossicles which lose their identity by fusing with certain cranial or periotic bones. Thus, each of the pterotic and sphenotic bones often includes a superficial dermal bone transmitting a section of a sensory canal, which has fused with it; and as the frontal bone is often similarly perforated, it may be taken that it also includes a canal-ossicle; and the same can often be said of the articular and dentary bones of the lower jaw.

Having now considered the general structure of a primitive cartilaginous type of skull, and the nature, disposition, and terminology of the various membrane- and cartilage-bones which may be added to, or more or less completely replace the former, reference will now be made to the more important features in the structure of the skull in the Cyclostomata and the Fishes.

In the Cyclostomata the skull presents a remarkable combination of characters, in some of which it is more primitive than in any other Craniates, while in others it has evidently attained a very high degree of specialisation on lines peculiar to the group, but differing in the two subdivisions.

{217}[Illustration: FIG. 124.—Skull, with branchial basket and anterior part of the vertebral column, of Petromyzon marinus. a.d.c, Anterior dorsal cartilage; a.lat.c, anterior lateral cartilage; an.c, annular cartilage; au.c, auditory capsule; br.b.1-9, vertical bars of the branchial basket; br.cl.1-7, external branchial clefts; cn.c, cornual cartilage; cr.r, cranial roof; l.c.1-4, longitudinal bars of branchial basket; lg.c, lingual cartilage; m.v.c, median ventral cartilage; n.a, neural arches; na.ap, nasal aperture; n.ch, notochord; Nv^2, foramen for optic nerve; olf.c, olfactory capsule; pc.c, cartilage surrounding pericardial cavity; p.d.c, posterior dorsal cartilage; p.lat.c, posterior lateral cartilage; sb.oc.a, subocular arch; st.p, styloid process; sty.c, styliform cartilage; t, teeth. (From Parker and Haswell, after W. K. Parker.)]

In the Lamprey (Fig. 124) the paired parachordals and trabeculae together form a trough-like chondrocranium, which has only a fibrous roof, except where a slender synotic band of cartilage extends between the two periotic capsules. The floor is also incomplete, a large pituitary fontanelle remaining to indicate the original separation of the trabeculae while transmitting the hypophysial or pituitary caecum. The notochord traverses the floor of the parachordal portion of the cranium as far as the pituitary fontanelle, and from the sides of the synotic ring the auditory capsules project in the shape of conspicuous lateral prominences. In front the otherwise open end of the cranial cavity is closed by the dorsally-placed and unpaired olfactory capsule, which is perforated behind by two apertures for the olfactory nerves, and has only a fibrous connexion with the cranial walls. Anteriorly to the olfactory capsule the cranial floor is prolonged forwards over the roof of the mouth as a large laterally-expanded plate, formed by the united anterior portions of the trabeculae, and no doubt representing the mesethmoid cartilage of the Dog-Fish. So far the cranium presents no special difficulty, and in its general features may be readily compared with that of an embryonic Elasmobranch. As for the rest of the skull, it is obvious that it has been greatly modified, partly to form and to support the skeletal framework of the remarkable suctorial buccal funnel, and partly to form the singular rasping lingual apparatus. Hence it is always difficult and sometimes impossible to identify with {218}certainty the component parts as being represented in other Craniates. On each side of the cranium, beneath the eye, there is a characteristic V-shaped subocular arch. Of its two legs the hinder one is continuous above with the periotic region of the cranium, and the other with the anterior trabecular region, while the pointed apex is directed obliquely downward and forward. From the hinder margin of the posterior limb a slender styloid process passes downward in the side wall of the pharynx, and terminates below in a forwardly directed cornual cartilage. A velum, fringed along its free margin with a series of tentacles, projects forwards into the oral cavity from between the oral apertures of the oesophagus and the branchial canal, and probably serves to prevent the entrance of foreign particles to the gill-sacs. This valve-like velum is supported by a velar skeleton, consisting of two lateral cartilages which are prolonged into the tentacles, and extend transversely between the inner surfaces of the two styloid processes. The apex of each subocular arch is connected with a small and somewhat triangular cartilage (postero-lateral cartilage), which is directed upward and forward, and lies in the side wall of the oral cavity. With some degree of probability the subocular arch may be compared to the palato-quadrate cartilage of a skull which has become "autostylic" in order to form a rigid support for the skeleton of the buccal funnel; the styloid processes and cornual cartilages to the hyoid arch; while the relations of the posterior lateral cartilages to the subocular arches suggest that they may possibly be regarded as Meckelian cartilages which have lost their primitive function of forming biting jaws. In the median line below, and projecting backward for some distance beneath the branchial canal, there is a long and stout lingual cartilage, carrying a small median and a still smaller pair of lateral cartilages at its anterior extremity, where it supports the lingual teeth and projects into the buccal funnel beneath the mouth. In front of the lingual cartilage, and connected by fibrous tissue with the inferior and hinder margin of the annular cartilage, there is a median T-shaped element, the median ventral cartilage. It has been conjectured that the lingual cartilage is a free basi-hyal element, and the median ventral cartilage the equivalent, elsewhere unknown, of the corresponding element of the mandibular arch.

{219}The remaining anterior skull elements are principally skeletal supports for the roof and walls of the buccal funnel. The roof is supported by an extended anterior dorsal cartilage, which is overlapped behind by the ethmoid cartilage, while the circular margin of the funnel is strengthened by a large ring-like annular cartilage. On each side of the latter there is a slender, rod-like, styloid cartilage, and above the latter a small anterior lateral cartilage. All these cartilages are usually termed labial cartilages, and it is at least possible that they possess representatives in the similarly named structures of the Dog-Fish and the larvae of some of the tailless Amphibia. It must not be forgotten, however, that the annular cartilage bears some resemblance to the ring of cartilage which encircles the lips of the buccal cavity in Amphioxus.

The complex supporting skeleton of the gill-sacs forms a basket-like structure. It consists on each side of nine unsegmented, irregularly curved, and slightly branched cartilaginous rods, situated in the outer margins of the inter-branchial septa, directly internal to the skin. The first lies directly behind the styloid process (hyoid arch), the second and third in front of and behind the first gill-sac, and of the remainder one lies just behind each of the six succeeding gill-openings; above and below each gill-aperture the rods are connected by longitudinal bars, and also in the median ventral line by a pair of similar partially united bars. The dorsal ends of the rods are also connected on each side by another longitudinal bar, which runs alongside the notochord and in front blends with the chondrocranium. The rods forming the last pair are continuous with a cup-like cartilage, supporting the lateral and hinder walls of the pericardium.

This singular branchial basket undoubtedly bears a superficial resemblance to the branchial arches of Fishes, but in any comparison of the two structures it is well to bear in mind that the branchial rods of the Lamprey are situated along the outer edges of the inter-branchial septa, and are therefore external to the gill-sacs and branchial arteries, and further, that they are developed in the somatic mesoblast of the embryonic protovertebrae, whereas true branchial arches are situated at the inner margins of the septa, internal to the gill-clefts and branchial arteries, and have their origin from the splanchnic layer of the mesoblast. So far as their position is concerned, the rods agree rather with the {220}extra-branchial cartilages of an Elasmobranch than with the more deeply-seated branchial arches.

While the skull of the Myxinoid Cyclostomes is constructed on the same general lines as that of the Lamprey, it is in some respects more primitive. It is also clear that in other features the skull has undergone marked specialisation on lines of its own, and in some points again it seems to deviate less from the more normal Craniate type. Of the more obvious differences, as illustrated by the skull of Bdellostoma (Figs. 125-127), it will be sufficient here to mention the following: (i.) The more primitive condition of the chondrocranium, the roof and side walls of the cranial cavity being entirely membranous. (ii.) The non-development of a suctorial buccal funnel and the presence of oral tentacles, associated with the absence of the complex system of labial cartilages and the substitution of a special tentacular skeleton. (iii.) The special modifications induced by the length and physiological importance of the naso-pituitary canal and by its communication with the pharynx after perforating the pituitary fontanelle in the cranial floor.

{221}[Illustration: FIG. 126.—View of the upper surface of the dental plate of Bdellostoma. t, Tendon of retractor muscle. (From Ayers and Jackson.)]

Under this head may be included the depression of the mesethmoid or hypophysial plate for the support of the naso-pituitary canal, the forward prolongation and median union of the palato-quadrate cartilages of opposite sides beneath the external portion of the canal, apparently for the support of the latter, and the encircling of the canal by supporting annular rings of cartilage. (iv.) The presence of two branchial arches, connected, as in Fishes, with a median basi-branchial segment which forms the middle one of the three divisions of the lingual apparatus. (v.) The reduction of the complicated extra-branchial basket to small isolated cartilages in relation with the gill-apertures and the œsophago-cutaneous duct. (vi.) The extraordinary development of the lingual apparatus, of which it has {222}been remarked that it "dominates the whole body, everything else yields to it." Meckel's cartilages are represented either by the cornual cartilages, as seems most probable, or by the dental plate (Fig. 125, c.c. and D).

In the generality of Elasmobranchs the skull resembles that of the Dog-Fish in essential structure. The more important modifications within the limits of the group relate to differences in the mode of attachment of the primitive upper jaw to the skull, and the number of branchial arches. In most Elasmobranchs the skull is hyostylic, as in Scyllium, but there are two genera which, in different ways, are exceptions to this rule. In Notidanus the hinder part of each palato-quadrate cartilage grows upwards into a strong post-orbital process, which articulates with the suitably modified post-orbital process of the periotic capsule (Fig. 128); hence the primitive upper jaw acquires a direct dorsal connexion with the cranium, and, as the hyoid arch is now relieved from taking any part in its support, the hyomandibular is reduced to the condition of a relatively slender rod of cartilage. By this arrangement both the mandibular and hyoid arches have their own separate and independent connexions with the cranium, and the skull is said to be amphistylic. The Port Jackson Shark {223}(Heterodontus) exhibits another and quite different modification. In this Fish the dorsal border of each palato-quadrate cartilage fits into a deep groove along the infero-lateral surface of the cranium, and is firmly attached thereto by ligament. Thus the first step is taken towards that more complete fusion of the two structures which is so characteristic a feature in the more typically autostylic Fishes like the Holocephali and the Dipnoi. Autostylism, whether incipient, as in Heterodontus, or complete, is to be regarded as a secondary modification, which may be independently acquired in widely different groups of Fishes, and is usually associated with the need of a firm and rigid support for an exceptionally massive dentition.

In the Holocephali (e.g. Chimaera) the cranium retains its primitively cartilaginous condition, and assumes a somewhat peculiar appearance owing to the lateral compression and vertical growth of its inter-orbital and nasal regions (Fig. 129). There is a complicated series of labial cartilages in relation with the ventrally-placed nostrils and the upper and lower jaws. In the males of Chimaera and Callorhynchus, but not in Harriotta, a movable cartilage is attached to the cranial roof, and supports the frontal clasper. The skull is typically autostylic. Along {224}the whole length of its dorsal border the palato-quadrate cartilage is fused with the inferior lateral margin of the cranium from the periotic to the olfactory region, thus forming a triangular plate of cartilage, the base of which is continuous with the cranium, while the downwardly directed apex provides an articular surface for the lower jaw. The hyoid arch is little better developed than the succeeding branchial arches, and includes a vestigial hyomandibular, an epi-hyal, and a cerato-hyal. As in other autostylic skulls the hyomandibular element is attached by ligament to the hinder margin of the palato-quadrate, instead of being directly connected with the periotic capsule, and obviously takes no part in supporting the jaws. Branchial rays for the support of the operculum are attached to the cerato-hyal, and some of them have their bases fused together. The five branchial arches resemble those of the Dog-Fish, except that they tend to become concentrated beneath the skull.

The existing Chondrostei, and especially the Sturgeon, are remarkable for the persistence and continuous growth of the chondrocranium, and the absence of true cartilage bones.

{225}[Illustration: FIG. 131.—Lateral view of the primary and secondary upper and lower jaws of Polyodon. b.br′, First basi-branchial; ch, cerato-hyal; d, dentary; hy.h, hypo-hyal; hy.m, hyomandibular; i.hy, inter-hyal; i.op, inter-operculum; lgs, ligaments connecting the palato-quadrate cartilage with the hyomandibular; mk.c, Meckel's cartilage; mx, maxilla; op, operculum; pa, palatine; pa.q, palato-quadrate; ps.l, pre-spiracular ligament; q, quadrate cartilage; sym, symplectic. (From Bridge.)]

Numerous dermal bones invest the dorsal surface of the chondrocranium, and only to a limited extent correspond with the less numerous membrane bones of the Salmon. To these are added a series of circum-orbital bones and a large parasphenoid. Undoubtedly the most striking feature in these Fishes is the primitive character of the upper jaw. In Polyodon (Fig. 131) the palato-quadrates are wholly cartilaginous, and, as in the Dog-Fish, they meet in front beneath the basis cranii, where the two are connected by ligament. The secondary upper jaw is but feebly developed, and is represented on each side by a thin splint-like maxilla in relation with the outer surface of each palato-quadrate cartilage, which meets its fellow in front. There are no premaxillae. The lower jaw is also very primitive. Meckel's cartilages are persistent, and except for a mento-Meckelian bone on each side, they are unossified, although membrane bones representing dentary and splenial elements are present. The skull is hyostylic. The hyoid and branchial arches are only partially ossified. Each opercular fold is supported by an operculum and an interoperculum, and both of these retain somewhat the shape of the cartilaginous hyoidean rays which they have replaced. In the Sturgeon (Fig. 130) the upper jaw is greatly modified in relation with the singular mouth of this Fish. The palato-quadrate cartilages meet not only in front, but also along their dorsal margins, and, with the help of the similarly opposed and somewhat fragmentary metapterygoid {226}cartilages, they form a complete concave roof for the protrusible spout-like mouth. Palatine, mesopterygoid, and pterygoid bones invest, and in some measure replace these cartilages. In brief, the skull of the Chondrostei occupies an interesting intermediate position between the purely cartilaginous and mainly bony types. While retaining a well-developed and unossified primary cranium, it has acquired a complete secondary cranium of dermal bones. Equally notable is the condition of the jaws. Unique among the Teleostomi in possessing the typical Elasmobranch union of the palato-quadrate cartilages beneath the basis cranii, the Chondrostei are so far specialised that they have acquired certain of the membrane bones which constitute the secondary jaws of the more typical bony Fishes.

As regards the general structure of the skull and the nature and disposition of its cartilage- and membrane-bones, the remaining living Teleostomi have much in common with the Salmon. In all the skull is hyostylic, and, unlike the Chondrostei, each half of the primitive upper jaw remains distinct from its fellow, and is separately articulated in front with the lateral ethmoid of the same side by its palatine element. The palato-quadrate cartilage is always more or less completely replaced by bones similar to those of the Salmon, and although they often carry teeth, as a rule they do little more than constitute a rigid buttress for the fixation of the quadrate condyle for the lower jaw. The secondary upper jaw is nearly always well developed, and includes a premaxilla as well as a maxilla on each side. There are, however, certain features in each of the minor groups which are either distinctive or highly characteristic.

In the surviving Crossopterygii (e.g. Polypterus) the chondro-cranium is complete in the ethmoidal and post-orbital regions, except where it has been partially replaced by cartilage bones, but in the inter-orbital region the continuity of the roof is interrupted by a large fontanelle, which is only closed by the investing frontal bones (Fig. 132, C). There is also a large basi-cranial fontanelle in the sphenethmoid, closed, however, by the underlying parasphenoid. A large "occipital" bone continuously ossifies in the occipital cartilage and completely surrounds the foramen magnum.

{227}[Illustration: FIG. 132.—A, side view of the skull of Polypterus; B, dorsal view, showing the chief dermal bones; C, similar view of the chondro-cranium after the removal of the dermal bones. An, Angular; Ar, articular; D, dentary; E, mesethmoid; f.m, foramen magnum; Fr, frontal; l.e, lateral ethmoid; Mx, maxilla; Na, Na′, nasal and accessory nasal bones; occ, occipital; ol, nasal aperture; Op, operculum; op.o, opisthotic; O.t, os terminate; Pa, parietal; Pm.x, premaxilla; P.t, post-temporal; Ptf, post-frontal; Qu, quadrate; S.b, S.b′, circum-orbital ossicles; S.Op, sub-operculum; Sp, splenial; sp.eth, sphenethmoid; sp.o, sphenotic; Spr, spiracular ossicles, between which is the spiracle; S.t, supra-temporals; Y, cheek-plate (pre-operculum); Y′, Y″, smaller cheek-plates; z, z, z, z, post-spiracular ossicles; z′, z′, prespiracular ossicles. In C the cartilage is dotted. (From Traquair.)]

Prootics and pterotics are absent, and the opisthotics seem to be confluent with their respective epiotics. The floor and side walls of the inter-orbital section of the cranium are formed by a remarkable "sphenethmoid" bone which occupies the position of the paired ali- and orbito-sphenoids in other bony Fishes; and in one species, P. lapradei, it forms in front distinct tubular investments round the olfactory nerves. In many respects this bone is singularly like the sphenethmoid bone of the Frog and other tailless Amphibia. A median ethmoid as well as lateral ethmoids are present. In addition to the ordinary dermal bones which invest the cranial roof there is a transverse row of supra-temporal plates crossing the cranial roof behind the paired parietals (Fig. 132, A). Fringing the outer margins of the frontals and parietals a row of pre- and post-spiracular ossicles extends nearly to the orbits, and between two of them, which form a {228}valve, is the spiracular aperture itself. There is a dentigerous splenial on the inner surface of the lower jaw. The hyoid arch has no separate symplectic bone. An operculum and a suboperculum are present, but no inter-operculum; and unless the hinder part of the large cheek-plate, which is traversed by the mandibulo-hyoid sensory canal, represents a pre-operculum, the latter is wanting. Branchiostegal rays are absent, but there is a single pair of large jugular plates.

Very little is certainly known about the cranial cartilage-bones in the fossil members of the group, but the investing dermal bones, which bear a general resemblance to those of Polypterus, are often somewhat more numerous, and they form a very complete dermal armature for the entire head. There is a very complete ring of circum-orbital bones, and very often a ring of sclerotic plates. Two large cheek-plates are often present. Nothing comparable to pre- and post-spiracular ossicles is known, but squamosal and supra-temporals can often be identified. To the ordinary bones of the lower jaw there may be added a series of infra-dentary plates, and besides the paired principal jugular plates there may also be present a small anterior median plate and a series of small lateral jugular plates on each side, as in the Carboniferous Rhizodopsis (Fig. 274). Most of the superficial dermal bones, both in the living and extinct Crossopterygii, are invested externally by a granulated or rugose layer of enamel-like ganoin.

In the Holostei, and especially in Amia, the skull approximates more closely to the normal Teleostean type as represented by the Salmon's skull. In Amia all the occipital cartilage-bones are present—a basi-occipital, two exoccipitals, and a supra-occipital; and, except for the absence of a pterotic, the periotic series of bones is also complete. Paired ali- and orbito-sphenoids form the lateral walls of the inter-orbital portion of the cranial cavity. Above, the complete cartilaginous roof of the cranial cavity is invested by a shield of suturally united and ganoin-covered dermal plates. The hyomandibular element has a symplectic bone at its distal extremity. There is a complete series of opercular bones, and the branchiostegal rays are numerous. A single median jugular plate is present. The lower jaw has on each side five dentigerous splenial bones in addition to dentary and angular bones, while cartilage-bones are {229}represented by articular and mento-Meckelian elements. In its essential structure the skull of Lepidosteus resembles that of Amia, but it has obviously undergone much specialisation. In some species (e.g. L. osseus) its appearance is greatly modified by the exceptional length and tapering shape of the beak, due to the elongation of that part of the skull which lies between the orbital and nasal regions; but in L. platycephalus the reduced length and greater width of the beak, combined with its somewhat flattened condition, impart an almost Crocodilian aspect to the head. Amongst other points of difference it may be mentioned that in Lepidosteus the continuity of the chondro-cranial roof is interrupted by a large superior fontanelle. There is no supra-occipital, and there are no lateral ethmoids, at all events in the usual position. The inter-orbital portion of the cranial cavity is largely obliterated by the formation of an inter-orbital septum, consisting of a thin vertical plate of bone, which either represents a pair of fused orbito-sphenoids or a pair of similarly modified lateral ethmoids. In addition to the ordinary investing dermal bones, including circum-orbitals, squamosal, and supra-temporals, there are numerous scale-like ossicles which take the place of the cheek-plates of Polypterus. The maxillae are segmented into numerous dentigerous bones fringing the margins of the upper jaw. The lower jaw has no mento-Meckelian bones, but there is a very complete series of dermal elements, including dentary, coronary, splenial, angular, and supra-angular bones in addition to an articular cartilage-bone. One of the most remarkable features in the skull of Lepidosteus is the existence of a secondary articulation between the metapterygoid bones and a pair of transversely elongated condyles formed on each side by a lateral outgrowth from the parasphenoid and alisphenoid bones. By a horizontal sliding movement of the former on the latter, provision is made for the lateral expansion and contraction of the walls of the oral cavity and the separation and approximation of the lateral halves of the upper jaw.

The generality of Teleosts more or less closely agree with Amia in the main features of their cranial structure. There are, however, certain minor features which are characteristic if not {230}always distinctive of the group. As a rule, to which, nevertheless, there are notable exceptions, there is little of the primary cartilaginous cranium in the adult, nearly the whole of it having become absorbed or converted into cartilage-bones. A supraoccipital is invariably present, and usually a mesethmoid and a basisphenoid. An additional bone is added to the periotic series, viz. a pterotic. Supra-temporal bones and jugular plates are always absent, and it may be doubted if mento-Meckelian bones and dentigerous splenials are ever developed in the lower jaw. Within the group itself the skull exhibits many notable modifications, of which only a few can here be mentioned. The shape, size, and character of the mouth and jaws, the extent to which they can be protruded and retracted, and the nature of the dentition, are the source of many characteristic modifications in the structure and appearance of the fore-part of the skull, and these again largely depend upon differences of habit and food. A protrusible mouth, or a mouth which is projected forwards, is usually associated with a suspensorium (hyomandibular) of considerable length, and so greatly inclined forwards as to make a more or less acute angle with the forepart of the cranium.

The presence or absence of an inter-orbital septum is also a feature in which considerable variation occurs. In some Teleosts there is no septum, and the cranial cavity is prolonged forwards between the orbits, where its lateral walls are formed by well-developed, paired ali- and orbito-sphenoid bones, as, for example, in the Carp and other Cyprinidae. In others the fusion of the cranial walls is accompanied by the median union of the orbito-sphenoids, so that a partly bony and partly cartilaginous inter-orbital septum is found, and the cranial cavity becomes largely obliterated in this region, as in the Salmon; or the orbito-sphenoids may be non-existent, the cartilage may undergo absorption, and the inter-orbital septum may become reduced to a vertical fibrous sheath extending between the frontals above and the parasphenoid below, as is the case in the Cod (Gadus).

An interesting modification of certain of the bones of the primary and secondary upper jaw occurs in the Siluridae. In these Fishes the maxillae are very small and edentulous, and serve no other purpose than forming basal supports for the maxillary barbels, while the rod-like palatine bone, losing its connexion with the pterygoid portion of the primitive upper jaw, {231}but retaining its articulation with the lateral ethmoid, serves to support the maxilla, and at the same time receives the insertion of the muscles by which the barbel is moved in various directions.

In the Plectognathi the premaxillae are co-ossified with the maxillae. Many other interesting cranial modifications occur in Teleosts, and to some of them reference is made in subsequent chapters.

In some respects the skull of Dipnoi is remarkably like that of the Holocephali, especially in its typical autostylism; but in possessing both cartilage- and membrane-bones it in some measure approaches the Teleostome skull. The investing dermal bones are not always easy to identify with those of other Fishes. In Neoceratodus an anterior median membrane-bone or dermal mesethmoid covers the ethmo-nasal region, and, on each side of it, forming the anterior boundary of the orbit, there is situated a pre-orbital or dermal lateral ethmoid. Behind the mesethmoid there is a much larger posterior median bone, and on each side a singular backward prolongation of the dermal lateral ethmoid separates it from a squamosal element. The latter bone descends on the outer surface of the quadrate portion of the palato-quadrate cartilage as far as the condyle for the lower jaw. Collectively, these bones form a fairly complete investment to the upper surface of the cranium, but the posterior median bone and the adjacent portions of the dermal lateral ethmoid and the squamosal are widely separated from the underlying chondrocranium by the powerful jaw muscles, and in this respect they differ from the ordinary roofing bones of other Fishes.

In Protopterus (Fig. 133) and Lepidosiren (Fig. 134) the posterior median bone is non-existent, and its place is taken by a large fronto-parietal, which forms the greater part of the cranial roof, internal to the jaw muscles, and is much larger in the latter Dipnoid than in the former. Circum-orbital bones are present only in Neoceratodus. A large parasphenoid supports the cranial floor. Vomers are absent, although there are two small vomerine teeth.

{232}[Illustration: FIG. 133.—Side view of the skull of Protopterus, with the pectoral girdle and fin. an, Angular; an.c, antorbital cartilage; c.c, coracoid cartilage (epi-coracoid); c.hy, cerato-hyal; cl, clavicle; c.r, cranial rib; c.sc, coraco-scapular cartilage; d.e, dermal ethmoid; d.l.e, dermal lateral ethmoid; e.g.f, external gills; eo, exoccipital; f.p, fronto-parietal; mk.c, Meckel's cartilage; n.a, neural arches; ol.c, fenestrated roof of the olfactory capsule; p.f, skeleton of the pectoral fin; p.pt, palato-pterygoid bone; p.q, palato-quadrate cartilage; s.cl, supra-clavicle; sp, splenial; sq, squamosal; 1-6, the branchial arches; the segmentation of the second and third arches is not shown. (From Wiedersheim.)]

Relatively small opercular and inter-opercular bones are present, and on the inner surface of each may be found vestigial remains of cartilaginous hyoidean rays. The chondrocranium is complete in Neoceratodus, but in the remaining genera it has undergone considerable absorption in the inter-orbital region, so that the roof and floor, and, in part, even the side walls of the cranial cavity, are formed by the fronto-parietal and parasphenoid bones. Two exoccipitals are present in all Dipnoi. There are small labial cartilages in relation with the ventrally-placed nostrils, and large lateral outgrowths from the ethmoid cartilage furnish the olfactory organs with conspicuous lattice-like roofs. A pair of strong palato-pterygoid bones fringe the lower margins of the palato-quadrate cartilage, and meeting in front beneath the ethmoid region their symphysial extremities support the large palatal teeth. The Meckelian cartilages are persistent in all Dipnoi. In Neoceratodus each is flanked by a dentary and an angular externally, and internally by a splenial; but in Protopterus and Lepidosiren distinct dentary bones are wanting. The hyoid arch is best {233}developed in Neoceratodus, and includes a small hyomandibular cartilage, a partially bony cerato-hyal and cartilaginous hypo-hyal and basi-hyal element. In the other genera (Fig. 133) only a cerato-hyal is retained. The branchial arches are but feebly developed in the Dipnoi. Neoceratodus has five, of which the first four are divided into epi-branchial and cerato-branchial segments, while the fifth is undivided. Protopterus has six, but only the second and third are segmented as in Neoceratodus. In Lepidosiren all the arches are simple undivided rods.

In all three genera the skull conforms to the same general type of structure, but it is much more primitive in Neoceratodus than in the other two genera.

With reference to the fossil Dipnoi, it may be stated that, so far as they are known, the cranial roofing bones are more numerous than in the existing genera, and they cannot readily be compared with those of the latter, or with the numerically reduced and more definitely arranged bones of most Teleostomi. There is also evidence that in some fossil Dipnoi (e.g. Dipterus) the chondrocranium and the mandibular suspensorium (palato-quadrate) must have been replaced by cartilage bones to an extent which has no parallel in any of the surviving types. Jugular bones were present in Dipterus and Phaneropleuron.

{234}MEDIAN FINS AND APPENDICULAR SKELETON

THE MEDIAN FINS.—Whether existing in the form of a continuous fin, or as discontinuous isolated fins, the median fins are provided with skeletal supports, and also with muscles, primitively formed from intrusive clusters of cells derived from a variable number of the neighbouring myotomes, for their varied movements. The skeletal structures of the dorsal and anal fins consist of a series of bony or cartilaginous, rod-like, and typically tri-segmented radial elements or pterygiophores, supporting distally a series of dermal structures in the shape of numerous slender horny fibres or ceratotrichia, as in the Elasmobranchii and Holocephali, or a smaller number of bony dermal fin-rays, which are probably modified scales or lepidotrichia, as in the Teleostomi. The typical tri-segmented character of the radialia is often retained in many existing Elasmobranchs (Fig. 135) and in Pleuracanthus, in Neoceratodus amongst the Dipnoi, in the Chondrostei, in existing Holostei (Fig. 136), and to a greater or less extent in several families of Teleosts (e.g. Salmonidae, Esocidae, Cyprinidae, and some Acanthopterygii); but in the latter group the radialia are greatly prone to reduction, and hence they are more generally bi-segmented, and sometimes consist of a single proximal segment only (e.g. Gymnotus). In all these Fishes the proximal segments are the longest and the most persistent, and when reduction occurs it is at the expense of the middle and distal segments.

{235}[Illustration: FIG. 136.—The tri-segmented radialia and the fin-rays of part of the dorsal fin of Amia calva. p.s, m.s, and d.s, The proximal, middle, and distal segments of a radial; f.r, fin-rays. (From Bridge.)

FIG. 137.—The first four radialia of the dorsal fin of Mesoprion gembra, showing the chain-links for the ring-like bases of the fin-rays. r.e^1, r.e^4, First and fourth proximal radialia.]

The cause of this reduction is often, but not always, to be found in the fact that, whenever the dermal fin-rays take the form of stout spines, as in the anterior dorsal fin in many Acanthopterygian Teleostei, the segmentation of their radialia would obviously detract from their value as skeletal supports, and hence they rarely consist of more than their proximal segments, although the radialia which in the same Fish support soft rays may be bi-segmented or tri-segmented. The radialia are, however, unsegmented, even slightly branched, cartilaginous rods in the Cyclostomata; short simple rods in the Holocephali; and equally simple bony rods in the dorsal fin of Polypterus, where they support the strong spines of the numerous finlets; but they are bi-segmented in the soft-rayed anal fin. As previously mentioned, the proportional share taken by the radialia and the horny fibres or the dermal fin-rays in the support of the fins differs greatly in different Fishes. In the Cyclostomata radialia are the sole, and in Elasmobranchs the main supports, and they may extend nearly to the free margin of the fin. In the more specialised Fishes, as in most Teleostomi, the reverse is the case. The radialia sink into the muscles of the body-wall and leave the strongly developed fin-rays as the sole support of the visible portions of the fins. In not a few Fishes there is an obvious segmental correspondence between the radialia and the vertebral neural or haemal spines, to the extent that the former equal the latter in number and articulate with their distal extremities, as, for example, in the caudal region of Pleuracanthus and in existing {236}Dipnoi. In others again, as in most Teleostomi, there is no such segmental relation, and the radialia are more numerous than the vertebrae whenever the two are co-extensive. The exoskeletal fin-supports exhibit similar relations to their radialia, but in inverse order. Much more numerous than the radialia in the Elasmobranchs, Holocephali, and the Dipnoi, the former become gradually reduced in the Teleostomi, until in the Holostei and Teleostei they correspond in number with the supporting radialia. Complete numerical correspondence between the neural and haemal spines and the radialia and fin-rays is very rare, and has only been observed in the caudal region of certain Crossopterygii (e.g. the Coelacanthidae).

In not a few Fishes the radialia of the median fins undergo modifications which offer an interesting parallel to an early stage in the evolution of the paired fins from primitively continuous lateral fins. The concentration of radialia which occurs in isolated median fins often results, through growth pressure, in the complete fusion of the proximal segments of more or fewer of the radialia into two or three basal supports, or even into a single basal piece. Examples of such basal fusion are frequent in the dorsal fins of Elasmobranchs, and the same modification may also be seen in the anal fin of Pleuracanthus, and especially in the {237}dorsal fin of the Devonian Crossopterygian, Holoptychius (Fig. 138), where several radialia, which are free distally, have their bases united into a single basal piece, or basipterygium. In most Teleostomi elevator and depressor muscles arise from the radialia, and are inserted into different points on the bases of the fin-rays, and by their contraction the latter may either be elevated into an erect position, or folded back like a fan along the middle line of the body, where, as in some Teleosts, there is a groove for their reception. When fin-rays are only capable of simple elevation or depression, the connexion between a radial element and its fin-ray is usually by some form of a hinge-joint, the cleft base of the ray clipping the distal segment of the radial (Fig. 139). In some Teleosts the articulation of the two is by means of a kind of chain-link (Fig. 137). In those Fishes in which the median fins are capable of lateral undulatory movements the articulation is of a more mobile character.

In the different types of caudal fin, diphycercal, heterocercal, and homocercal, the supporting elements of the ventral lobe are formed by the haemal spines of the terminal caudal vertebrae which are inclined backwards, and are often greatly expanded for the purpose (Fig. 140). The dorsal lobe may be supported either by the adjacent neural spines, or by radialia, or by both.

THE APPENDICULAR SKELETON.—It is probable that the skeleton of the paired fins and the pectoral and pelvic girdles have been formed from the supporting radialia of the isolated and enlarged anterior and posterior portions of primitively continuous lateral {238}fins, by a sequence of structural modifications in the same direction as in the median fins. The initial stage was probably marked by the fusion of the proximal portions of the radialia to form a basal support or basipterygium for the free distal portions. Subsequently, it may be, a rudiment of the future limb-girdle became segmented off from the inner extremity of the basipterygium, and by its dorsal and ventral growth in the body-wall the lateral half of a girdle was developed. The subsequent union of the two halves across the mid-ventral line resulted in the evolution of the dorsally incomplete hoop of cartilage which is the primary form of the complete limb-girdle in Craniates. The primitive fin skeleton or "archipterygium" was formed from the residue of the basipterygium in conjunction with the free distal radialia which it carried. The precise structure of the archipterygium is purely hypothetical. Possibly it was a biserial fin of the Pleuracanthus or Neoceratodus type, consisting of a cartilaginous segmented axis, fringed along its anterior and posterior, or pre-axial and post-axial margins, by a series of slender, simple, or jointed radialia (Fig. 147); or it may have been a uniserial structure, somewhat resembling the pelvic fin of Pleuracanthus, or the pectoral and pelvic fins of existing Elasmobranchs (Figs. 250, 141), in which an axis formed by the residue of the basipterygium or metapterygium had a fringe of radialia on its anterior or preaxial side only. If the archipterygium was biserial then the uniserial fin was probably derived from it by the subsequent suppression of all the post-axial radialia; or, if uniserial, the biserial fin was evolved by a later extension of radialia on to the post-axial margin. The evidence of comparative anatomy is not conclusive as to the nature of the archipterygium, and palaeontology seems to support either view with puzzling impartiality. It may be admitted that the lateral fin theory offers the best solution of the problem of the origin of the paired fins, but it must be borne in mind that no Fish, living or fossil, is known to possess fins of this nature, unless the singular lateral lobes of some Ostracodermi (e.g. the Coelolepidae) are kindred organs; neither do continuous lateral fins ever exist as vestiges, unless, indeed, the bilateral series of spines, which extend between the pectoral and pelvic {239}fins, in some of the Lower Devonian Acanthodei (e.g. Climatius), may be regarded in that light.

The Pectoral and Pelvic Girdles.—The pectoral girdle is more primitive in Cladoselache and Pleuracanthus than in any other Elasmobranch. In the former (Fig. 145, A) it may be doubted if the girdle has passed beyond the basipterygial stage, and although a definite girdle is present in the latter genus (Fig. 250) its lateral halves retain their primitive distinctness. Existing Elasmobranchs, including the Holocephali, have a pectoral girdle in the form of a dorsally incomplete hoop of cartilage imbedded in the muscles of the body-wall, close behind the last branchial arch (Fig. 141). The upper or dorsal portion of each half is the scapula, and the ventral is the coracoid. Between these two portions of the girdle, and defining their limits, there are articular surfaces for the basal cartilages of the pectoral fin.

Cladoselache (Fig. 145, B) had no pelvic girdle, nor does it appear that this primitive Elasmobranch had acquired even a basipterygium. Pleuracanthus, on the contrary, had a pair of pelvic rudiments distinct from well-developed basipterygia. In other Elasmobranchs there is a distinct girdle, formed by the median union of primitively distinct lateral rudiments, consisting of a simple transverse bar of cartilage, imbedded in the ventral abdominal wall, just in front of the cloacal aperture, and having articulated to each of its outer extremities the basal cartilage (metapterygium) of the pelvic fin.

{240}[Illustration: FIG. 142.—The left half of the pelvic girdle and the right pelvic fin of Chiloscyllum. meta, Metapterygium; pelv, pelvic girdle. (From Parker and Haswell.)]

Sometimes there is a rudiment of a dorsally-directed "iliac" process at each extremity of the girdle, but in no Fish do these processes ever acquire a dorsal connexion with the vertebral column. In the Holocephali the iliac processes are better developed than in any other Fishes, but ventrally the lateral halves of the girdle are united by ligament alone. In the Teleostomi important differences are observable in both girdles. The primary cartilaginous pectoral girdle now consists of distinct lateral halves which have no ventral connexion with each other. In addition, there is developed on the outer surface of each half a series of membrane bones, which form a secondary girdle (Fig. 143). From above downward the series includes a supraclavicle and a cleithrum (clavicle of Teleosts) which are always present, and to these may be added in the {241}Crossopterygii and Chondrostei an infraclavicle or clavicle proper, while one or two "post-clavicles" may be present in relation with the hinder margin of the cleithrum. The infraclavicles, or in their absence the cleithra (e.g. Holostei and most Teleostei), usually meet in a median ventral symphysis, so that the secondary girdle tends to acquire the characteristic hoop-like arrangement of its parts which has been lost in the primary girdle. With the development of a bony secondary girdle, the primary girdle (scapula and coracoid) becomes much reduced, and, as a rule, does little more than connect the fins with the cleithra. The secondary girdle acquires a dorsal connexion with the skull on each side by means of the post-temporal bone, which is attached below to the supra-clavicle and above to the periotic capsule. In the Chondrostei and the Dipnoi the primary girdle retains its primitive cartilaginous condition, but in the Crossopterygii, Holostei, and in all Teleosts it is ossified as distinct scapulae and coracoids. To these may be added in some Teleosts a mesocoracoid formed by a separate ossification of the coracoid cartilage (Fig. 143).

With the possible exception of small paired or median cartilages inserted between the inner extremities of the basipterygia in Polypterus and a few other Teleostomi, the pelvic girdle is absent in all the existing members of this group, having either become completely suppressed, or remaining unseparated from the basipterygia of the pelvic fins. In the Dipnoi (Fig. 144) there is a true pelvic girdle which has some points of resemblance to that of certain of the caudate Amphibia. It is represented by a median, lozenge-shape, cartilaginous {242}plate, produced in front into a long tapering epipubic process, and on each side of this into a forwardly inclined prepubic process. The hinder part of the plate bears two short processes for the basal cartilages of the pelvic fins. There is no trace, however, of iliac processes.

THE PECTORAL FINS.—The skeleton of the pectoral fins exhibits remarkable structural variations in different Elasmobranchs. In the existing members of the group two large basal cartilages, the propterygium and the mesopterygium, are formed by the concentration and fusion of the proximal portions of certain of the preaxial radialia, and they, with the metapterygium, articulate with the pectoral girdle; hence the fin is tribasal as well as uniserial (Figs. 141 and 146, A, B). In striking contrast to all other Elasmobranchs the pectoral fin of Cladoselache (Fig. 145, A) is far more primitive than in any other Fish. Each fin is supported by a distal series of slender, more or less parallel, unjointed, cartilaginous radialia, and basally by a similar series of shorter, stouter, and less numerous cartilages, which apparently were imbedded in the body-wall, the entire fin skeleton presenting a striking resemblance to an isolated median fin in which the supporting radialia have concentrated by growth pressure, and their proximal portions have been reduced in number by partial fusion. Pleuracanthus, on the other hand, had a biserial fin, the preaxial and postaxial radialia supporting fan-like clusters of horny fibres at their distal ends (Fig. 250).

{243}[Illustration: FIG. 146.—Pectoral fins of various Fishes. A, Acanthias vulgaris; B, Raia sp.; C, Chimaera monstrosa; D, Acipenser rhynchaeus; E, Amia calva; F, Lepidosteus platyrhynchus; G, Polypterus bichir; H, Salmo salvelinus. The preaxial side of each fin is to the left and the postaxial to the right. f.r, Dermal fin-ray; ms, mesopterygium; mt, metapterygium; p, propterygium; r, free radialia; 1, 5, the preaxial and postaxial basal elements in a Teleost, which may be mesopterygial and metapterygial pieces respectively, the three remaining basal pieces probably being intrusive metapterygial radialia directly articulating with the pectoral girdle. In B, D, E, and F, similar intrusive radialia are shown. (From Gegenbaur.)]

The broadly lobate pectoral fin of the existing Crossopterygii (Fig. 146, G) is uniserial, closely resembling that of the more typical Elasmobranchs. There are three basal elements, a propterygium, a mesopterygium, and a metapterygium, each of which supports a series of partially ossified radialia. Little is known of the endoskeletal elements of the broadly or acutely lobate fins of the fossil Crossopterygii, but it seems probable that their disposition was uniserial and abbreviate in obtusely lobate fins and biserial in acutely lobate fins. In the remaining Teleostomi (Actinopterygii) the endoskeletal elements become {244}gradually reduced in number and importance, their place as fin-supports being usurped by the dermal fin-rays. In addition, more than three, usually several, basal elements articulate directly with the pectoral girdle, and hence the fins become multi-basal. In the Chondrostei and the Holostei a metapterygium is always recognisable, supporting several radialia along its preaxial border, as in Acipenser (Fig. 146, D) and Amia (Fig. 146, E), or only a single one, as in Lepidosteus (Fig. 146, F). The anterior part of the fin is supported by a variable number of cartilaginous or bony radialia, which, with the metapterygium, articulate with the limb-girdle. In Teleosts the process of reduction reaches its maximum. Usually there is but a single row of short, hour-glass-shaped ossicles, of which the postaxial one may represent a vestigial metapterygium, and sometimes there is also a distal row of small cartilages or ossicles, partially hidden in the cleft bases of the dermal fin-rays (Fig. 146, H). In all these Fishes the fin is a much reduced uniserial fin, in which more or fewer of the preaxial radialia have acquired a direct secondary connexion with the pectoral girdle. Of living Dipnoids Neoceratodus has a nearly typical biserial fin, but, as seems to be the case in all fins of this type at present known, there is a marked absence of symmetry in the number and disposition of the radialia on the two sides of the axis. There is also much individual variation. No two fins are precisely alike, and the radialia may sometimes divide. In the very acutely lobate fins of the remaining Dipnoids it is evident that great reduction has taken place. Protopterus has lost all trace of postaxial radialia, and in Lepidosiren even the preaxial have atrophied, leaving only the long jointed axis to represent the originally biserial fin.

{245}THE PELVIC FINS.—In the simplicity of their endoskeletal supports the pelvic fins of Cladoselache are the most primitive type of paired fins at present known (Fig. 145, B). In general structure they resemble the pectorals, but the radialia are fewer in number, less modified by concentration, and exhibit little, if any, trace of basal fusion. Add to such features as these the apparent absence of any trace of pelvic rudiments, or of basipterygia, and it will be obvious that the pelvic fins differ but little from the median fins of the same Fish except that they are paired. In Pleuracanthus the pelvic fins differ from the corresponding pectorals in being uniserial instead of biserial (Fig. 250). All other Elasmobranchs, including the Holocephali, have uniserial fins, which consist of a large metapterygium, supporting a preaxial fringe of segmented radialia. A propterygium is sometimes present, notably in some of the Skates and Rays, and, like the metapterygium, it is directly connected with the pelvic girdle.

The skeleton of the pelvic fins of the Teleostomi is often extremely degenerate. It is perhaps best developed in the Chondrostei, where each fin is supported by numerous segmented radialia, more or fewer of which fuse towards the base of the fin, and those form a large and slightly ossified basipterygium (Fig. 148). In the living Crossopterygii, Holostei, and Teleostei, the pelvic fins are similar in essential structure, but are very degenerate. The basipterygium is usually well developed and is always bony (Fig. 149), and in many Teleosts it acquires so extensive a sutural connexion with its fellow that, physiologically, it supplies the place of a true pelvic girdle. At its distal end there may be a single row of small cartilaginous or bony nodules, representing vestigial radialia, as in the Crossopterygii, Holostei, and {246}Teleostei, but even these may be absent, and the dermal fin-rays then articulate directly with the basipterygium. Little is known of the skeleton of the pelvic fins in the fossil Crossopterygii, but there is evidence of the existence of a higher grade of structure than in their surviving allies. In Eusthenopteron, for example, the fin is supported by an axis of at least three bony segments, with at least three ossified preaxial radialia; hence, it has obviously undergone less degeneration than in Polypterus, where the fin-skeleton is essentially Teleostean. In the Dipnoi the pelvic fins are similar to the corresponding pectoral fins, but individual variation is more marked and even the central axis may divide. In the males of all existing Elasmobranchs, including the Holocephali, certain of the more distally situated metapterygial radialia become modified to form a supporting skeleton for the copulatory organs, the claspers, or mixipterygia. In the latter group the anterior claspers are also provided with cartilaginous supports articulating with the pelvic girdle directly in front of the pelvic fins.

{247}CHAPTER IX

THE DENTITION, ALIMENTARY CANAL, AND DIGESTIVE GLANDS

The alimentary canal is a muscular tube with an epithelial lining, formed for the reception and the digestion of the food. It begins with a mouth, and from thence it extends backwards through the coelom, finally communicating with the exterior either by a cloacal or by an anal orifice. The oral or buccal cavity into which the mouth leads is a stomodaeum, and is lined by inpushed epidermis, while the hinder portion of the cloaca and the anus are lined by a somewhat similar inpushing of the epidermis which forms the proctodaeum. The rest of the alimentary canal, consisting in succession of a pharynx, an oesophagus, a stomach, and an intestine, constitutes the mesenteron, and is lined by endoderm. Teeth are developed from the walls of the stomodaeum, and glands for the secretion of digestive fluids from the endoderm of the mesenteron.

DENTITION.

In the Lampreys among the Cyclostomata teeth are developed in the form of yellow conical structures on the inner surface of the buccal funnel, and on the extremity of the rasping "tongue" (Fig. 91, A). Each tooth consists of an axial papilla of the dermis, sometimes enclosing a pulp-cavity, and invested by the epidermis, and also by a stratified horny cone which forms the projecting hard part of the tooth. The dermal papilla with its ectodermal investment bears a superficial resemblance to the germ of a true calcified tooth, but no odontoblasts are formed, nor any calcic deposit, the laminated horny teeth being formed by the gradual conversion of the successive strata of the {248}epidermic cells into horny layers. The old teeth are vertically replaced by new teeth developed beneath the functional teeth. With the exception of a median tooth above the oral aperture, Myxine and its allies have only lingual teeth. These are comb-like, and they are formed by the basal fusion of primitively distinct tooth-germs. The structure and development of the teeth of the Cyclostomes lend no support to the view that the teeth are degenerate calcified structures. With greater probability they represent a stage in the evolution of teeth and dermal spines, which has been succeeded by a later stage in which calcification superseded cornification as a method of hardening.

True calcified teeth first make their appearance in Fishes, where they assume the form of modifications of exoskeletal structures. The teeth of Elasmobranchs are identical in essential structure, as well as in the manner of their development, with the ordinary dermal spines of the skin, and in the embryo the {249}dermal spines form a continuous series with those which invest the jaws and eventually become teeth (Fig. 151). It is only later, when lips become apparent, that the continuity of the teeth and dermal spines is interrupted, and the two structures assume their distinctive characters.

The tissues of which the teeth of Fishes are composed are (1) dentine, which is a non-vascular, calcified tissue, traversed by numerous radiating, branched, dentinal tubuli, into which extend protoplasmic prolongations from the cells (scleroblasts) by which the dentine is secreted. Dentine forms the greater part of the body of a tooth. (2) vasodentine and (3) osteodentine are modifications of ordinary dentine, the former containing blood-vessels ramifying in its substance but no dentinal tubules, and the latter more closely resembling bone. (4) enamel, an exceptionally dense, non-vascular, non-tubular tissue, which may or may not exhibit traces of the prismatic structure so characteristic of this tissue in the higher Vertebrates, forms the outer investment of the teeth.

As regards their fixation, the more primitive forms of teeth, such as those of Elasmobranchs, are simply embedded in the gums, and are only connected with the jaws by fibrous tissue; but in some of the older fossil Sharks the fixation of the teeth is effected by the mutual articulation of the basal plates of the teeth with one another. The Chondrostean Polyodon, so {250}shark-like in many other respects, also has teeth implanted basally in the gums, and quite free from any special connexion with the jaw-bones. In some Teleosts with movable teeth, the latter are merely attached to the jaws by fibrous, and often elastic, ligaments, as in the Pike (Esox) and the Angler-Fish (Lophius). As a rule, however, the teeth are directly ankylosed to the bones developed in relation with the jaws. Very rarely, as, for example, in some Characinidae, are the teeth implanted in sockets.

Nearly all Fishes are polyphyodont, that is, the old teeth are constantly replaced by new teeth as fast as they become worn down or fall out. In the Sharks and Dog-Fishes, for example, where the teeth are arranged in rows parallel to the axis of each jaw, the functional teeth along the upper edge of the jaw are usually erect, while those in the rows more internally situated point inwards towards the oral cavity; and behind these again there are rows of developing teeth in different stages of growth, and partially hidden beneath a projecting fold of the oral mucous membrane (Fig. 152). As the teeth in use become lost they are successively replaced by the inner rows, which, with the mucous membrane in which they are embedded, move forwards to the edge of the jaw, where they become erect and functional. The teeth of the Holocephali and of the Dipnoi are not shed, but the loss which they sustain through wear and tear is made good by persistent growth at their bases. In the Teleostomi the succession is less regular, new teeth being formed between or at the bases of the old teeth. In the case of socketed teeth the succession is usually vertical, the new teeth being formed at the sides of the old ones; and by the absorption of the bases of the latter, the former come to lie directly below them, and eventually they occupy the same sockets.

As might be expected from the remarkable diversity in the {251}habits and in the food of different Fishes, the teeth exhibit an equally striking diversity in form, size, and structure. The most primitive type of tooth resembles an ordinary dermal spine, and is little more than a simple pointed cone. A few Elasmobranchs and many Teleostomi possess teeth of this kind. By the flattening of the cone parallel to the axis of the jaw, the tooth becomes triangular, and then the margins may either remain smooth and trenchant, or they may become complicated by the formation of marginal serrations or of accessory basal cusps, and by such modifications the characteristic teeth of most Elasmobranchs are formed. The simple cone may also be modified to form crushing teeth—short, blunt, more or less hemispherical teeth—or even transformed into a mosaic of hexagonal plates, as in the Myliobatidae amongst Elasmobranchs. Massive, flattened, scroll-like crushing teeth are also formed by the fusion of adjacent teeth, or of several successional teeth, and of such composite teeth we have examples in the Heterodontidae and in the Palaeozoic Cochliodontidae. By a somewhat similar process of concrescence the anomalous composite teeth of such Teleosts as the Diodons and Tetrodons, and of the Parrot-Fish (Scarus), have been evolved. The singular dental structures of the Holocephali are probably composite teeth, and it is certain that the highly characteristic teeth of the Dipnoi have resulted from the basal fusion of primitively distinct simple conical denticles. The dentition is often heterodont. In Heterodontus (Cestracion), for example, the anterior teeth in each jaw are pointed and prehensile, while the hinder ones are scroll-like and crushing. Prehensile and crushing molar-like teeth are also present in such Teleosts as many of the Sparidae, and in the Wolf-Fish (Anarrhichas). The existence of sexual differences in the dentition is illustrated in the Skates and Rays (Raia), where teeth which are simple and pointed in the male become flattened and plate-like in the female. A few Teleosts, like the Syngnathidae, Cyprinidae, and some Siluridae, are entirely devoid of jaw-teeth.

In addition to jaw-teeth, many Teleosts possess pharyngeal or gill-teeth, developed in connexion with the inner margins of the branchial arches, to which they are usually firmly ankylosed (Figs. 352, 412 and 413). As a rule "the pharyngeal dentition is inversely proportional to the extent of tooth development {252}on the jaws." Pharyngeal teeth differ greatly in size and structure in different Teleosts, and, like the jaw-teeth, they are capable of replacement by vertical succession. The teeth are sometimes restricted to the inferior pharyngeal bones (cerato-branchials of the last branchial arch), and then, as in the Carp (Cyprinus), they may bite against a callous pad on the under surface of the basioccipital bone; or, as in some of the Wrasses (Labrus), the inferior teeth are opposed to superior teeth on the upper pharyngeal bones (pharyngo-branchials of more or fewer of the branchial arches). When pharyngeal teeth are present it is probable that they are the principal masticatory organs, the jaw-teeth being used for seizing or holding the prey.

ALIMENTARY CANAL.

A protrusible tongue is never developed in Fishes. A rudiment of that organ is present in the Elasmobranchs (Fig. 153) and Dipnoi, and also in the Crossopterygii, and usually consists of an elevated area of mucous membrane provided with free lateral edges and a forwardly projecting apex; it is supported by the basi-hyal element of the hyoid arch. In the Crossopterygii (e.g. Polypterus) the tongue contains muscle fibres, and in the Dipnoi, where the organ is better developed than in any other Fishes, special lingual muscles are present.

The pharynx succeeds the oral cavity, and is perforated on each side by the branchial clefts (Figs. 153, 154). The rest of the alimentary canal differs considerably in various Fishes in the degree of distinctness of its several regions, and in the extent to which it is convoluted. As a rule the pharynx is followed in succession by an oesophagus, a stomach, and an intestine (Fig. 153), the latter terminating in a portion usually termed the "rectum." The boundaries of these regions are not always very obvious, but are indicated by variations in calibre, by changes in the character of the lining epithelium, by special valves or sphincter muscles, or by the entrance of the ducts of certain glands like the pancreas and liver.

{253}[Illustration: FIG. 153.—Dissection of a male Dog-Fish (Scyllium). The left side of the body is cut away to the median plane so as to expose the abdominal and pericardial cavities and the neural canal in their whole length. The alimentary canal and the liver have been drawn downwards, and the oral cavity, the pharynx, part of the intestine, and the cloaca have been opened. The cartilaginous parts of the skeleton are dotted, and the calcified portions of the vertebral centra are black. abd.cav, Abdominal cavity; au, auricle; b.br, basi-branchial; b.hy, basi-hyal; c.art, conus arteriosus; cd.a, caudal artery; cd.st, cardiac part of the stomach; cd.v, caudal vein; cl, cloaca; cn, centrum; cr, cranium; crb, cerebellum; d.ao, dorsal aorta; dien, thalamencephalon; epid, epididymis; fon, fontanelle; gul, oesophagus; h.a, haemal arch; i.br.a^1-i.br.a^5, internal gill-clefts; int, intestine; kd, kidney; l.j, lower jaw; l.lr, left lobe of liver; med.obl, medulla oblongata; mes, mesentery; n.a, neural arch; n.cav, neural canal; olf.l, olfactory lobes; opt.l, optic lobes; pan, pancreas; pcd.cav, pericardial cavity; pct.a, pectoral arch; ph, pharynx; pin, pineal body; p.n.d, vestigial Müllerian duct; prs, prosencephalon; pty, pituitary body; pv.a, pelvic arch; pyl.st, pyloric portion of the stomach; r, rostrum; r.lr, right lobe of liver; rct.gl, rectal gland; sp, spiracle; sp.cd, spinal cord; spl, spleen; sp.s, sperm sac; sp.vl, spiral valve; s.v, sinus venosus; tng, tongue; ts, testis; u.g.s, urino-genital sinus; u.j, upper jaw; ur, metanephric duct; v, ventricle; v.ao, ventral aorta; v.def, vas deferens or mesonephric duct; vs.sem, vesicula seminalis. (From Wiedersheim, after T. J. Parker.)]

The oesophagus is occasionally separated from the stomach by a slight constriction, but more frequently the replacement of the squamous epithelium of the oesophagus by the columnar epithelium of the stomach and the appearance of gastric glands in the wall of the latter cavity afford the only distinction between the two regions. The {254}commencement of the intestine is usually indicated by a pyloric "valve" (Fig. 155, A, B), in the form of a ring-like, inwardly projecting thickening of the circularly-disposed muscle fibres of the terminal extremity of the stomach, and usually also by the entrance of the distinct or united ducts of the liver and pancreas; sometimes, as in certain Elasmobranchs and in the Dipnoi, by a special dilatation or "Bursa Entiana" (Fig. 155, A). The rectum, or terminal portion of the intestine, is distinguished from the rest of the gut by its straight course to the cloacal aperture or the anus, and sometimes by an increase in calibre. In Box vulgaris and a few other Teleosts a caecal diverticulum indicates the commencement of the rectum, while in a few cases the pre-rectal portion of the intestine communicates with the enlarged rectal segment by a much constricted valvular orifice which is suggestive of the ileo-colic valve of the higher Vertebrates, as in the Teleosts Amiurus catus, Trigla gurnardus, and Cyclopterus lumpus.

The relation of the regional divisions of the intestine in Fishes to those of other Vertebrates are somewhat difficult to determine. If we may regard the "rectal" gland of Elasmobranchs and the intestinal caecum of certain Teleosts as homologous with each other, and with the caecum coli of the higher Vertebrates, then it would seem that by far the greater part of the intestine of Fishes, including that portion in which a spiral valve may be developed, is homologous with the pre-caecal segment of the gut or small intestine in other Vertebrates, and that the post-caecal section, or large intestine, of the latter is represented in Fishes only by that relatively short portion of the gut which lies posterior to the rectal gland or its homologue in Teleosts, the equivalent of the colon of Mammalia being, as in Amphibia, Reptiles, and Birds, practically undifferentiated.

In the Cyclostomata the alimentary canal retains much of its primitive simplicity. It pursues a straight course from mouth to anus, and the usual regions are very obscurely indicated. The same remarks apply also to the Holocephali and a few Teleosts, although in these Fishes the limits of the different regions are somewhat more clearly defined. In the Dipnoi (Fig. 155, A), a contracted sigmoid curve between the somewhat dilated stomach and the spacious intestine is the only departure from the straight course of the preceding groups.

{255}[Illustration: FIG. 154.—Dissection of a male Teleost (Salmo fario) from the left side. a.bl, Air-bladder opened; an, anus; au, auricle; b.a, bulbus aortae; B.HY, basi-hyal; B.OC, basioccipital; cd.a, caudal artery; cd.v, caudal vein; CN, centrum; crb, cerebellum; d.f.1, first dorsal fin; D.F.R, dermal fin-rays; du, duodenum or anterior segment of the intestine; FR, frontal; g.bl, gall-bladder; gul, oesophagus or gullet; H.SP, haemal spine; int, intestine; kd, kidney; kd′, "head-kidney"; lg, tongue; lr, liver; N.SP, neural spine; opt.l, optic lobes; PA.SPH, parasphenoid; ph, pharynx; pn.b, pineal body; pn.d, bristle passed into ductus pneumaticus; prsen, prosencephalon; pty.b, pituitary body; PTG, pterygiophores, or radial elements of dorsal and ventral fins; pv.f, pelvic fin; py.c, pyloric caeca; S.ETH, supra-ethmoid; S.OC, supra-occipital; spl, spleen; st, stomach; ts, testis; u.bl, urinary bladder; u.g.s, urino-genital sinus and its external aperture; ur, ureter or kidney-duct; v, ventricle; v.ao, ventral aorta; v.df, vas deferens; v.f, ventral fin; VO, vomer. (From Parker and Haswell.)]

{256}In the remaining Fishes the degree of convolution varies within rather wide limits. The oesophagus is usually straight and wide, but in Lutodeira, among Teleosts, it is long and even convoluted, and in the Plectognath Teleosts it gives off a large sac-like outgrowth ("air-sac"), which extends anteriorly as far as the head, and posteriorly to the beginning of the tail, and communicates with the oesophagus by two apertures. The stomach may be U-shaped with the concavity directed forwards, and consisting of a right limb passing backwards from the oesophagus, and a left limb curving forwards to its junction with the intestine (Fig. 153). In such instances as these the stomach and the adjacent section of the intestine describe a characteristic siphonal curve. In certain other Fishes (Fig. 160), the oesophageal portion of the stomach terminates behind in a tubular or sac-like dilatation at some distance posterior to the laterally situated pylorus, which indicates the origin of the intestine. The intestine is straight, or nearly so, in Elasmobranchs, Crossopterygii, and Dipnoi, and also in a few Teleosts; but sometimes, and very generally in Teleosts, it is more or less convoluted, notably in some of the Mugilidae, and in the Loricariidae, where, as in Plecostomus, it is disposed in numerous spiral coils like a watch-spring. The terminal portion of the intestine or rectum either opens into a cloaca, which also receives the urinary and genital ducts, as in Elasmobranchs (Fig. 153), and Dipnoi (Fig. 155, A), or opens externally by an anus, situated in front of the separate or united urinogenital ducts, as is the case with all the remaining groups of Fishes (Fig. 154). The cloacal aperture is invariably situated near the junction of the caudal and trunk regions, and as a rule is median in position, rarely, as in the Dipnoi, displaced to the right or left of the middle line; but the anus differs greatly in position, sometimes retaining its primitive position at the hinder end of the trunk, as in the Holocephali, Chondrostei, Crossopterygii, Holostei, and many Teleosts, or occupying almost any position between that point and, as in the "Electric Eels" (Gymnotidae), the ventral surface of the throat (Fig. 351.)

{257}[Illustration: FIG. 155.—A, alimentary canal and liver of a female Protopterus, from the left side. Part of the left wall of the stomach and intestine, and the peritoneal investment of the spleen have been removed. a.p, Abdominal pore; b.d, bile-duct; b.ent, Bursa Entiana; cl, cloaca; cl.ap, cloacal aperture; cl.c, caecum cloacae; c.m.a, coeliaco-mesenteric artery; cy.d, bile duct; k.d, kidney duct; m.a, mesenteric arteries; od, oviduct; pt.c, post-caval vein or inferior vena cava; p.v, portal vein; the other reference letters as in B. (From Newton Parker.) B, viscera of an adult female Lepidosteus, ventral view. The oesophagus, the commencement of the intestine and the rectum have been laid open. ab, air-bladder; an, anus; b.d, intestinal aperture of the bile-duct; g.b, gall-bladder; gl, oesophageal aperture of the air-bladder; h.d, hepatic duct; l, liver; oes, oesophagus; py, pylorus; py.c; pyloric caeca; py.c′, the four intestinal orifices of the pyloric caeca; r, rectum; s, spleen; sp.v, spiral valve; st, stomach. (From Balfour and Newton Parker.)]

{258}[Illustration: FIG. 156.—Transverse section of a Fish, diagrammatic. cn, Centrum; coel, coelome; d.a, dorsal aorta; d.f, dorsal fin; d.m, dorsal muscles; d.ms, dorsal mesentery; f.r, fin ray; gon, gonad; int, intestine; l.v, lateral vein; msn, mesonephros; msn.d, mesonephric duct; n.a, neural arch; p, parietal layer of the peritoneum; p′, visceral layer; p.c.v, posterior cardinal vein; pn.d, Müllerian duct; r, ventral rib; r′, dorsal rib; sp.c, spinal cord; t.p, transverse process; v.m, ventral muscles; v.ms, ventral mesentery. (Modified, after Parker and Haswell.)]

The whole length of the alimentary canal from the oesophagus to the rectum is invested externally by the visceral layer of the peritoneum (Fig. 156), which histologically consists of a stratum of connective tissue, supporting on its free surface an epithelial stratum (coelomic epithelium). Primarily, the investing peritoneum is continued both dorsally and ventrally into bilaminar suspensory folds, the dorsal and ventral mesenteries (d.ms, v.ms), which extend to the mid-dorsal or mid-ventral line of the abdominal cavity. The two layers then separate and become continuous with the parietal layer of the peritoneum lining the whole of the inner surface of the body-wall. Embryologically, the two mesenteries owe their formation to the fusion above and below the mesenteron of the contiguous walls of two laterally situated and primitively distinct coelomic cavities. The dorsal mesentery in the adult is occasionally complete, as in the Myxinoid Cyclostomata and in the Elasmobranch Hypnos subnigrum, and also in some Dipnoi and in a few Teleosts, but much more frequently it is reduced by absorption to anterior and posterior remnants, or to a series of isolated bands, or even, as in the Lamprey (Petromyzon), to a few filaments accompanying the intestinal blood-vessels. The ventral mesentery, on the contrary, is rarely present, and if present is never complete. In Lepidosteus a ventral mesentery {259}is said to be present in connexion with that part of the intestine which contains the spiral valve. In Protopterus, and also in Neoceratodus, there is a well-developed ventral mesentery in relation with the greater part of the length of the intestine, although in the former Dipnoid its continuity is interrupted by one or two vacuities, and in the latter the mesentery is incomplete posteriorly. A ventral mesentery is also present in the intestinal region of some of the Muraenidae among Teleosts.

Internal to its peritoneal investment the wall of the alimentary canal consists in succession from without inwards of (1), a {260}muscular coat, (2) the submucosa, and (3) an epithelial stratum or mucous membrane, the first two of these layers, with the addition of the peritoneum, being derivatives of the inner or splanchnic portion of the embryonic mesoblast.

Excluding the oesophagus, where the muscular coat is mainly composed of striated fibres, the musculature of the alimentary canal usually consists solely of non-striated, spindle-shaped fibres disposed in two layers, an external stratum of longitudinally arranged fibres, and an inner stratum of circularly disposed fibres (Fig. 157), with the addition, in the stomach, of an oblique layer between the two. In the oesophagus the reverse arrangement may exist, the circular layer being external and the longitudinal internal. The muscular coat varies considerably in thickness in different regions and in different Fishes, and in the Cyclostomata, the Holocephali, some Teleosts, and the Dipnoi may be very feebly developed, or even entirely absent, as in the intestine of the Hag-Fish (Myxine). In the Gillaroo Trout (Salmo stomachicus), on the contrary, the distal section of the siphonal stomach has its musculature unusually thickened, so as to form an incipient gizzard for the crushing of the shells of the freshwater Molluscs on which the Fish feeds. In some of the Mullets (Mugilidae), a true gizzard is developed by the enormous thickening of the muscular coat of the caecal stomach, the cavity of which, in consequence, is reduced to a mere vertical fissure, and is lined by an exceptionally thick, horny epithelium.

There are a few exceptions to the rule that the muscular fibres are of the non-striated variety. Thus in some Teleosts, as in the Tench (Tinca vulgaris), striated fibres are continued from the oesophagus into the walls of the stomach and intestine, and there form an outer longitudinal and an inner circular layer, situated externally to the corresponding layers of the non-striated stratum. {261}In the Siluroid, Amiurus, the striated fibres of the outer circular layer of the oesophagus are continued, although but sparsely, into the inner circular layer of the stomach.

The submucosa (Fig. 157) lies between the muscular layer externally and the epithelial lining internally, and is characteristically developed in the stomach, and even more so in the intestine. Histologically, it consists of a framework of connective tissue, enclosing in its meshes masses of leucocytes (lymphoid tissue), some of which are amoeboid and migratory, and may even be found between the cells of the intestinal epithelium (including in some instances the cloacal epithelium), probably actively participating in the transmission of food material from the alimentary canal to the lymphatics and blood-vessels; while other and somewhat similar, but larger, leucocytes (phagocytes), are concerned with the elimination of waste substances or noxious micro-organisms. In addition to the diffused lymphoid tissue of the submucosa, special rounded or oval, and sometimes encapsuled, masses of this tissue (lymph follicles) are common in the intestinal wall (Fig. 157) of Acipenser, the Dipnoi and some Elasmobranchs, and are perhaps the only representatives in Fishes of the solitary follicles or "Peyer's patches" of the higher Vertebrates. A mass of lymphoid tissue exists in the axis of the spiral valve of Acipenser, which has been compared with a similarly situated structure in Lepidosiren. In some Elasmobranchs a large lymphoid organ is imbedded in the submucosa of the oesophageal wall, while a local thickening of the tissue is met with in the pyloric sphincter. Protopterus is remarkable among Vertebrates for the extraordinary development of lymphoid tissue, which, apart from its distribution in the submucosa, is abundantly present between the longitudinal and circular muscle layers, and the peritoneal and muscular coats of the intestine.

In addition to the lymphoid tissue the submucosa contains non-striated muscle cells and plexuses of capillary blood-vessels, which in certain Loaches (e.g. Misgurnus), where intestinal respiration occurs, extend between the cells of the intestinal epithelium. A network of lymphatic spaces or vessels surrounds the blood-vessels. In some Elasmobranchs the small arteries of the submucosa of the stomach are provided with singular sphincter muscles, which {262}occasionally encircle both the artery and the corresponding vein.

The lining epithelium differs considerably in character in different portions of the alimentary canal. The epithelium of the mouth, pharynx, and anterior section of the oesophagus is often squamous and is succeeded in the hinder part of the oesophagus, and in the stomach and intestine, by a columnar epithelium. As a rule the epithelium of the rectum is also columnar, but in Elasmobranchs it may become squamous. Goblet cells are of very frequent occurrence throughout the whole length of the alimentary canal, from the mouth to the rectum inclusive, interspersed between the superficial epithelial cells; in the same position in the intestine migratory leucocytes have been found. The primitive ciliation of the Vertebrate alimentary canal is retained to a greater or less extent in many Fishes, and is sometimes, but not always, associated with a feeble development of the musculature. In the larval form of Petromyzon (Ammocoetes), the whole canal is ciliated except the pharynx and rectum; but in the adult ciliation is retained only in places which gradually become fewer as the rectum is approached. In the Myxinoids, however, cilia are said to be absent.

In the Dipnoi (e.g. Protopterus) the epithelium of the stomach and intestine is largely ciliated, but in Elasmobranchs, ciliation is usually restricted to the posterior portion of the oesophagus and the edge of the spiral valve. Among the more generalised Teleostomi (e.g. Acipenser, Lepidosteus, Amia), the oesophagus, stomach, and intestine may be ciliated, but to an extent which varies in different genera. The pyloric appendages, when present, are also more or less extensively ciliated. In Teleosts, however, the recorded instances of ciliation are relatively rare. Nevertheless, ciliated epithelium has been found in the intestine of a few species (e.g. Rhombus aculeatus and Syngnathus acus), and also in the pyloric appendages; in the stomach (e.g. Perca and Esox), and even in the oesophagus (e.g. Perca).

The mucous membrane, including the submucosa, is frequently developed into variously arranged ingrowths projecting into the lumen of the alimentary canal; these are generally of the nature of longitudinal or transverse ridges, or a combination of the two, giving rise to retiform structures. The simple longitudinal {263}folds, which are sometimes found in the oesophagus, stomach, and rectum, often disappear on distension, and probably merely provide for the enlargement of these cavities during the deglutition of relatively large prey, or for the accumulation of faeces. On the other hand, the permanent and often complicated folds of the intestinal mucous membrane are probably related to an increase in the secretive or absorptive area of this portion of the alimentary canal. In the stomach the mucous membrane is usually smooth, rarely, as in the "Electric Eel" (Gymnotus), reticulate. In the intestine the folds assume a highly characteristic and often complicated disposition. In the Cyclostomata the folds are simple and longitudinally arranged. In Elasmobranchs (Fig. 158, A), obliquely transverse folds are present in addition, and, uniting with the longitudinal ridges, bound linear depressions.

In various Teleostomi (Fig. 158, B, C, D), the union of the two series of folds becomes more or less retiform, and the network of intersecting ridges bounds a series of deep tubular crypts which appear to penetrate to a considerable distance into the intestinal wall, and possibly foreshadow the characteristic Lieberkühn's glands of Mammalia. Crypts may also be found in the stomach, where they receive the apertures of the gastric glands, as in Amiurus, but more usually they are restricted to the intestine. In the Dipnoi (e.g. Protopterus) the mucous membrane of the {264}stomach, and—excluding the Bursa Entiana where a number of oblique folds are present—of the intestine also, is, on the contrary, perfectly smooth.

In addition to transverse and longitudinal folds the mucous membrane of the various sections of the alimentary canal is often developed into outgrowths which are more or less linear. In the oesophagus these may be papilliform, as in Box and Caesio; obtuse in Acipenser, hard and almost spine-like in species of Rhombus; or in the form of pyramidal retroverted processes with jagged or fringed edges, as in the Spiny Dog-Fish (Acanthias vulgaris). In the Basking Shark (Selache) similar processes are present, which, near the stomach, become unusually long and branched, so that the entrance to that cavity is surrounded by a series of backwardly-directed arborescent tufts. Peculiar papillose or tag-like processes of the mucous membrane are frequently present on the spiral valve of Elasmobranchs, in the intestine of such Teleosts as Balistes, Mugil and some Pleuronectidae, and also in the rectum of Rhombus maximus.

Of all the outgrowths from the mucous membrane of the alimentary canal the so-called "spiral valve" of the Cyclostomata, Elasmobranchs, Holocephali, Chondrostei, Crossopterygii, Amiidae, Lepidosteidae and Dipnoi is the most characteristic. The first appearance of this structure was probably in the form of a straight longitudinal fold or ridge projecting into the cavity of the intestine, similar, perhaps, to the typhlosole of many Invertebrata. This primitive condition is not retained in any existing Fishes, although it may be closely approached in the larval Cyclostome (Ammocoetes), and is perhaps also indicated in the straight anterior portion of the spiral valve of Polypterus. Absent altogether in the Myxinoids, the valve is represented in its simplest condition, as in certain other Cyclostomata (e.g. Petromyzon), by a ridge of mucous membrane which commences anteriorly on the dorsal side, and, after describing a partial spiral as it passes backwards, terminates posteriorly on the ventral side, the width of the valve not exceeding half the diameter of the intestine. This simple type of valve is repeated in embryo Elasmobranchs, but in the adults of these Fishes the valve becomes much more complicated, and exhibits a wide range of structural variation. The increased complexity of the valve {265}seems to depend on several factors, the effect of which, in different Elasmobranchs, is best studied in a series of valves of progressively higher differentiation.

In a hypothetical simple type of valve, easily derivable from the more primitive type of Petromyzon, it may be conceived that, while not exceeding in width the semi-diameter of the intestine, the valve becomes disposed in several complete and more or less closely approximated spiral turns, the free edge of the valve being on the same level as its attached margin, and leaving an open axial canal along the centre of the gut. The nearest approach to this hypothetical type, which has been compared, not inaptly, to un escalier tournant sans noyau, is perhaps to be found in the Thresher-Shark (Alopecias vulpes).

The structure of the more complicated spiral valves of other Elasmobranchs are well illustrated within the limits of the single genus Raia.

In one specimen of Raia sp. (Fig. 159, A) the last four coils of the valve are similar to those of the hypothetical type, but the more anterior ones, owing to the greater width of the valve, which here exceeds the semi-diameter of the intestine, have their free margins deflected downwards, while that portion of the valve which forms the first half turn is coiled inwards upon itself, so as to form a hollow cone, open dorsally, and having its apex directed forwards. In other examples a further modification is introduced by the increasing width of the valve, which now, throughout its whole length, equals the semi-diameter of the intestine; and by the formation of an axial columella by the thickened free edge of the valve, which is traversed by a central band of unstriped muscle, as well as by the intra-intestinal artery and vein, and takes the place of the central canal of the preceding types. The valve is, however, still regular, and its free margin remains on the same level as the corresponding portion of the attached edge. In other specimens, again, additional complications are introduced by a still further increase in the width of the valve, which now exceeds, often considerably, the semi-diameter of the intestine, and the consequent deflection of the free edge of the valve either forwards or backwards (C and D). As shown in C the valve, in consequence of the backward deflection of its free margin, presents the appearance of a nest of {266}imperfect truncated cones with their apices directed backwards, the successive cones adhering so closely to one another that they combine to form a central conical chamber with a spirally disposed cavity winding round it. In D, on the contrary, the free edge of the valve is deflected forwards, so that, as in C, a nest of cones is formed, but the apices of the successive cones are directed forwards instead of backwards. Notwithstanding these variations in the structure of the valve as a whole, the first coil or half coil nearly always resembles that described in A.

It is obvious that the structure of the valve varies considerably within the limits of the genus, and it may be added that various intermediate types of structure occur between A and B, {267}A and C, and A and D. The individual variations are perhaps even more remarkable, and appear to be quite independent of age and sex. By way of example it may be mentioned that valves approximating to one or other of those represented by C and D occur in different individuals of Raia maculata of the same sex and similar in size, even in young specimens not more than three inches in length.

As regards other Elasmobranchs, the common Dog-Fish (Scyllium canicula) has a well-developed spiral valve disposed in twelve coils, which structurally represents a more highly developed example of the type D. The existence of considerable individual variation is nevertheless indicated by the fact that in one specimen examined the valve was intermediate between C and D, five of the eight cones projecting forwards and three backwards. In a specimen of Notidanus sp. there were as many as twenty coils, which in disposition were intermediate between B and C, approximating, however, more nearly to B. In a specimen of the Port Jackson Shark (Heterodontus) the valve had eight coils, and in structure was also intermediate between B and C, but approached more nearly to C. Some of the Hammer-headed Sharks (e.g. Sphyrna malleus) possess a type of spiral valve which differs considerably from any of those hitherto described, and is termed a "scroll" valve (Fig. 159, E). The attached edge of the valve pursues a straight longitudinal course, or at any rate only describes a half turn and back again in passing from the pyloric to the cloacal extremity of the gut. In the middle of its course the width of the valve is about equal to two-thirds of its length, but towards either extremity it gradually diminishes until the free and attached margins meet. The valve thus constituted is rolled upon itself from left to right, the successive coils being comparable to a series of cylinders placed one inside the other, and becoming gradually larger both in length and diameter from within outwards. A similar valve is present in some of the Carchariidae.

In the Holocephali (e.g. Chimaera monstrosa) the valve describes only three and a half coils, and is further remarkable in that the attached margin, for a considerable portion of its {268}extent, does not form a regular spiral but describes only a slightly sinuous course. Posteriorly, the valve is more normal, and consists of about two cones with their apices directed forwards.

In the Dipnoi the spiral valve is well developed, and in Neoceratodus describes nine coils, and in Protopterus six or seven. The structure of the valve in the latter Dipnoid resembles that of Scyllium canicula, except for the smaller number of cones.

In the more generalised Teleostomi the valve is best developed in the Sturgeon (Acipenser) and in Polypterus. In the former the valve is restricted to the posterior half of the total length of the intestine, often extending to within an inch of the anal aperture, and describing in its backward course about seven or eight coils. The width of the valve is about equal to the semi-diameter of the intestine, and the thickened free margin forms a well-marked axial columella, round which the cavity of the gut winds, as in the type B, except that the spiral is a more open one. In Polypterus the valve begins close to the solitary pyloric caecum, and for some distance pursues a straight longitudinal course, but eventually forms a few spiral coils, ceasing, however, at a considerable distance from the anus. The evidence afforded by petrified faeces or "coprolites" proves that certain extinct Crossopterygii (e.g. Macropoma, Megalichthys), like their living representative, Polypterus, possessed a spiral valve. In Amia and Lepidosteus the valve is almost vestigial, being restricted to the terminal portion of the intestine, and is somewhat variable as to the precise number of its coils. In Amia there are nearly four coils, extending over 3 cm., that is less than a tenth of the total length of the intestine, but in some specimens the coils do not exceed two and a half or three in number. Lepidosteus has a still shorter valve which, in specimens of 7-10 cm. in length, may not consist of more than three and a half coils, and in much larger specimens may be reduced to less than two coils, a variation which suggests that a reduction takes place in the number of coils as the fish increases in age and size. The structure of the valve in the three last-mentioned genera resembles that described in Acipenser, and in none of them does {269}the width of the valve so far exceed the semi-diameter of the intestine as, by forward or backward deflection, to give rise to the highly characteristic cones of Elasmobranchs and Dipnoi.

In the more specialised Teleostomi (Teleostei) the spiral valve is wholly wanting, except perhaps as a vestigial structure in certain Clupeoids, as, for example, Chirocentrus, and possibly also in some Salmonidae.

From what has been said as to the structure of the spiral valve in the different groups of Fishes, it may be concluded that the valve most nearly retains its primitive condition in the Cyclostomata; attains its maximum development in the Elasmobranchs, especially in the Notidanidae, and shows no indication of degeneration in the Dipnoi. In the Holocephali and the lower Teleostomi, on the other hand, the valve exhibits various stages of retrogressive modification, and in the Teleosts is either absent altogether or persists only as a vestigial structure in a very few species.

From a physiological point of view the object of the spiral valve is to increase the absorptive inner surface of the intestine, but, from what has been said as to the structural variability of the valve, it is obvious that its efficacy from a functional standpoint must be equally variable. The value of the valve as an absorptive mechanism necessarily depends on the area of absorption-surface which it provides, as well as on the degree of resistance which it offers to the passage of food material along the cavity of the intestine. These factors will in turn depend on the number of coils, on the width of the valve, and on the extent to which its free margin is deflected in forming the series of cones, but these again are precisely the structural features which are most liable to variation. The total absorption area in the four types of valve characteristic of the genus Raia has been calculated, and may be expressed in square centimetres as follows:—A, 136.64; B, 143.82; C, 254.3; and D, 276.7. Hence as regards mere absorption area a spiral valve of the type D has twice the extent of a valve of the type A, and if, in addition, account be taken of the retardation of the food due to the increased obstruction offered by the columella and cones in D, it is clear that the {270}difference in physiological value between the two types must be far more considerable than is indicated by a comparison of their relative superficial areas alone.

The evolution of the spiral valve was probably due to the necessity of increasing the absorptive area of an almost straight unconvoluted intestine, a result which in other animals is often obtained by an increase in the length and concurrent convolution of the intestine itself. Any attempt to correlate the variations in the degree of perfection or imperfection of the valve considered as an absorptive mechanism with any special variations in the nature or quality of the food is, however, a very difficult problem, and a satisfactory explanation has yet to be found. The difficulty, moreover, is increased by the fact that the majority of Fishes with a spiral valve are mainly carnivorous; the Elasmobranchs, in which this structure is at the same time most highly developed and most variable, exclusively so. On the other hand, the term "carnivorous" covers a multiplicity of minor differences in the nature and relative digestibility of different forms of animal food, and it is quite possible that it is with differences of this kind that the specific or individual variations in the development of the spiral valve are associated. The absence of the valve in the variously nourished Teleosts, save perhaps as a vestige in one or two, is also difficult to account for, although it is not improbable that compensating structural modifications exist in this group. As a rule, the intestine is much more convoluted in these Fishes, but to an extent which varies greatly in different species, while the characteristic pyloric caeca and the spiral valve appear to a certain extent to be developed in inverse proportion to one another.

THE GLANDS.

The glands associated with the alimentary canal in different Fishes are (1) the gastric glands, (2) the liver, (3) the pancreas, (4) the pyloric appendages, and (5) the "rectal" gland.

Oral salivary glands are wanting in all Fishes, the only secretory structures in the mouth being numerous mucus-secreting goblet cells, which here, as elsewhere throughout the alimentary canal, are intermixed with the ordinary epithelial cells.

THE GASTRIC GLANDS.—The Cyclostomata and Dipnoi do not possess any specially differentiated gastric glands, and it is {271}probable that in these Fishes the secretion of the digestive fluids is effected by the ordinary lining epithelium of the stomach or intestine, or both. In the remaining groups gastric glands are generally present in the form of simple caecal structures embedded in the submucosa and opening on the surface of the mucous membrane into the cavity of the stomach. The glands differ in different Fishes in the character of their lining epithelium and in the extent to which their component cells are differentiated from the epithelium of the stomach. There does not appear, however, to be any distinction into "central" (pepsin-forming) and "parietal" (acid-secreting) cells, as is the case in the higher Vertebrata. Towards the pyloric end of the stomach the true gastric glands are often replaced by mucous glands. There are, nevertheless, not a few Teleosts in which special gastric glands are absent, as, for example, Syngnathus acus, and several species of Cyprinidae, Labridae, and Blenniidae, etc. In at least two genera (Gastrosteus and Cobitis), belonging to widely different families, gastric glands are present in certain species but absent in others. As suggested by Edinger, the absence of these glands may possibly be due to degeneration.

It may be remarked that the formation of such digestive ferments as pepsin and trypsin, which are associated with the stomach and pancreas respectively, in the higher Vertebrates, is not nearly so strictly localised in Cyclostomes and Fishes. So far from peptic digestion being limited to the stomach, it may take place in the pharynx, stomach, and intestine of Ammocoetes, and in some Elasmobranchs (e.g. Scyllium), and in such Teleosts as the Pike, Eel, and Carp, the peptic region extends from the stomach for some distance along the intestine, while trypsin has been obtained from the mucous membrane of the stomach, intestine and pyloric caeca, as well as from the pancreas.

Intestinal glands analogous to the glands of Lieberkühn in the higher Vertebrates seem to be entirely wanting in Fishes, unless represented by the sac-like or tubular crypts which are so generally present in the Teleostomi.

THE LIVER.—Phylogenetically the oldest gland in connexion with the Vertebrate alimentary canal, and in size by far the {272}largest, the liver arises as a caecal outgrowth from the embryonic mesenteron, and in this primitive stage recapitulates a condition which is retained throughout life in Amphioxus. By the subsequent division and branching of this outgrowth the massive compound tubular gland of the adult Fish is eventually formed.

The liver of Fishes (Figs. 153, 154) is very variable in size, shape, colour, and degree of lobulation. Anteriorly, it is usually moulded to the posterior face of the transverse septum between the pericardial and abdominal portions of the coelom, and from thence extends backwards in the abdominal cavity to a varying distance, in some Sharks as far as the cloaca. Externally, the gland is invested by the peritoneum, which extends on to it from the pericardial septum and forms a suspensory fold, and also from the oesophagus and stomach. The shape of the liver usually bears some relation to that of the body, being, for example, longest in the Eels and broadest in the Rays. In the great majority of Fishes the liver is bilobed, consisting of two sub-equal lateral lobes, disposed longitudinally and confluent anteriorly for a portion of their extent. From this normal type there are a few minor variations. In Petromyzon, Lepidosteus (Fig. 155, B), and a few Teleosts (e.g. the Gymnodontes, Lophobranchii, and some Salmonidae) the liver is unilobed. In the Myxinoids and in the Dipnoi (e.g. Protopterus), the organ is bilobed, but the small anterior lobe lies immediately in front of the much larger posterior lobe, with the gall-bladder between the two (Fig. 155, A). In some Teleosts (e.g. Scomber), the liver is trilobed. A gall-bladder is invariably present in either the larval or adult Cyclostomata, in the Chrondrostei, Holostei, Crossopterygii and Dipnoi, and generally also in Elasmobranchs and Teleosts. In the Elasmobranchs it is rarely entirely wanting, as in Sphyrna and Pristis, and in the Teleosts in some of the Gurnards (Trigla). The gall-bladder and bile-duct of Petromyzon fluviatilis atrophy after the metamorphosis which follows the larval Ammocoetes stage, but in Petromyzon marinus the duct, although usually absent, is sometimes retained. In the Ammocoetes the epithelium lining the gall-bladder is ciliated. In some Fishes, as, for example, in many Elasmobranchs, the gall-bladder is more or less completely embedded in the substance of the liver; in others, as in most Teleostomi, the organ is quite distinct from the gland (Fig. 154).

{273}A simple arrangement of the ducts from the liver and gall-bladder is that found in the common Dog-Fish (Scyllium canicula). In this Elasmobranch a cystic duct leaves the gall-bladder, and, after receiving several hepatic ducts from the lobes of the liver, becomes the bile-duct and opens into the commencement of the intestine. In the Myxinoids and in the Dipnoi (e.g. Protopterus), there are but two hepatic ducts, one from each lobe of the liver; these unite and then meet the cystic duct to form the bile-duct (Fig. 155, A). The number of hepatic ducts may, however, be considerably increased, as, for example, in the Siluroid Amiurus, where 8-10 separate ducts join the cystic duct. In a few instances one of the hepatic ducts opens directly into the intestine, independently of that which unites with the cystic duct in forming the bile-duct. In the Dipnoi (e.g. Protopterus), and in some Teleostomi (e.g. Lepidosteus), the bile-duct receives the duct from the pancreas before opening into the intestine.

THE PANCREAS.—In the Cyclostomes (e.g. Petromyzon, Bdellostoma, Myxine) a rudimentary pancreas is apparently present, but the evidence as to its identity is not wholly conclusive. A well-developed pancreas occurs in Elasmobranchs, in at least one of the Dipnoi, and probably in most Teleostomi.

In Elasmobranchs the pancreas is a compact structure, uni- or bi-lobed, and entirely distinct from the liver. In Scyllium canicula (Fig. 153), the bilobed gland lies in the angle between the distal limb of the stomach and the adjacent portion of the intestine, and from the smaller of its two lobes the duct issues to pass to its intestinal aperture near the commencement of the spiral valve. In most of the Teleostomi in which its existence has hitherto been recorded, the pancreas is a singularly diffuse gland; and usually a considerable portion, or even the whole of it, is embedded in the substance of the liver, its lobules accompanying the ramifications of the hepatic artery and duct, and the portal vein. The pancreatic duct usually opens into the intestine near the aperture of the bile duct (e.g. Amiurus); sometimes the two ducts open on the apex of a common papilla (e.g. Acipenser and Amia), or by their union form a common {274}duct (e.g. Lepidosteus). Among the Dipnoi a well-developed pancreas is present in Protopterus, embedded in the wall of the stomach and intestine, internal to the peritoneal investment of these organs, and extending even into the first fold of the spiral valve. The gland is traversed by fine ductules which unite together and open into the bile-duct just before the latter enters the intestine. In the remaining Dipnoi the existence of a pancreas has yet to be ascertained. Developmentally, the pancreas resembles the liver, and, histologically, is very similar to that of the higher Vertebrates, consisting of terminal glandular alveoli continuous with intermediary tubular portions, and eventually with the finer ductules, which, by their union, form the main efferent duct.

THE PYLORIC CAECA.—These structures are caecal outgrowths from the intestine, and are situated close to the pyloric extremity of the stomach and the intestinal apertures of the bile and pancreatic ducts. Wholly wanting in the Cyclostomata and Dipnoi, and, unless represented by a pair of caeca opening into the long, tubular, non-valvate anterior portion of the intestine in the Greenland Shark (Laemargus borealis), in the Elasmobranchs also, they are very generally present in the Teleostomi, although extremely variable both in number and arrangement in different families. In Amia there is no trace of pyloric caeca. Polypterus has a single short caecum with a thick muscular wall. In Acipenser, Polyodon, and Lepidosteus, on the contrary, pyloric caeca are unusually well developed. In Acipenser the caeca are not only numerous, but are so connected together by connective tissue and blood-vessels, and so invested externally by the peritoneum, as to form a large, compact, gland-like mass, communicating with the intestine by a single wide duct. In Polyodon the organ is essentially similar, but is lobed externally. In Lepidosteus (Fig. 155, B, py.c), the caeca are also very numerous, but relatively short, and, although united into a compact mass, open by four pit-like orifices into the intestinal cavity. In Teleosts the caeca are subject to extraordinary variations in number, size, and arrangement. In some families, and even in groups of higher taxonomic value, they are entirely absent, as is the case with the {275}Siluridae, Esocidae, Cyprinodontidae, Labridae, Plectognathi, and Lophobranchii. The "Sand-eel" (Ammodytes) has but a single caecum; the Turbot (Rhombus maximus) two, and other Pleuronectidae three to five; and the Perch (Perca), three (Fig. 160, py.c).

In other Teleosts, on the contrary, these structures are much more numerous. In Labrus labrax there are about 60, in the Whiting (Gadus merlangus) 120, while in the Mackerel (Scomber scombrus) there are no fewer than 191. If few in number the caeca open separately into the intestine, but when numerous, more or fewer of them may unite to form a smaller number of efferent ducts, as in the Whiting, where four such ducts are formed. In some instances, as in the Tunny (Thunnus), the union of the caeca by connective tissue leads to the formation of a compact mass. As regards their arrangement, the caeca may either be disposed in a whorl round the intestine, as in the Whiting, or in a linear series, as in the Salmon (Salmo) and in some of the Clupeidae.

The mucous membrane lining the anterior pyloric caeca is often developed into a network of ridges, limiting crypt-like or tubular depressions; and not infrequently the epithelium is ciliated.

The precise function of these organs, whether digestive or absorptive, is still uncertain. That they may be digestive is suggested by the presence of certain amylolytic and proteolytic enzymes, but this obvious conclusion is to some extent vitiated by the close proximity of these organs to the stomach, and more especially to the intestinal orifice of the pancreatic duct. It is by no means improbable, however, that the caeca are both digestive and absorptive organs. An attempt has been made to show that the pyloric caeca and the spiral valve vary inversely as regards the extent of their development in different groups of {276}Fishes. To some extent the reciprocal variation of these structures supports this view, but it is also evident that there are obvious objections to its unqualified acceptance. Thus, in some Teleostomi (e.g. Acipenser, Polyodon), exceptionally well-developed and numerous caeca and a spiral valve are both present. Amia with an almost vestigial spiral valve has no trace of pyloric caeca, and in Teleosts the absence of a spiral valve is associated with the complete suppression of the caeca in many large and important groups.

THE RECTAL GLAND.—The "rectal" gland, or appendix digitiformis, is a small organ of unknown function with complex glandular walls, and a central duct opening dorsally into the terminal portion of the intestine. The organ is generally present in Elasmobranchs (Fig. 153, rct.gl), in which group the intestinal orifice of its duct may either be close to the termination of the spiral valve, or, as in Chlamydoselachus, near the cloacal outlet of the gut. An apparent representative of the gland, the "caecum cloacae," is also present in the Dipnoi, but communicates directly with the cloaca (Fig. 155, A, cl.c). The "rectal" gland is perhaps homologous with the intestinal caecum which is to be found in some Teleosts (e.g. Box vulgaris), and possibly also with the "caecum" (caecum coli), and its vermiform appendix in the higher Vertebrata. The caecum cloacae, on the contrary, is morphologically a urogenital sinus, formed as a dilatation of the fused hinder portions of the mesonephric ducts, and probably comparable with the sperm sacs of male Elasmobranchs, and also with the urinary bladder of Teleostomes.

{277}CHAPTER X

THE RESPIRATORY ORGANS

The principal respiratory organs consist of a series of pairs of branchial clefts in the form of perforations in the side walls of the throat, which place the pharynx in free communication with the exterior. The first and most anterior of these clefts, the mandibulo-hyoid cleft or "spiracle," is situated between the mandibular and hyoid arches; the second, the hyo-branchial or hyoidean cleft, between the hyoid arch and the first branchial arch; and the remaining clefts between the succeeding branchial arches. On the anterior and posterior walls of more or fewer of the clefts highly vascular plate-like, or variously shaped filamentous outgrowths of their lining membrane are developed, which subserve the purpose of exposing the blood to the influence of the oxygen-containing water, and are termed branchial lamellae or "gills." In addition to their usual respiratory organs, the gills, a few Fishes utilise the air-bladder either as a functional lung or as an oxygen reservoir, and in others accessory breathing organs of various kinds are developed.

The arrangement of the branchial clefts and the gills may be conveniently studied first in the Elasmobranchs. Excluding the spiracles, there are usually in this group (Fig. 161, A), five pairs of branchial clefts, but in certain primitive members of the group the number may be larger. Thus, in Notidanus griseus (Hexanchus) and in Chlamydoselachus there are six, and in Notidanus cinereus (Heptanchus), seven clefts. The pharyngeal apertures of the clefts are relatively wide, but their external openings, which are freely exposed on the lateral surface of the head between the eye and the pectoral fin, are usually narrow and slit-like.

{278}[Illustration: FIG. 161.—A, Horizontal section through the head of an Elasmobranch; B, similar section of a Teleost (diagrammatic). b.c, Branchial cavity; b.l, branchial lamellae; c, coelom; e.b.a, external branchial aperture; hy.a, hyoid arch; hy.c, hyo-branchial cleft; l.s, interbranchial septum; n, nasal organ; oes, oesophagus; op, operculum; p.q, palato-quadrate cartilage; Ph, pharynx; sp, spiracle; s.ps, spiracular pseudobranch; 1-5, 1st to 5th branchial arches. (From Boas, slightly altered.)]

The successive clefts are separated from one another by a series of inter-branchial septa, each of which consists of the lining membrane of two contiguous clefts and a median fibrous sheet; it is further strengthened on its pharyngeal margin by a branchial arch, and more externally by the fringe of cartilaginous rods (branchial rays) with which the outer convex edge of each arch is provided. The anterior and posterior walls of each septum are produced into a number of outwardly-radiating vascular plates or folds (branchial lamellae or "gills"), which by their free edges project into the cavity of the cleft (Fig. 161, A). Although slightly free at their outer extremities, the lamellae do not extend so far as the external margin of the septum to which they are attached (Fig. 164, B). Each series of lamellae is termed a "hemibranch," and, from what has been said, it is obvious that each inter-branchial septum and its supporting branchial arch carry two hemibranchs, an anterior and a posterior, the two forming a complete biserial gill or "holobranch." The hyoid arch, however, has only a single hemibranch, viz. that pertaining to the anterior wall of the hyo-branchial cleft, and as the fifth or last cleft has a hemibranch only on its anterior wall, the fifth arch is {279}gill-less. The spiracle is a vestigial cleft. At an early stage of embryonic growth it differs but little from its fellows, but subsequently degenerating it is represented in the adult by a tubular passage between the oral cavity and the exterior, which, however, is often complicated by the development of caecal outgrowths. The anterior wall of the spiracle often retains a rudiment of a hemibranch in the shape of more or fewer vascular lamellae, which, as they are supplied with arterial blood, and not with venous blood like the ordinary gills, are said to form a mandibular or spiracular "pseudobranch." The spiracle varies greatly in size in different families, being largest in the Trygons and Torpedos, and very small, or even absent in the Lamnidae. Its pseudobranch is best developed in the Notidanidae, where it has the essential structure of a true hemibranch, and, as in other Elasmobranchs, but to a greater extent, probably aids in the additional aeration of the blood which is distributed to the eye and brain. The characteristic opercular covering of the external apertures of the gill-clefts in the Teleostomi and Dipnoi is wanting in Elasmobranchs. It is interesting to note, however, that in Chlamydoselachus curious frilled cutaneous folds are developed as extensions of the outer edges of the inter-branchial septa, as well as of the hyoid region, and, like a series of incipient opercula, project backwards over the successive branchial clefts (Fig. 252).

While in many respects more primitive than in Elasmobranchs the branchial system of the Cyclostomata presents certain special and peculiar features. The branchial clefts assume the form of oval, antero-posteriorly flattened pouches or sacs, varying, however, in number, and in their mode of communicating with the exterior, in different genera. In the Lamprey (Petromyzon) there are seven pairs of obliquely-disposed gill-sacs opening externally by small rounded orifices, and by similar apertures, not directly into the pharynx, but into a branchial canal (Fig. 162, r.t), which underlies the oesophagus, and, while ending blindly behind the last pair of sacs, communicating in {280}front with the oral cavity. The first of the series of gill-sacs corresponds to the hyo-branchial or hyoidean cleft of Elasmobranchs and other Fishes. Spiracles are absent in the adult, but in the embryo are represented by pouch-like outgrowths of the hypoblast of the oral cavity, which subsequently undergo singular changes. Thus, the outgrowths become converted into the lateral halves of a complete ciliated circum-oral groove, which is retained even in the Ammocoetes stage, and recalls the ciliated peripharyngeal ring of Ascidians. Another archaic feature is also to be noted in the continuity of the groove with a ciliated mid-dorsal pharyngeal ridge, which has been compared to the "dorsal lamina" of Ascidians, and to the equally characteristic hyperbranchial groove of Amphioxus. Ventrally also, the lateral halves of the groove unite to form a single groove, which, after receiving the median aperture of the thyroid rudiment, is continued backwards in the mid-ventral line of the pharyngeal wall as far as the last branchial arch. No trace of these ciliated structures is, however, to be met with in the adult.

The branchial lamellae are represented by a series of vascular horizontal and parallel ridges radiating outwards along the roof, floor, and lateral walls of each gill-sac, and invested by an {281}epithelium which is partially ciliated. The inter-branchial septa are much thicker than in Elasmobranchs, and include not only the walls of adjacent sacs and the branchial muscles, but also contain cavernous peribranchial lymph-sinuses. The cartilaginous branchial skeleton is situated wholly external to the gill-sacs, the so-called branchial arches lying between the external apertures of the sacs, and directly beneath the superficial skin, or, in other words, on the outer margins of the inter-branchial septa, and not on the inner, as is invariably the case with the branchial arches of Fishes.

In the Hag-Fish (Myxine) (Fig. 163), there are usually six, very rarely seven, pairs of gill-sacs, all of which open directly into the pharynx, and not into a branchial canal as in the Lampreys. On the other hand, Myxine is unique in having the outer extremities of its gill-sacs produced into a corresponding number of tubular canals which, after a longer or shorter course obliquely backwards and outwards, unite to form on each side a ventrally-situated external aperture (Fig. 163). In the same genus a short canal, or oesophageo-cutaneous duct, passes from the pharynx {282}behind the last gill-sac of the left side, and opens externally with the common external branchial aperture of that side.

In Bdellostoma there are usually six or seven pairs of gill-sacs, but some species have ten or even fourteen pairs. They agree with those of the Lamprey in having independent external apertures, but resemble the corresponding organs in Myxine in opening directly into the pharynx. An oesophageo-cutaneous duct is also present.

In the Holocephali there are but four branchial clefts, the fifth cleft being closed. Spiracles are absent in the adult, although present in the young of Chimaera. The branchial lamellae resemble those of Elasmobranchs, but the inter-branchial septa are somewhat shorter, so that the lamellae project slightly beyond their outer margins (Fig. 164, B). A hyoidean hemibranch is present. A noteworthy feature is the development of a cutaneous fold from the outer surface of the hyoid arch, which grows backwards over the gill-clefts, and, uniting above and below with the body-wall, terminates in a free posterior margin, just behind the last gill-cleft. By the growth of this opercular fold the gills become enclosed in a spacious branchial cavity, and the clefts communicate with the exterior through a slit-like opening between the free margin of the fold and the body-wall.

The reduction in the extent of the inter-branchial septa which is initiated in the Holocephali is carried to a still further extent in the Teleostomi. Commencing with the Chondrostei, and passing thence to the more specialised Teleostei, the septa become gradually reduced in length, and the branchial lamellae project freely beyond their outer margins to an increasing extent.

This modification, least marked in Acipenser (Fig. 164, C) and Polyodon, attains its maximum in the Teleosts (Fig. 164, D and E), where the branchial lamellae take the form of a double series of free filaments disposed along the convex outer margin of each branchial arch, and attached by their bases only to the reduced and inconspicuous septa. As a general rule each of the first four arches supports two hemibranchs, forming a {283}biserial gill or holobranch. In shape the branchial filaments are usually somewhat triangular, and consist of an axial supporting cartilage or bone, invested superficially by a highly vascular mucous membrane. As in most of the preceding groups the fifth branchial arch is gill-less. All Teleostomi possess a well-developed movable operculum, supported by a more or less complete series of opercular bones, with or without the addition of branchiostegal rays (Fig. 161, B). The size of the external branchial aperture varies considerably. Usually the hinder and lower margins of the operculum are free, and then the aperture is spacious. Not infrequently, however, the more or less extensive fusion of the ventral and hinder edges of the operculum with the body-wall reduces the aperture to a narrow slit, as in the Eels and some Siluridae, or to a small upwardly directed pore, as in the "Sea-Horse" (Hippocampus). In the Symbranchidae the branchial apertures close dorsally, but fuse ventrally, leaving a single median orifice on the under side of the throat.

Open spiracles are wanting in most adult Teleostomi, but are, nevertheless, retained in the Crossopterygii (Polypterus), and in the Chondrostei (Acipenser and Polyodon). They have been observed, however, in the embryos of some Teleosts, as in the Salmon (Salmo), and even in the adults of Amia, Lepidosteus, {284}and a few Teleosts are represented by pouch-like recesses of the oral cavity. A few vestigial branchial lamellae may be developed on the anterior wall of each spiracle in Acipenser and Polyodon, but are wanting in Polypterus, and, as in Elasmobranchs, represent a mandibular or spiracular pseudobranch.

The structure usually regarded as a hyoidean hemibranch in the Teleostomi differs greatly in its development in different members of the group. In Acipenser it is undoubtedly the hemibranch of the hyoid arch and is a true gill, receiving venous blood from the ventral aorta and returning arterial blood to the dorsal aorta, as in Elasmobranchs. In Polyodon and in Polypterus the hemibranch is suppressed. Lepidosteus, on the other hand, has two series of lamellae on the inner surface of the operculum, a dorsal and a ventral series meeting at an angle (Fig. 197). The ventral lamellae are supplied with venous blood, the dorsal with arterial, so that while the former retain their primitive character as a functional hyoidean hemibranch, the latter is a pseudobranch. It is interesting to note, however, that the development of this pseudobranch and its blood-vessels proves that it does not represent any portion of a true hyoidean hemibranch, but is really a spiracular pseudobranch. In most other Teleostomi a degenerate hemibranch occupies a similar position. In Amia it is very feebly developed, and is lodged in a canal communicating with the branchial cavity by a small aperture, and situated directly anterior to the dorsal end of the first branchial arch. Its blood supply is arterial, and the organ is therefore a pseudobranch. In Teleosts the hemibranch is invariably a pseudobranch; nevertheless, its primitive condition as a gill is indicated either by its structure or by its embryonic history. In some genera the pseudobranch consists of short free lamellae, as in some Pleuronectidae; or it is partly free and partly concealed, as in some of the Horse Mackerels (Caranx) and in Salmo; or it may be completely hidden beneath the oral epithelium, as in the Cod (Gadus), where the organ is very degenerate, and is little more than a "rete mirabile" of blood-vessels. The nature of the Teleostean pseudobranch is not in {285}all cases quite clear. In Salmo it is said that there is no hyoidean hemibranch, and that the pseudobranch is really a persistent spiracular pseudobranch; hence it is probable that a like significance must be attached to this singular structure in other Teleosts. The evidence of the cranial nerves on this point is conflicting. If the pseudobranch pertains to the spiracular cleft its nerve supply should be derived from the nerve of that cleft—viz. the seventh or facial nerve; but if it represents a hyoidean hemibranch, then one would expect it to be innervated by the ninth or glossopharyngeal nerve. As a matter of fact, however, the organ is said to be supplied by the seventh in some Teleosts, and in others by the ninth nerve.

In the Dipnoi the branchial system is best developed in Neoceratodus, the increasing importance of the lungs as respiratory organs in Protopterus and Lepidosiren being associated with a corresponding reduction in the structural and functional development of the gills. There is no trace of spiracles in the adult.

In Neoceratodus there are five branchial clefts, including the hyobranchial. Each of the first four branchial arches carries a pair of hemibranchs, and, as in the Holocephali, the gill-lamellae are attached along nearly their whole length to a well-developed interbranchial septum (Fig. 165). A peculiarity of Neoceratodus, which has no counterpart in any other Fishes, is the extension of the branchial lamellae on to the dorsal and ventral walls of the branchial clefts, so that the hemibranchs on opposite sides of each cleft are continuous both dorsally and ventrally (Fig. 166). The fifth arch is gill-less. {286}In addition to the normal gills there is also a hyoidean pseudobranch. As in other Dipnoi, an operculum forms the outer wall of the branchial cavity, and leaves but a narrow, slit-like external branchial aperture.

In Protopterus the number of branchial arches is increased to six, but, in consequence of the closure of the hyobranchial cleft, there are but five open clefts. The first, second, and third arches are wholly devoid of branchial filaments: the fourth and fifth support each a biserial gill, while the sixth arch retains only an anterior hemibranch, which, however, as the source of its blood supply seems to indicate, may consist of "emigrant" gill-filaments from the posterior hemibranch of the fifth arch. Interbranchial septa are practically non-existent, the flattened, leaf-like gill-lamellae being free except at their attached bases, and thus repeating a characteristic Teleostean feature. A "hyoidean" hemibranch or pseudobranch, supplied from the ventral aorta, is present, but as the hyobranchial cleft is closed it projects into the branchial cavity immediately in front of the cleft between the first and second branchial arches. In Lepidosiren the branchial arches are reduced to five and the clefts to four, the hyobranchial and fifth clefts being closed. There is a "hyoidean" hemibranch resembling that of Protopterus.

The facts furnished by the study of the numerical and structural variations in the gill-clefts, gills, and gill-arches of different groups of Fishes prove that atrophy of these structures takes place at opposite ends of the series. We have examples of this anteriorly in the suppression of the hyo-mandibular cleft and its hemibranch, and of the hyoidean hemibranch, as the result of {287}the conversion of the mandibular and hyoid arches into jaws, or into skeletal supports for the jaws; and posteriorly, in the reduction which is evident when the generality of Fishes are compared with such primitive Elasmobranchs as Chlamydoselachus and Notidanus.

In most Fishes the concave pharyngeal margins of the branchial arches are fringed with a double series of either cartilaginous or bony tubercles or filaments, the "gill-rakers" (Figs. 161 and 164). The anterior row of gill-rakers on each arch usually interdigitate with those of the posterior row on the preceding arch, and in this way the two rows form a sieve-like mechanism to prevent any solid particles, which may enter the pharynx with the respiratory current of water, from passing into the gill clefts and clogging or otherwise injuring the branchial filaments.

In a few Fishes the gill-rakers are enormously developed, and subserve a function similar to that of the baleen plates of the Whalebone Whales in acting as a filter for straining from the water the small pelagic organisms on which the Fish feeds. This is notably the case in the great Basking Shark (Selache maxima) in which the closely-set, flattened, tapering gill-rakers may be so long as four or five inches, and, while somewhat resembling "whalebone" in appearance, have the histological structure of vascular dentine. The nature of the food, which in the stomach of one specimen examined consisted solely of an immense quantity of plankton, including Copepods and the larvae of other Crustaceans, affords clear evidence of the great value of such a filtering mechanism to this Shark, and, at the same time offers an explanation of the striking and significant reduction in the size of the teeth, which, relatively to the dimensions of the Fish, are so small as to be almost vestigial. A similar filter has been observed in an extinct Selache (S. aurata) from the Antwerp Crag, and also in an existing South African Shark (Rhinodon typicus); and in the latter, as in the Basking Shark, is associated with a marked reduction in the importance of the dentition. The long slender gill-rakers of the Chondrostean {288}Polyodon also constitute an efficient filter, and the same may be said of several plankton-eating Teleosts.

THE MECHANISM OF RESPIRATION.—The aeration of the blood is effected by the rhythmical suction of water into the oral cavity, and its subsequent expulsion through the gill-clefts, bathing the highly vascular gill-lamellae in its course. In any single act of inspiration the mouth is opened, and the oral cavity enlarged by the lateral expansion of its walls. When the oral cavity is filled with water, the mouth is closed and the expiratory process begins. By the lateral contraction of the oral walls the water is driven outwards through the gill-clefts, and over the gill-lamellae. During this process the branchial arches become widely separated by the contraction of their muscles, the operculum is elevated, and the oesophagus is closed by the contraction of its muscular wall. In many Fishes the course of the expiratory water-current is controlled by special valve-like folds of the oral mucous membrane, the maxillary and mandibular "breathing-valves."

The rate of "breathing" varies considerably in different Fishes, even in allied species. In the Blue Wrasse (Labrus), and the Rockling (Motella), the number of respirations per minute is 15, in the Minnow (Leuciscus), and Stickleback (Gastrosteus), as many as 150. A deficiency of oxygen in the water accelerates the respiratory movements, and the Fish appears to "pant" or breathe hurriedly. In the Lampreys, both inspiration and expiration may take place through the external gill-apertures by the alternate expansion and contraction of the gill-sacs, more especially when the suctorial buccal funnel is used for the attachment of the animal. On the other hand, the singular habits of the Myxinoids involve a further modification of the respiratory process. In these Cyclostomata the inspiratory current enters the external naso-pituitary aperture and reaches the pharynx through the naso-pituitary canal, and thence, as an expiratory stream, traverses the gill-sacs on its way outwards. The pharynx is closed behind the last pair of gill-sacs by a constrictor muscle, which prevents the entrance of the water into the oesophagus, and converts the pharynx into a respiratory tube for the time {289}being; but, when food is being swallowed, the pharyngeal constrictor is relaxed and the internal apertures of the gill-sacs are closed by the contraction of their own sphincter muscles.

In addition to the usual respiratory organs it is probable that in not a few Fishes the superficial skin may share with the gills the function of breathing. In this connexion may be mentioned the fact that in Periophthalmus the tail is used for respiration. Hickson observed that a species of this genus, frequenting the extensive sandy shores of the Island of Celebes, often rests with its tail in the water, the head and trunk being exposed. Under such circumstances the gills are probably of little use, and the tail is utilised as a breathing organ, principally, as Haddon subsequently pointed out, through the agency of its extremely vascular caudal fin.

Some Fishes possess larval breathing organs; others, even when provided with gills, either utilise the air-bladder, or develop special accessory organs, for aquatic or, more usually, for aerial respiration.

{290}[Illustration: FIG. 168.—Head of young Polypterus. ex.g, External gill of the left side. (From Steindachner.)]

LARVAL GILLS.—In early life many Fishes acquire larval gills, either as the result of the precocious growth of the normal gills, or by reason of the development of evanescent structures. In the embryos of Elasmobranchs "external gills," in the form of long filiform processes invested by hypoblast, are developed from the walls of all the branchial clefts, including the spiracles, and protrude outwards for some distance through the external apertures of the clefts (Fig. 167, B). They perhaps facilitate respiration within the egg, as they completely disappear after hatching; but there is also reason for believing that they aid in the absorption of nutriment. Similar gills are present in young Holocephali. In some larval Teleosts, as in certain genera of the Osteoglossidae and Mormyridae (e.g. Heterotis and Gymnarchus) these structures are remarkably developed (Fig. 239). The young of the Loach (Misgurnus) and of the Salmon (Salmo) also have the ordinary gill-filaments prolonged externally as filiform structures, which subsequently become reduced to their normal size. In its larval state Polypterus has a pair of pinnately-fringed ectodermal or cutaneous gills projecting from the lateral surfaces of the head behind and above the external branchial apertures (Figs. 168 and 281). Apparently as an individual peculiarity the right gill has been retained in a specimen of P. congicus so large as 22 cm. in length, although the left one had entirely disappeared. Each gill is supplied with blood from the ventral aorta by a vessel which ascends the {291}hyoid arch, and is apparently the representative of the artery supplying the hyoidean hemibranch in Elasmobranchs. The efferent vessel of each gill joins the common trunk formed by the union of the efferent vessels of the normal gills of the same side.

The cutaneous gills of the Dipnoid Protopterus may also be included in the category of larval breathing organs. They consist of three simple unbranched filaments on each side of the head, and, as in Polypterus, are situated at the dorsal extremity of the external gill aperture (Fig. 309). Although usually represented in the relatively young or half grown specimens which, so far, have reached Europe, it is extremely probable that these organs atrophy in older individuals. Similar gills are present in the larval Lepidosiren (Fig. 311), but disappear at a much earlier stage. At no period of its development are larval gills present in Neoceratodus.

THE AIR-BLADDER AS A RESPIRATORY ORGAN.—In certain Fishes the air-bladder may become subservient to the function of respiration. In Amia and Lepidosteus the internally sacculated and vascular air-bladder is obviously adapted for air-breathing, and there are not wanting observations which suggest that the organ is actually used for this purpose after the fashion of a lung. According to Jobert, this is also the case with the sacculated air-bladder of certain Brazilian Teleosts, viz. Sudis gigas, Erythrinus taeniatus and E. braziliensis, since these Fishes die of asphyxia when the organ is cut off from communication with the exterior by the ligature of its ductus pneumaticus. It is in the Dipnoi, however, that the air-bladder becomes most completely a true lung. In Neoceratodus the lung is probably of the greatest use to the Fish when the rivers are low during the hot season and the water is charged with foul gases from decomposing vegetable matter, and possibly also when the water is filled with sediment in the rainy season. In Protopterus, and more especially in Lepidosiren, the partial atrophy of the gills renders it highly probable that the lungs are the principal breathing organs at all times. Nevertheless, it must be emphasised that in all these Fishes respiration by means of the air-bladder {292}necessarily involves a transit of air to and from that organ through the ductus pneumaticus, and at present nothing is known as to the method by which such inspiratory and expiratory currents can be produced.

There is also some experimental evidence for the belief that the air-bladder of some Teleosts may be subsidiary to respiration by acting as a reservoir for the superabundance of oxygen which is taken into the blood through the gills, and subsequently reabsorbed into the blood when the Fish is in water containing relatively little oxygen. It is clear, however, that the conditions under which the air-bladder can be used in this way are by no means fully understood, for, under experiment, such Fishes died of asphyxia even though after death the air-bladder still contained upwards of fifty per cent of oxygen.

ACCESSORY ORGANS OF RESPIRATION.—In certain Fishes of peculiar habits, or living under special external conditions, accessory respiratory organs are developed.

Although in this particular instance no special organs are formed, mention may first be made of the singular method of intestinal respiration in vogue in some Teleosts. In one of the Loaches (Misgurnus fossilis), air is swallowed and passed along the alimentary canal until it is finally voided at the anus. The mucous membrane of the intestine is extremely vascular, and hence the blood comes into sufficiently intimate relations with the swallowed air to admit of it exchanging carbon dioxide for oxygen. Intestinal respiration also occurs in species of the South American freshwater genera of Siluridae and Loricariidae, Callichthys, Doras, Loricaria, and Plecostomus; and in some cases the area of respiratory surface is considerably increased by the development of folds and processes of the intestinal mucous membrane.

In a few tropical Teleosts curious labyrinthiform organs are developed in connexion with certain of the branchial arches, and serve as accessory breathing organs. In the Indian "Climbing Perch" (Anabas scandens), of the family Anabantidae, the organ (Fig. 169) consists of three or more concentrically-arranged bony {293}laminae, with wavy, crenulated margins, attached by a common bony base to the upper extremity of the fourth branchial arch, and enclosed in a special dorsal enlargement of the branchial cavity. The vascular membrane which invests the laminae is abundantly supplied with venous blood by a branch of the fourth afferent branchial artery, the equivalent efferent vessel joining the dorsal aorta. Essentially similar organs are found in several genera of Osphromenidae (e.g. Polyacanthus, Osphromenus, and Trichogaster). A simpler form of respiratory organ of somewhat the same type occurs in the Indian family Ophiocephalidae. In these Fishes there is, on each side, an accessory branchial cavity, situated above that which contains the gills, but freely communicating with it (Fig. 170). The cavity is lined by a thickened and puckered vascular membrane, but otherwise contains no special respiratory structures.

In the Siluroid genera Clarias and Heterobranchus the accessory organ takes the form of branched, arborescent and highly vascular structures, developed as outgrowths from the dorsal extremities of one or two branchial arches, and enclosed within a posterior and dorsal expansion of the proper branchial cavity (Fig. 171).

{294}Another example of these interesting structures occurs in Chanos salmoneus and a few other Clupeidae in the shape of a coiled gill-like organ ("gill-helix"), which is supported by the dorsal segment of the fourth branchial arch, and enclosed in a similarly curved caecal extension of the branchial cavity. Each gill derives its blood from the fourth afferent branchial artery, the corresponding efferent vessel joining the fourth efferent branchial artery. A similar spirally-coiled "gill-helix" is found also in Heterotis ehrenbergii, amongst the Osteoglossidae, and in several species of Characinidae.

In other Teleosts the accessory breathing organ assumes the condition of paired lung-like outgrowths of the branchial cavity. Thus, in one of the Symbranchidae, the Indian "Cuchia Eel" (Amphipnous cuchia), there is a pair of small bladder-like sacs, with membranous and vascular walls, each of which opens into the branchial cavity above the first gill-cleft, and is supplied with blood by the afferent branchial artery of the gill-less first branchial arch. An extreme modification in the same direction {295}is presented by the Indian Siluroid Saccobranchus. In this Fish a long caecal diverticulum of the branchial cavity extends backwards on each side from the dorsal region of the first branchial cleft to the tail, and in its course is situated internally to the lateral trunk musculature, and close to the vertebral column (Fig. 172). The walls of the caeca are vascular, but no special respiratory structures are developed within their cavities, which, during life, only contain air. In S. singio the right caecum is supplied with blood by an extension backwards of the dorsal portion of the first afferent branchial artery of that side; the left, on the contrary, being supplied by the corresponding portion of the fourth afferent artery of the same side. In S. fossilis both air-sacs are supplied by the fourth afferent branchial artery. The efferent vessels join the fourth efferent branchial artery, right or left as the case may be.

With perhaps one or two exceptions, the accessory respiratory organs of Fishes seem to exist for the purpose of enabling their possessors to breathe in air. This is certainly the case with the labyrinthiform organs of Anabas and its allies, and also in such Fishes as Amphipnous, Saccobranchus, and the Ophiocephalidae, and probably in others. Nearly all these Fishes are tropical in geographical distribution, more or less amphibious in their habits, and usually possess a remarkable capacity for sustaining life out of water, under conditions which are promptly fatal to ordinary Fishes. Thus, Anabas scandens may be kept alive for days in earthen pots without water, and when free is able to travel short distances on land, especially in the early morning when the dew is on the ground, while Amphipnous frequents {296}marshes, lurking in holes in the grass and about the sides of ponds. In fact, even when in the water, access to air, which is probably swallowed and passed over their accessory breathing organs, is indispensable to their existence. Experiments conclusively prove that if the Fish is artificially prevented from obtaining air in this way asphyxiation speedily ensues.

In addition to breathing air through the agency of special organs evolved for the purpose, there are many freshwater Fishes which, like those just mentioned, periodically rise to the surface and swallow air in order to saturate the water which bathes the gills with oxygen.

{297}CHAPTER XI

THE AIR-BLADDER

In the Crossopterygii, Chondrostei, and Holostei, in the Dipnoi, and in the great majority of Teleosts, there is situated on the dorsal side of the coelom, between the alimentary canal below and the kidneys and vertebral column above, a more or less elongated sac with membranous walls, an internal epithelial lining and gaseous contents—the air-bladder (Figs. 154 and 173). Usually developed in the embryo as a caecal outgrowth from the dorsal surface of the oesophagus, the air-bladder grows anteriorly and posteriorly, and may either retain throughout life its primitive connexion with the alimentary canal by means of a longer or shorter tubular canal, the ductus pneumaticus, or become completely separated therefrom in the adult by the atrophy of the duct. Its walls sometimes, but rarely, contain muscle-fibres, as in Lepidosteus, Amia, and the Dipnoi, and are always more or less vascular, while laterally and ventrally the organ is invested externally by the peritoneum (Fig. 173). In addition to the muscle-fibres distributed in its walls, the bladder is often provided with powerful extrinsic muscles, more especially in those Fishes in which it is used as an organ for {298}sound-production. In the different groups of Fishes in which it is present the air-bladder frequently undergoes remarkable structural modifications and becomes adapted for various distinct functions.

In the Cyclostomata there is no trace of an air-bladder, and, unless represented in certain Sharks (e.g. Mustelus, Galeus, and Acanthias), by a small caecum embedded in the dorsal wall of the oesophagus and communicating with its cavity, it is also absent in all Elasmobranchs. In the Crossopterygii (e.g. Polypterus), the air-bladder is double, but while the right sac is long and somewhat tubular, the left is much smaller and oval in shape (Fig. 174). Near their anterior extremities the two sacs fuse into a single unpaired chamber, beyond which they again project in the form of two short caeca. The median chamber opens into the oesophagus on the ventral side by an orifice (gl) bounded by prominent lips and furnished with a muscular sphincter. The organ is devoid of internal sacculations. In the Chondrostei (e.g. Acipenser) the air-bladder is oval in shape, with a smooth, non-sacculated, inner surface, and a lining of ciliated epithelium, and it communicates with the oesophagus by means of a relatively wide, dorsally placed, funnel-like orifice.

In the Lepidosteidae the single air-bladder extends the whole length of the abdominal cavity, and, as in Polypterus, communicates with the exterior through a larynx-like vestibule provided with a glottis, which, however, opens dorsally into the oesophagus (Fig. 175). A strong fibrous band runs along the median line of the inner surface of its dorsal wall, from which extends ventrally on each side a series of transverse fibro-muscular ridges, forming the boundaries of a double row of regularly arranged alveoli (Fig. 176). The bottom of each alveolus {299}is still further sacculated by finer branches of the principal fibrous bands. In the Amiidae the bladder is very large, and, except that a short median cleft divides it in front into two short caeca, it is unpaired. Internally, its walls are much sacculated, but the alveoli are smaller and arranged less regularly than in Lepidosteus. The aperture of communication with the oesophagus is dorsally situated.

It may be mentioned that in all the preceding Teleostomi the ductus pneumaticus is remarkably short, the connexion between the air-bladder and the oesophagus being almost direct by means of a larger or smaller orifice, which, except in Acipenser, is more anteriorly placed than in most other Teleostomi; and further that, unlike many Teleosts, there are no special "retia mirabilia," "red bodies," or "red glands."

In the Dipnoi the structural resemblance of the air-bladder to a true lung, which to some extent is indicated in Polypterus, Amia, and Lepidosteus, becomes still more marked.

In Neoceratodus the organ is not unlike that of Lepidosteus, and takes the form of a spacious unpaired sac, extending from one end of the abdominal cavity to the other. On its inner surface two fibrous bands, one of which is dorsal and the other ventral, traverse the whole length of the bladder, and project slightly into its cavity. {300}Between these median ridges extend a number of transverse septa, forming the boundaries of a series of pairs of bilaterally symmetrical oval alveoli, the walls of which are still further sacculated by a network of finer ridges (Fig. 177). The short ductus pneumaticus seems to be an anterior continuation of the right half of the bladder, and opens into the oesophagus by a small glottis, situated on the ventral side, a little to the right of the median line.

The more complicated and much more lung-like air-bladder of Protopterus (Fig. 178) is essentially double, consisting of an anterior unpaired portion, and of two sac-like prolongations which extend backwards the whole length of the coelom, gradually tapering towards the cloaca. Anteriorly, the unpaired portion of the organ is continued into a vestibule or pneumatic duct, which, after passing ventrally on the right side of the oesophagus, opens into the latter by a ventrally-situated, slit-like glottis, immediately behind the last pair of gill-clefts. The margins of the glottis are provided with radially-arranged dilator muscles, and in connexion with its anterior border there is an epiglottis-like fibro-cartilaginous plate. The central cavity of each lung (Figs. 178 and 179) communicates with a series of larger or smaller alveoli in the lung-wall, and each of the latter opens in succession into smaller tubular cavities, and then into still smaller terminal caecal sacculi. Hence, much more than in Neoceratodus, the lungs approximate in structure to those of the higher terrestrial {301}Vertebrata. Non-striated muscle cells, pigment cells, and blood capillaries are abundantly present in the connective tissue external to the lining epithelium of the lung-cavities.

The air-bladder of Lepidosiren closely resembles that of Protopterus, and, as in the latter Dipnoid, the glottis seems to be furnished with an epiglottis.

In all the Dipnoi the air-bladder is highly vascular, but nevertheless presents no trace of "red bodies" or "red glands."

The most striking features in the remarkably polymorphic air-bladder of Teleosts relate to (a) its presence or absence; (b) differences in shape and relative size; (c) the development of caecal outgrowths; (d) the subdivision of its cavity by the formation of internal septa; (e) the retention or suppression of the ductus pneumaticus, and the occasional development of secondary ducts communicating directly with the exterior; (f) the presence of "red glands" or "red bodies"; (g) its connexion with the auditory organ; (h) its adaptation as an organ for sound-production.

{302}(a) The air-bladder is by no means universally present in Teleosts. It is absent in several entire families, such as, for example, the Flat Fishes or Pleuronectidae, the Scopelidae, and the "Lump-suckers" (Cyclopteridae). In a few families, as in the Mackerels (Scombridae), the "Blennies" (Blenniidae) and the Polynemidae, the organ is present in most genera, but absent in a few, or even present or absent in different species of the same genus. Thus, of the three British species of Mackerel, viz. the Spanish Mackerel (Scomber colias), S. pneumatophorus, and the common Mackerel (S. scombrus), an air-bladder is present in the first two, but absent in the third.

(b) As might be anticipated, the shape of the air-bladder is extremely different in various Teleosts, and usually conforms to the shape of the body, while differences in relative size are of frequent occurrence, even in closely related species. Sometimes the organ is more or less tubular, fusiform, ovoid, or heart-shaped; occasionally it is shaped like a "dumb-bell," consisting of two lateral sacs connected by a median tubular portion, as in the Siluroids Clarias and Callichthys; or it may be horse-shoe-shaped, as in the Silurid Ailia. Not unfrequently a transverse constriction divides the air-bladder into two intercommunicating sacs, as in most of the Carp family (Cyprinidae), or three such sacs may be formed by two constrictions (e.g. Ophidium). In the "Electric Eels" (Gymnotidae) there are two sacs, connected by a slender canal, from which the ductus pneumaticus takes its origin.

The air-bladder is either more or less free in the abdominal cavity, or firmly attached to the vertebral centra and their rib-bearing processes by fibrous extensions passing between the two structures. Not rarely the organ extends from the abdominal {303}cavity into the tail, sometimes penetrating for a short distance into the expanded haemal canal of the anterior caudal vertebrae, or extending unsymmetrically along either the right or left side of the tail. More frequently, perhaps, where the air-bladder is prolonged into the tail, it assumes the form of two bilaterally arranged and symmetrical caeca, which extend backwards for a variable distance internal to the caudal muscles and in contact with the adjacent skeletal elements, as in Notopteridae, and in some Sparidae, Carangidae, and Scombridae. The extension of the air-bladder into the tail is often associated with a short, laterally-compressed trunk, which, if the bladder is to attain its normal degree of development, necessitates its prolongation into the caudal region.

(c) A characteristic feature in the air-bladder of many Teleosts belonging to widely different families is the development of a more or less complex system of simple, or variously branched, caecal outgrowths, which, like the internal septa, are specially characteristic of those Fishes in which the bladder is used as a vocal organ without, however, being peculiar to them.

In some of the Gadidae, as in the Cod (Gadus morrhua), the air-bladder divides anteriorly into a pair of caecal prolongations which extend forwards to the head, and are often curiously coiled. Somewhat similar caeca are also present in species of Berycidae, Sparidae, Siluridae, Clupeidae, and Notopteridae. Caecal prolongations may also be developed from the hinder end of the bladder, and, as already mentioned, extend into the tail; or even from both ends in the same species (e.g. Notopterus). In the Silurid, Rita crucigera, a long tubular caecum is developed from each side of the heart-shaped bladder, and thence is prolonged backwards to the anus. In certain species of Doras of the same family (e.g. D. maculatus), an elegant series of variously sized branched caeca fringe each of the lateral margins of the bladder. It is, however, in the Physoclist family of the Sciaenidae that the branching of the air-bladder attains its greatest development in extent and variety.

{304}[Illustration: FIG. 180.—Air-bladder of Otolithus. (From Cuvier and Valenciennes.)]

In Otolithus (Fig. 180) two short tubular canals are given off from the antero-lateral angles of the bladder, each subsequently dividing into two elongated, tapering sacs, of which one is directed forwards and the other backwards. In Corvina lobata (Fig. 181) the lateral margins of the bladder are everywhere fringed with a series of tufts of caeca, each tuft being connected by a short common canal with the cavity of the organ. In the "Drum" (Pogonias chromis) (Fig. 182) each side of the anterior third of the air-bladder has a series of digitately branched caecal appendages, the most posterior of which on each side are connected by a tubular canal, also bearing branched caeca, with the corresponding postero-lateral extremity of the bladder.

Collichthys has a still more remarkable arrangement. In this Sciaenoid (Fig. 183) twenty-five tubular branches are given off from each side of the bladder, all of which soon subdivide into a dorsal and a ventral division. These still further divide, and their branches either end blindly or are prolonged into a series of arches to the mid-dorsal or mid-ventral line as the case may be, where they become continuous with the corresponding branches of the opposite side. The series of dorsal branches, enveloped in their peritoneal investment, extend between the body of the air-bladder and the roof of the body-cavity, while the corresponding ventral branches, similarly invested, surround that part of the coelom which contains the stomach, intestine, and liver.

(d) In addition to the subdivision of the cavity of the air-bladder by the externally obvious, {305}transverse, or longitudinal constrictions already described, or by the growth of simple or branched prolongations, the organ is often chambered or sacculated by the development of internal septa or partitions.

In many of the Gurnards (Trigla) the cavity of the bladder is divided into two intercommunicating compartments by a transversely-disposed and centrally-perforated diaphragm. The large air-bladder of some species of Erythrinus is subdivided internally into numerous alveoli or sacculi. In Notopterus a longitudinal septum divides the cavity of the abdominal portion of the bladder into two lateral chambers, which, however, freely intercommunicate anteriorly. In the great majority of the Siluridae the cavity of the organ is divided by a characteristic T-shaped arrangement of a primary transverse and a longitudinal septum into three communicating chambers, of which one is anterior and transversely disposed, and two are posterior and longitudinally arranged (Fig. 222). The posterior compartments in many genera are still further divided by the growth of secondary transverse septa, extending outwards from the median longitudinal septum, without, however, reaching the external lateral walls of the chambers. In a few genera, as in certain species of Pangasius, additional fibrous bands and ridges passing between the primary and secondary {306}septa give to the cavities of the lateral compartments the appearance of being occupied by a coarse spongy network.

(e) In its relations to the oesophagus and to the air-bladder the ductus pneumaticus exhibits striking modifications in different Teleosts. With very rare exceptions, an open ductus is wanting in the Heteromi, Catosteomi, Acanthopterygii, Opisthomi, Pediculati, Jugulares, and the Plectognathi, for which reason the term "Physoclisti" has often been used as a collective name for these Fishes. On the other hand, a permanently open ductus is generally present in the Malacopterygii, Ostariophysi, Apodes, and the Haplomi, which, in consequence, have been designated "Physostomi." It must be emphasised, however, that all Teleosts are Physostomous in the embryonic condition, and whether they eventually become Physoclistous or remain Physostomous depends entirely on the abortion or retention of the primitive communication between the air-bladder and the alimentary canal. When present in Teleosts, the ductus pneumaticus, with a few exceptions (e.g. Notopterus), where it is both short and relatively wide, is almost invariably much longer and narrower than in the other orders of Teleostomi and in the Dipnoi, sometimes passing directly from the air-bladder to the oesophagus, but not infrequently describing a sigmoid curve, as in some Cyprinidae, or an even more tortuous course. The opening into the alimentary canal is, with perhaps a single exception, dorsal, but may vary from the commencement of the oesophagus to the hinder end of the stomach. In Erythrinus the oesophageal aperture is lateral. In two instances the air-bladder has acquired secondary openings to the exterior, and of these one occurs among the Physostomi and the other {307}in the Physoclisti. In the Herring (Clupea harengus), in addition to the proper ductus, which is connected with the distal end of the caecal stomach, a tubular canal leaves the hinder extremity of the bladder and opens externally on the left side of the genital aperture; consequently, in this Fish the air-bladder has a secondary and direct connexion with the exterior in addition to the primary and indirect communication by means of its proper duct. The Horse-Mackerel (Caranx trachurus) is even more peculiar. This Teleost has no true pneumatic duct, but instead a special duct which passes from the bladder to open into the right branchial cavity by a very minute aperture. In neither case is anything known of the mode of origin or morphological nature of the secondarily acquired duct.

(f) The air-bladder differs greatly in its degree of vascularity in various Teleosts, as well as in the extent to which its capillary blood-vessels accumulate at special points on the inner surface to form the so-called "red bodies" or "red glands." In some Teleosts the distribution of capillaries is uniform or nearly so; in others, as in the Carp (Cyprinus carpio) the vessels are arranged in fan-like, radiating tufts over almost the whole extent of the inner surface; in others again, as in the Pike (Esox lucius) the tufts are larger and more definitely localised. A more extreme modification occurs in some of the Physostomi, in which a remarkable concentration of capillaries takes place at one or more points on the inner surface of the bladder, which project into the cavity of the organ in the form of variously shaped blood-red masses. These "red bodies" are essentially retia mirabilia, consisting of masses of interlacing, tightly-packed capillaries with their afferent arteries and efferent veins. The flattened lining epithelium of the bladder is continued over these bodies without undergoing any special modification. In the common Eel (Anguilla vulgaris) there are several of these bodies, of which the largest are near the entrance of the pneumatic duct.

In the Physoclisti the "red bodies" seem to be replaced by true glands, which nevertheless in appearance closely resemble the former. Some of the Gadidae, such as the Cod (Gadus {308}morrhua), the Haddock (G. aeglefinus), and the Hake (Merluccius vulgaris), have a single large "red gland" projecting into the interior of the bladder from its dorsal or ventral wall (Fig. 184, A). The John Dory (Zeus faber) has five such glands, worm-like and curved in shape, with their concavities facing a central point between them (Fig. 184, B). In these Fishes a "rete mirabile" of blood-vessels forms the vascular basis of the glands. The ordinary pavement epithelium of the bladder becomes replaced by faintly granular, columnar, and evidently glandular cells as it passes over the retia mirabilia, and at the same time becomes invaginated into the mass of capillaries in the form of a number of simple caecal glands (Fig. 185). So far as is at present known, the "red glands" are only found in those Teleosts in which the air-bladder has no ductus pneumaticus, whereas in those Fishes which retain the ductus throughout life there are either no special retia mirabilia, or, as in the Eel, only the so-called "red bodies."

{309}(g) and (h) The structural modifications involved in the connexion of the air-bladder with the auditory organ, and its adaptation for sound-production, as well as its use in respiration, are considered elsewhere.

THE GASES OF THE AIR-BLADDER.—The gaseous contents of the air-bladder consist of oxygen and nitrogen, but the relative proportions of the two gases differ in different Fishes, and even in the same Fish, under different conditions. Normally the proportion of oxygen is considerably less in freshwater than in marine Fishes, and amongst the latter the proportion of oxygen is often enormously greater, amounting in some cases to 87 per cent., in deep-sea species as compared with their shallow water congeners. A trace of carbon dioxide is also usually present. The gases are derived from the blood as the latter circulates through the capillaries in the walls of the bladder, and it is highly probable that the "red glands" take an important part in the process; at all events, experimental research has shown that the "secretion" or diffusion of gas into the air-bladder, as well as the absorption of gas from the bladder into the blood, take place most rapidly in those Fishes in which "red glands" or "red bodies" are present.

THE FUNCTIONS OF THE AIR-BLADDER.—Probably no single organ in any group of Vertebrata is associated with the performance of a greater variety of functions than the air-bladder of Fishes. Originally evolved, it may be, as a glandular caecum in certain {310}Sharks, the air-bladder in the Dipnoi, and some of the more generalised Teleostomi (e.g. Amia and Lepidosteus), and perhaps also in a few of the more specialised members of the latter group (e.g. certain Teleosts), is to a greater or less extent an accessory respiratory organ. In not a few Teleosts it is an organ for sound-production, and in others again it is sometimes regarded as having an important relation to the sense of hearing. But omitting such subordinate functions which, as it were, have been grafted on to the air-bladder, there can be no doubt that in the great majority of Fishes its primary use is to act as a hydrostatic organ or "float." From this point of view experimental investigations seem to justify the following conclusions:—

The function of the air-bladder is to render the Fish, bulk for bulk, of the same weight as the water in which it lives. In this condition of equilibrium, or plane of least effort, the Fish floats in the water, and therefore it is able to swim with a minimum of muscular effort. It is obvious, however, that as a Fish rises or sinks it becomes exposed to an increase or a diminution of hydrostatic pressure, which will necessarily bring about the expansion or contraction of the volume of gas in the air-bladder, and, therefore, by decreasing or increasing the specific gravity of the animal, will tend to remove the Fish from its plane of least effort. To counteract this, and to restore the Fish to a plane of equilibrium at the new level, gas is either absorbed from the air-bladder, or more gas is secreted into the bladder, as the case may be. According to Moreau, by this process of automatic adjustment a Fish will always find, sooner or later, a plane of least effort, whatever may be its depth in the water; and further, this process takes place much more readily in those Fishes which possess "red glands" or "red bodies", and with extreme slowness in those in which these organs are absent. Nevertheless, it seems doubtful if this process of adjustment can be of much use to a Fish in ordinary vertical movements, inasmuch as gaseous secretion and absorption are comparatively slow processes, the length of which in different Fishes, and under different conditions, varies from a few hours to several days. On the whole it seems more probable that adjustment to the varying pressures of different depths by such means is far more likely to be useful during such slow and gradual changes of level as are encountered {311}in the course of diurnal, seasonal, or other periodic migrations than during the rapid changes of level which may take place in ordinary vertical locomotion. In the generality of Fishes, and more especially in the Physoclisti, it may be concluded that the possession of an air-bladder restricts freedom of movement in the vertical direction, and confines ordinary locomotion within more or less well-defined vertical limits above or below the plane of least effort for the time being. As illustrating this point, and as a proof of the danger incurred by a too rapid rise in the water, the following remarks with reference to the "Kilch," a small Salmonoid (Coregonus) inhabiting the Lake of Constance, and a favourite article of food, may be quoted:—The Fish "are caught in nets, and brought to the surface of the water; they come up invariably with the belly much distended, the air in the swimming-bladder, being relieved from the pressure of the column of water, has expanded greatly and occasioned this unnatural distension, which renders the Fish quite incapable of swimming. Under these conditions the Fish is naturally unable to live for any length of time. But the fishermen of the lake have a very simple remedy; they prick into the air-bladder with a fine needle; the air escapes with some force, the distension subsides, and the fishes are enabled to live under totally changed conditions as to pressure, even in quite shallow water and at the surface, swimming quite as freely as their companions, the natives of the surface water. Hence the Kilch is confined to a certain depth, because it is not capable of accommodating the tension of its swimming-bladder to the change of pressure in the column of superincumbent water."

It is not improbable that the Physostomi, or at any rate most of them, are somewhat more advantageously placed in this respect. From the general absence of "red glands" in this group, it may be inferred that whatever capacity for gaseous secretion or absorption they possess must be exercised with exceptional slowness, and, therefore, as a means of pressure-adjustment may be neglected. On the other hand, they seem to possess the compensating advantage of being able to substitute for absorption the mechanical liberation of gas through the ductus pneumaticus. It would seem, therefore, that the Physostomi have a distinct advantage over the Physoclisti in that during ascent in the water they {312}can more readily adapt themselves to the diminished pressure of a higher level by ejecting the needful amount of gas than by relying upon the process of gaseous absorption. This conclusion is in harmony with the results of experiment and with much that is known of the habits of these Fishes and their greater freedom of locomotion in the vertical direction.

These briefly summarised conclusions as to the hydrostatic function of the air-bladder must, however, be accepted only in a general sense. There are many structural anomalies in the air-bladder of Fishes which are very difficult to explain, or to correlate with any variations in the habits or in the locomotor activities of its possessor.

In this connexion it may be mentioned that the presence or absence of an air-bladder in different Fishes seems to some extent to be governed by two causes. First, whenever the requirements of a Fish necessitate exceptional freedom of locomotion in all directions the restrictions imposed by the presence of an air-bladder are removed by its partial or complete suppression; a result produced, secondly, by the assumption of a bottom feeding or ground habit on the part of the Fish. Fishes like the Flat Fishes or Pleuronectidae, when not in motion by the exercise of their fins, habitually rest on the sea-bottom, and, as an air-bladder is useless under such conditions, it has, in consequence, undergone complete atrophy. Not a few Siluridae, and some Cyprinidae, inhabit the comparatively shallow waters of rapidly flowing mountain torrents, and are often provided with suckers for attachment to stones or rocks. To such Fishes as these a hydrostatic organ is obviously useless, and it has hence become greatly reduced in size, and in other respects approaches the condition of a vestigial organ.

{313}CHAPTER XII

THE VASCULAR SYSTEM, THE LYMPHATICS, AND THE BLOOD-GLANDS

The Cyclostomata and Fishes possess a closed vascular system, consisting of a heart, arteries, capillaries, and veins, the whole forming a continuous series of blood-containing channels provided with definite limiting walls, through which the blood is propelled in a constant direction by the rhythmical contractions of the heart. In the course of the circulation the blood flows from the heart through a single large trunk, the ventral aorta, to the capillaries of the gills. From the gills the arterialised blood is collected into a large dorsally-situated vessel, the dorsal aorta, and thence is distributed through a system of arteries to the capillaries of the various organs of the body. Finally, the blood is collected from the capillaries and returned to the heart by the veins.

Although in most instances the organs of the body are supplied with arterialised blood conveyed to them by arteries, there are nevertheless cases in which an organ may receive venous blood by a vein in addition to arterial blood supplied by an artery. For example, the capillaries of the liver not only receive blood from the hepatic artery, but also venous blood by a large vein (hepatic portal vein), formed by the union of a number of smaller veins by which venous blood is collected from the capillaries of the stomach, intestine, spleen, and pancreas. In this and similar instances, where a vein formed by the union of the capillaries of an organ, or of a series of organs, instead of uniting with other veins and proceeding towards the heart, becomes continuous with a second set of capillaries in some other organ, a "portal" system is said to be formed, and in the particular example of the liver it is termed the "hepatic portal" system. A similar, or "renal {314}portal," system also exists in connexion with the kidneys in the majority of Fishes.

There is little doubt that, primarily, the vascular system of Vertebrate animals consisted of a dorsal artery (dorsal aorta), running along the median dorsal line of the alimentary canal, and a ventral or subintestinal vein similarly related to the ventral surface of the digestive tube. The two vessels were connected by a series of pairs of lateral branches, which had their origins from the dorsal vessel, and, by their subdivision, formed a capillary network in the walls of the alimentary canal. From these networks paired veins issued and opened into the subintestinal vein. The simplicity of this primitive arrangement was somewhat disturbed in the region of the pharynx by the development of gill-clefts, in the walls of which the blood circulated for respiratory purposes from the ventral to the dorsal vessel; and also by the development of a hepatic portal circulation in connexion with the liver. In the latter instance the subintestinal vein entered the liver and subdivided into capillaries in the substance of that organ, the corresponding efferent vessel, or hepatic vein, becoming continuous with the anterior or pharyngeal section of the subintestinal vein, or, as it is usually termed, the ventral aorta. In this low grade of vascular system, which is perhaps most completely retained in Amphioxus, the circulation of the blood was probably effected by the wave-like contractions of more or fewer of the larger vessels; but subsequently a definite chambered heart was developed at the origin of the ventral aorta.

Of Fishes in general it may be said that the primitive dorsal and ventral vessels are present in the embryo, and for a time retain their original relations and physiological importance. To a very unequal extent they may also be retained in the adult, where, however, they co-exist with numerous other vessels, which the increasing differentiation of the body has called into existence. Thus, at a later period of embryonic life, the subintestinal vein becomes somewhat fragmentary. Its caudal section (caudal vein) ceases to be continuous with the precaudal portion, and the blood collected from the muscles and other structures of the tail is conveyed to the heart by a pair of posterior cardinal veins, which are either directly continuous with the caudal vein, or indirectly through the intervention of a renal portal system in the kidneys. {315}The precaudal portion of the subintestinal vein is represented by a vein which runs forwards in the intestinal wall, and is one of the minor affluents of the hepatic portal vein, while its prehepatic section is represented in succession by the hepatic vein, the heart, and the ventral aorta. Of the additional veins which supplement these remnants of a primitively continuous subintestinal vein, the largest and most constant are (a) the posterior cardinal veins which, commencing in the kidneys and receiving the blood from those organs, pass forwards to the heart; (b) a pair of anterior cardinal veins, formed by the union of smaller veins from the head, including the brain, and passing backwards towards the heart. At the level of the latter organ each anterior cardinal vein joins the posterior cardinal of the same side of the body to form a short but wide transverse vessel, the Cuvierian duct or precaval vein, which opens into the hindermost of the cavities of the heart, viz. the sinus venosus; (c) a pair of inferior jugular veins by which the nutrient blood of the branchial apparatus is returned to the right and left Cuvierian ducts. In addition to these principal veins there may also be a pair of lateral veins collecting the blood from the lateral walls of the trunk, and also opening into the Cuvierian ducts; and subclavian and femoral veins from the pectoral and pelvic fins.

On the other hand, the primitive dorsal vessel (dorsal aorta), retains not only its original position and relations, but also its primary function as the main channel for the distribution of arterialised blood to all parts of the body. The system of lateral and probably segmentally arranged vessels, by which the dorsal and subintestinal vessels were connected in the primitive Vertebrata, have undergone considerable modification in all existing Fishes, but nevertheless retain much of their original disposition and relations in the pharyngeal region of the alimentary canal, where they are represented by the afferent and efferent vessels of the gills.

A more detailed account of the condition of the vascular system in the Cyclostomata and Fishes will now be given.

THE VENOUS SYSTEM.—The Cyclostomata, as might be expected, exhibit a more primitive condition of the venous system {316}in certain features than is the case in any other group. The precaudal portion of the subintestinal vein retains much of its original importance and runs in the rudimentary intestinal spiral valve as far as the liver, where it becomes the hepatic portal vein. From the liver the blood is collected into a single hepatic vein, and by it is conveyed to the sinus venosus. The caudal section of the subintestinal vein, now known as the caudal vein, bifurcates near the anus, and its two branches become directly continuous with the right and left posterior cardinals, without forming a renal portal system. In their forward course to the heart the posterior cardinals are situated directly beneath the notochord, and after receiving the blood from the kidneys and gonads, and from the numerous pairs of segmental veins of the body-wall, join the corresponding anterior cardinal veins, and form on each side a short transverse Cuvierian duct which opens into the sinus venosus. There is also a pair of inferior jugular veins which, however, unite opposite the fifth pair of gill-sacs to form a single trunk; this vessel is continued backwards, externally to the medio-ventral cartilage of the branchial basket, and finally opens directly into the sinus venosus.

In Elasmobranchs (e.g. Mustelus antarcticus) the caudal vein (Fig. 186) lies in the haemal canal of the caudal portion of the vertebral column. On reaching the kidneys the vein divides into two renal portal veins, which, however, are not directly continuous with the posterior cardinal veins as in the Cyclostomata, but, on the contrary, after receiving the posterior segmental and oviducal veins, become continuous with the capillaries of the kidneys.

From the latter organs the blood is collected by a series of renal veins, and by them conveyed to the posterior cardinals, and thence to the Cuvierian ducts. In the adult, therefore, there is a well-developed renal portal system, but it is worthy of note, nevertheless, that this system is developed comparatively late in embryonic life, and that at an earlier stage the caudal vein is directly continuous with the two posterior cardinals, precisely as is the case in the Cyclostomata throughout life. The posterior cardinal veins are situated in the dorsal wall of the coelom (Fig. 187), beneath the vertebral column. For the hinder portion of {317}their extent they are embedded in the kidneys (Fig. 186); and in this region the two veins are in close relation in the median line, and here and there freely communicate with each other. More anteriorly, they enlarge so much that they present the appearance of cavernous sinuses. In addition to the anterior segmental and oviducal veins, the posterior cardinals receive the spermatic or the ovarian vein from the male or female gonad.

The precaudal section of the primitive subintestinal vein, now termed the internal intestinal vein (Figs. 186 and 187), traverses the spiral valve as it passes forwards to the liver, but from a physiological point of view is now merely one of the factors of the great hepatic portal vein, the principal tributaries of which are the veins from the stomach and intestine, including the rectal gland, and the pancreas and spleen. On entering the liver the hepatic portal vein divides into two principal branches for the right and left halves of the gland. From the liver the blood is conveyed by two hepatic veins to the sinus venosus.

The lateral veins (Fig. 186) are situated in the lateral walls {318}of the abdomen, immediately external to the peritoneum (Fig. 187). Each vein begins near the pelvic fin, where it is connected with its fellow across the dorsal face of the ischio-pubic cartilage, and thence runs forward towards the pectoral fin. At its origin the lateral vein receives a femoral vein from the pelvic fin and a cloacal vein, and also, near its anterior end, a brachial vein from the pectoral fin, finally joining the Cuvierian duct of its side.

The anterior cardinal vein is situated directly above the gill-arches of its side of the head, and extends forwards from its junction behind with the Cuvierian duct to the outer side of the auditory capsule, where it communicates by a valvular orifice with a large sinus surrounding the eye-muscles (orbital sinus), and ventrally, by means of a similar aperture, with another large sinus, the hyoidean sinus, which lies on the outer face of the corresponding hyoid arch, and is continuous ventrally with its fellow of the opposite side. Into the orbital sinus open the anterior facial vein from the anterior and external regions of the head, and the anterior cerebral vein from the lateral half of the brain, and, into the hyoidean sinus, the nutrient veins from the hyoidean hemibranch.

The inferior jugular veins are situated beneath the branchial apparatus. Each vein begins anteriorly by communicating with {319}the hyoidean sinus of its side, and, after receiving the nutrient veins from the holobranchs of the first four branchial arches, opens into the corresponding Cuvierian duct.

The venous blood from the heart itself is collected into two coronary veins, which open into the sinus venosus.

In addition to the more important veins already described, there is also a series of median and lateral cutaneous veins communicating at different points with certain of the more deeply seated veins (Fig. 187).

Characteristic features in the venous system of Mustelus, as also of Elasmobranchs in general, are the development of transverse connexions between certain of the principal paired veins, and the tendency of many of the main veins to enlarge into more or less irregularly-shaped sinuses.

In its broad outlines the venous system of the Teleostomi agrees with that of Elasmobranchs, but is nevertheless characterised by several more or less important modifications, while at the same time exhibiting many differences in minor details.

A renal portal system is usually present, but is singularly variable in the source of its tributary veins, even in closely allied forms. In the Sturgeon (Acipenser) and in some Teleosts, as in the Siluroid, Amiurus catus, it resembles that of Elasmobranchs. In other Teleosts, on the contrary, the renal portal system presents various grades of degeneration, or, possibly, of imperfect evolution, as will be seen from the following illustrations of its condition in different genera.

In Amiurus the caudal vein, after giving off right and left renal portal veins to the renal capillaries, emerges from the ventral surface of the kidneys, and is then continued forwards between the gonads, the veins from which it receives, as the radicle of the hepatic portal vein.

In the Eel (Anguilla vulgaris) the caudal vein (Fig. 188) traverses the fused hinder portions of the kidneys, receiving several segmental veins from the body-wall and also giving off from each side numerous renal portal branches. More anteriorly, where the two kidneys become distinct, the caudal vein also divides into two renal portal veins and, as each vein extends {320}forwards along the outer border of the kidney of its side, it receives a number of segmental veins, and, at the same time, gives off branches to the renal capillaries. In addition, each renal portal vein is connected with the hepatic portal vein by a series of singular arch-like vessels into which the ovarian or spermatic veins open.

It is obvious, therefore, that in both Amiurus and Anguilla the primitive direct continuity of the caudal and posterior cardinal veins has been interrupted by the formation of a well-developed renal portal system, and further, that the residue of the caudal venous blood finds its way to the liver through the hepatic portal vein; hence it follows that, as in so many of the lower air-breathing Vertebrates, the whole of the venous blood from the tail is distributed either to the kidneys or liver in the course of its return journey to the heart.

The Tench (Tinca vulgaris) exhibits the interesting anomaly of possessing two caudal veins, a dorsal and a ventral (Fig. 189). The dorsal vein is directly continuous with the right posterior cardinal, while the ventral one divides into three branches, two forming right and left renal portal veins and receiving numerous segmental veins, and the third becoming one of the affluents of the hepatic portal vein. In this Teleost it is clear that a portion of the caudal blood passes directly to the heart through the right posterior cardinal without traversing either the renal portal or hepatic portal system.

In the Cod (Gadus morrhua) the caudal vein divides into two branches. The larger right vein retains its direct continuity {321}with the corresponding posterior cardinal; the left, on the contrary, has ceased to be continuous with the greatly reduced left posterior cardinal and forms a renal portal vein, the distribution of which is, however, restricted to the hinder portion of the left kidney (Fig. 190). As in Amiurus, a branch of the caudal vein forms one of the tributaries of the hepatic portal vein. In the Cod it would therefore seem that only a relatively small proportion of the caudal blood flows through the imperfectly developed renal portal system, the bulk of it traversing the right posterior cardinal and passing directly to the heart, leaving, nevertheless, a modicum for transmission to the liver. Finally, it may be mentioned that in some Teleosts the caudal vein retains its embryonic continuity with one, usually the right, posterior cardinal, without giving off a renal portal affluent, as in the Perch (Perca fluviatilis); or, after division, with both posterior cardinals, as in the Lump-sucker (Cyclopterus lumpus). In such instances as these no portion of the caudal blood traverses the kidneys, and if a renal portal system exists at all, the only true renal portal veins are the adjacent segmental veins, which transmit venous blood directly to the kidneys, instead of first uniting with renal portal branches of the caudal vein as in the Tench and the Eel.

Whatever may be the condition of the renal portal system, all the renal blood is eventually collected by renal veins and conveyed to the posterior cardinals, which are often connected by one or {322}by several transverse anastomoses (Fig. 190). In the region of the heart each posterior cardinal joins the corresponding anterior cardinal to form a short but wide Cuvierian duct, which finally opens into the sinus venosus.

A subintestinal vein is present in the embryo (e.g. Lepidosteus, Acipenser, and some Teleosts), but in the adult Teleostome its precaudal section is usually absorbed, or at all events ceases to be recognisable except, perhaps, as one of the minor tributaries of the hepatic portal vein.

The hepatic portal vein is formed as in Elasmobranchs, but in different Teleostomi it may also receive the veins from the pyloric caeca, from a portion of the air-bladder, the gonads, and, as previously mentioned, a tributary from the caudal vein. There are usually two hepatic veins opening into the sinus venosus, and generally of equal size (Fig. 190).

Most of the veins from the air-bladder join the hepatic portal {323}vein, as already mentioned (Fig. 190), but more or fewer of them, especially those from the dorsal wall of the organ, open into the posterior cardinals. They may, as in Polypterus, even join the hepatic veins.

The veins from the gonads are very variable in their destination, sometimes joining the posterior cardinals, as in the Salmon (Salmo salar); or the hepatic portal vein, as in Amiurus; or, as in the Perch (Perca fluviatilis), forming by their union a single trunk, which communicates directly with the left Cuvierian duct.

Representatives of the great lateral veins of Elasmobranchs appear to be absent in the Teleostomi, the veins from the pectoral and pelvic limbs joining the Cuvierian duct and the posterior cardinal veins respectively.

The two large anterior cardinal veins, which collect the blood from the head and brain, occupy their usual position directly above the branchial apparatus, and are sometimes connected by transverse anastomoses as they pass backwards to join the Cuvierian ducts. The inferior jugular vein is either single (e.g. Gadus); or paired, as in Perca (Fig. 190).

In the Dipnoi the venous system is distinguished by an interesting combination of characters, some of which are either primitive or peculiar to the group, while others exhibit a distinct transition to the embryonic or the adult condition of the lower air-breathing Vertebrates.

In Neoceratodus (Fig. 191) the renal portal system is unusually complex, the veins distributing venous blood to the kidneys being derived from several sources, as follows: (1) from each of the two branches into which the caudal vein divides on its exit from the haemal canal (af.r.v); (2) from a common trunk (pt.v) which, on each side, is formed by the union of segmental veins from certain of the post-cloacal myotomes and is united with its fellow by a transverse anastomosis; (3) from more anteriorly situated intercostal or segmental veins (i.c.v) which enter each kidney directly; and (4) from a vein on each side corresponding to the renal portal vein of Amphibia. The latter vein (rp.v) is formed by one of the two branches of the iliac or femoral vein, and joins {324}the corresponding vein from the caudal myotomes; from the common trunk numerous branches enter the kidney.

In the derivation of renal portal veins from each of the two veins into which the caudal vein divides, Neoceratodus approaches the Elasmobranchs. On the other hand, the utilisation of ordinary segmental veins from the caudal and pre-caudal myotomes, some of which directly enter the kidney, is a feature which has already been remarked in some Teleosts; while the formation of a renal portal affluent by a branch of the femoral vein is an even more striking Amphibian characteristic.

The efferent renal veins join the root of the left posterior cardinal and the adjacent portion of the caudal vein.

Of the two great venous trunks into which the caudal vein divides, the right is much the larger and behaves somewhat differently to the left. The former (i.v.c) passes forwards in relation with the right kidney, receiving in its course the spermatic or ovarian veins from the gonad of its side, and then traverses the liver, finally opening into the median portion of the sinus venosus, between the orifices of the two hepatic veins. The left branch of the caudal vein (l.p.c) also passes forwards in relation with the left {325}kidney and receives veins from the corresponding gonad; but, instead of traversing the liver, it passes above that organ, and finally opens into the left Cuvierian duct. The course of the left vein, and the relations of the vessel to the caudal vein and the left Cuvierian duct, point to the conclusion that it represents the left posterior cardinal of other Fishes. From its continuity with the caudal vein it is also obvious that the hinder or renal portion of the right trunk is a remnant of the right posterior cardinal; but the more anterior section so closely resembles the postcaval vein, or inferior vena cava of the higher Vertebrates, in its relations to the liver, the hepatic veins, and the sinus venosus, that its identity as such seems beyond doubt, and this interpretation is supported by well-known observations on the mode of origin of the inferior vena cava in Amphibia, and especially the union of the independently formed inferior vena cava with the posterior or inter-renal portion of the embryonic right posterior cardinal vein, combined with the atrophy of the anterior portion of the latter vein. The singular connexions and relations of these two great veins afford an additional illustration of the significant transitional condition of the venous system in the Dipnoi. On the other hand, the direct continuity of the caudal vein with vessels which, wholly or in part, represent the two posterior cardinals, is a feature alike characteristic of the adult Cyclostome and the embryonic Elasmobranch, Teleost, and Amphibian.

As in the Cyclostomes and Elasmobranchs, the precaudal section of the embryonic subintestinal vein is represented in the adult by an intra-intestinal vein which traverses the spiral valve near its free edge and is a tributary of the hepatic portal vein.

The two veins from the undivided air-bladder unite to form a single vessel, which, instead of joining the hepatic portal or posterior cardinal veins as in other Fishes, opens into the left auricle, like the pulmonary veins of the Amphibia.

A further resemblance to the Amphibia is to be found in the presence of an anterior abdominal vein. After leaving the pelvic {326}limb each femoral vein divides into two branches; one of these forms a renal portal vein as previously described; the other, which may rightly be termed a pelvic vein (pv.v), unites with its fellow to form a median anterior abdominal vein (a.ab). Pursuing its course forwards in the ventral abdominal wall, the vein eventually reaches the heart and opens into the sinus venosus. The direct connexion of the anterior abdominal vein with the heart is yet another example of the retention in the adult Neoceratodus of a transitory embryonic feature in the developing Amphibian.

As in other Fishes, the blood from the head is conveyed to the Cuvierian ducts by an anterior cardinal and an inferior jugular on each side. There are no lateral veins, the blood from the pelvic fins flowing into the renal portal system or into the anterior abdominal vein, and that from the pectoral fin through subscapular and brachial veins into the Cuvierian ducts. Lateral cutaneous veins are, however, present; and, as in Elasmobranchs (e.g. Mustelus antarcticus), anastomose anteriorly with the subscapular vein and behind with the caudal vein.

{327}Less is known of the venous system of Protopterus, but it is certain, nevertheless, that it presents a more advanced grade of evolution than in Neoceratodus, and, except for the doubt as to the existence of an anterior abdominal vein, it is essentially similar to that of a Urodele Amphibian in which the right posterior cardinal vein has aborted.

The caudal vein (Fig. 192) divides into right and left renal portal branches, neither of which, however, is directly continuous with the inferior vena cava or the left posterior cardinal; on the contrary, each renal portal vein is joined by the corresponding iliac or femoral vein, and also by numerous segmental veins, and then distributes the whole of its venous blood to the kidney. The radicles of the inferior vena cava and the left posterior cardinal are formed by the renal veins from the two kidneys, and in their forward course to the heart both veins receive in addition genital and segmental veins. In its course through the liver the inferior vena cava receives several hepatic veins, and finally opens into the sinus venosus, while the left posterior cardinal vein joins the corresponding Cuvierian duct, which also receives anterior cardinal, inferior jugular, and subclavian veins. There is an intra-intestinal vein as in Neoceratodus, but an anterior abdominal vein has yet to be discovered. The two pulmonary veins from the double air-bladder form a single trunk before communicating with the left auricle.

With the exception of certain doubtful details which need further investigation, the venous system of Lepidosiren seems to resemble that of Protopterus.

THE HEART.—The heart is more anteriorly placed than in other Vertebrates, being situated directly behind and beneath the last pair of branchial clefts and internal to the ventral portion of the pectoral girdle. The organ is enclosed in a pericardial cavity, which, in the adult, is separated from the abdominal portion of the coelom by a transverse pericardio-peritoneal septum, and in the Lamprey (Petromyzon) is partially enclosed within a cartilaginous, cup-like modification of the hinder part of the branchial basket. In the Ammocoetes-stage of the Lamprey the pericardium is in communication behind with the general coelom, but the connexion is lost in the adult. In Elasmobranchs the {328}two cavities are connected by a single pericardio-peritoneal canal, or by two such canals; and in Chimaera, and in the Sturgeon (Acipenser) and Polyodon, by a single canal.

The heart consists of at least three chambers, a sinus venosus which receives the venous blood from the body, an auricle and a ventricle, to which is added a conus arteriosus in the Elasmobranchs, certain Teleostomi (Crossopterygii, Chondrostei, and Holostei), and in the Dipnoi. Through these cardiac chambers the blood is forced in the order mentioned. In the Dipnoi the auricle is subdivided by a more or less complete interauricular septum into a right and left auricle, the former receiving the venous blood from the sinus venosus, and the latter the aerated blood from the lung-like air-bladder.

The sinus venosus and the auricle have very thin walls; the ventricular walls, on the contrary, are very thick and in great measure are composed of a sponge-like network of muscular bundles which generally encroaches considerably on the ventricular cavity. Membranous valves, the sinu-auricular, and the auriculo-ventricular valves, are developed at the junctions of the sinus venosus with the auricle, and the auricle with the ventricle respectively. The conus arteriosus is muscular and contractile, and is interposed between the ventricle and the root of the ventral aorta. Internally, the conus is provided with several transverse rows of pocket-shaped or semilunar valves. In Teleosts the conus is non-muscular and vestigial, and has but a single row of valves, corresponding to the most anterior of the multiple rows of valves in the Elasmobranchs. In these {329}Fishes the vestigial conus is succeeded by a non-contractile, bulb-like dilatation, or bulbus aortae, of the root of the ventral aorta. In only a single Teleost, viz. Albula, one of the Albulidae, is the vestigial conus muscular, and at the same time provided with two rows of valves. In the Cyclostomata there is a bulbus with a single row of two valves, but no true conus.

In the Dipnoi (e.g. Protopterus) the heart, like the rest of the vascular system, exhibits certain interesting resemblances to the Amphibian heart. In addition to a more or less complete interauricular septum separating right and left auricles, there is a median longitudinal ridge, partly muscular and partly fibrous, which incompletely subdivides the cavity of the ventricle. The spirally-twisted conus arteriosus is furnished with several transverse rows of valves, certain of which coalesce longitudinally to form a complete septum dividing the cavity of the conus into two distinct lateral channels: with this septum there coalesces another septum, which cuts off the origins of the anterior two pairs from the remaining afferent branchial arteries. The formation of these septa has the physiological effect of subdividing the series of cardiac cavities into two parallel channels, of which one has its origin behind in the sinus venosus and transmits venous blood to the posterior afferent branchial vessels; while the other, commencing with the left auricle, conveys arterial blood to the first two pairs of afferent branchial arteries. In Neoceratodus, however, the longitudinal septum in the conus is incomplete, and hence the blood which is sent to the anterior afferent vessels is mixed.

THE ARTERIAL SYSTEM.—The ventral aorta is a median artery situated beneath the floor of the pharynx, and having its origin, behind, either directly from the ventricle or from the conus arteriosus.

In the Cyclostomata (e.g. Petromyzon) the ventral aorta (Fig. 194) is continued forwards from the heart as a single vessel to the fourth pair of gill-sacs, where it divides into right and left branches which extend as far as the anterior walls of the first pair of gill-sacs. Eight pairs of afferent branchial arteries arise from the ventral aorta and its two branches, of {330}which the first and last supply the anterior walls of the first pair of sacs and the posterior walls of the last pair respectively. Each of the remaining afferent vessels extends into an interbranchial septum, and supplies the gill-lamellae of the posterior wall of one sac and those of the anterior wall of the next sac behind. The corresponding efferent branchial vessels have a similar distribution, and unite dorsally to form a median dorsal aorta. Beneath the base of the skull the latter vessel divides into two branches which, after receiving the first pair of efferent branchial vessels, pursue a divergent course forwards, but subsequently converge and unite to form a "circulus cephalicus," as in Teleosts. From the cephalic circle are given off on each side (1) an "internal carotid" artery for the brain and eye; (2) an "external carotid" for the lateral and ventral walls of the head; and (3) a large ventral branch which supplies the lingual apparatus; while from the abdominal portion of the dorsal aorta are derived, first, a coeliaco-mesenteric artery for the liver and alimentary canal, and subsequently branches for the myotomes, kidneys, and the gonad. The terminal portion of the aorta then enters the tail and forms the caudal artery.

In Elasmobranchs (e.g. Mustelus antarcticus) the undivided ventral aorta gives off five pairs of afferent branchial arteries which, on each side, ascend in succession the outer convex sides of the hyoid and first four branchial arches (Fig. 195).

{331}[Illustration: FIG. 195.—The branchial arterial system of Mustelus antarcticus. Left lateral view. The ventral aorta and afferent branchial vessels are in solid black, the efferent arteries and their branches have double contours. The branchial clefts have fringed borders to indicate their hemibranchs, and the arches are in simple outline. a.c.a, Anterior carotid; a.d.a, anterior dorsal aorta; af.b.a, afferent branchial artery; br.a, brachial artery; c.m.a, coeliaco-mesenteric; d.a, dorsal aorta; E, eye; ep.a, epibranchial artery; H, heart; h.b.a, hypobranchial artery; hy.a, afferent pseudobranchial or hyoidean artery; md.a, mandibular artery; op.a, ophthalmic artery; p.c.a, posterior carotid; sb.a, subclavian; sp, spiracle; v.a, ventral aorta; 1-5, the hyobranchial and four succeeding branchial clefts. The hypobranchial artery is seen immediately beneath the ventral aorta. (After T. Jeffery Parker, diagrammatic.)]

The first or most anterior of these arteries supplies the hyoidean hemibranch, while the succeeding four supply the holobranchs of the four branchial arches. The blood is collected from the capillaries of the branchial lamellae by a series of efferent branchial vessels, a pair for the two hemibranchs of each branchial arch and a single vessel for the hyoidean hemibranch, which unite with one another in a somewhat singular fashion. The efferent arteries from the anterior and posterior hemibranchs of each branchial cleft unite above and below each cleft in such a way as to form a series of complete vascular loops round the hyoidean cleft and the three succeeding branchial clefts, which are connected by short longitudinal trunks in each arch and also by a longitudinal commissural vessel between their ventral extremities. As the fifth arch is gill-less, there is no complete loop round the fifth cleft, the blood collected by the efferent vessel of the posterior hemibranch of the fourth arch being conveyed to the corresponding vessel of the anterior hemibranch of the same arch by one of the short longitudinal vessels above mentioned. Dorsally, each arterial loop is continuous with an epibranchial artery; and by the dorsal union of the four {332}epibranchial arteries of the two sides the dorsal aorta is formed. It may be pointed out that the anterior efferent vessel of each arch, which is usually larger than the posterior one, is to be regarded as the primary efferent artery of the corresponding holobranch, and as such is directly continuous with an epibranchial artery, the posterior efferent artery being a secondary vessel which opens not into the primary trunk of its own branchial arch, but into that of the succeeding arch. The principal arteries which supply the various parts of the head with blood are derived from the first efferent branchial vessel. From the ventral end of this artery a mandibular artery is given off, which subdivides into branches for the muscles of the lower jaw as well as into nutrient vessels for the hyoidean hemibranch. At about the middle of its length the same artery gives off an afferent pseudobranchial or hyoidean artery, to the spiracular or mandibular pseudobranch. From the latter organ the blood is collected by an anterior carotid artery which, after giving off an ophthalmic branch to the eye, perforates the orbital wall and enters the cranial cavity, where it is joined by an anastomotic trunk from the posterior carotid of the opposite side; finally, the anterior carotid divides into anterior and posterior cerebral arteries for the brain. The third and last of the cephalic arteries is the posterior carotid; this artery arises from the dorsal extremity of the first efferent branchial vessel, and, on entering the orbit, gives off the anastomotic trunk previously mentioned. The latter vessel enters the cranial cavity, and, after crossing its fellow, joins the anterior carotid of the opposite side, as described above. The main trunk is then continued forwards in the orbit, and its various branches eventually supply the eye-muscles, the mandibular adductor muscle, and some other parts of the head.

It is worthy of note that the median dorsal aorta is prolonged forwards in front of the first pair of epibranchial arteries as a slender median vessel (a.d.a), which ultimately divides into two branches, each branch uniting with the posterior carotid of its side.

A remarkable system of arteries for the supply of nutrient blood to the gills and heart has its origin in the following {333}manner. On each side, the longitudinal commissural vessel, which connects the ventral ends of the arterial loops surrounding the different gill-clefts, gives origin to a series of pairs of short transverse vessels, and by their union these combine to form a median longitudinal hypobranchial artery which lies beneath the ventral aorta. From the hypobranchial artery are derived the coronary arteries for the heart; and from the same artery, or from its lateral connexions with the longitudinal commissural artery, and, in the case of the hyoidean hemibranch, from the mandibular artery, are derived the various nutrient vessels for the gills.

The arteries for the trunk, and for the pectoral and pelvic limbs, arise in succession from the dorsal aorta. The first of the series is the subclavian artery, which has its origin from the aorta close to the dorsal extremities of the fourth pair of epibranchial arteries. Each subclavian artery gives off a brachial artery to the pectoral fin, and is then continued forwards as a lateral hypobranchial artery, which, with its fellow of the opposite side, eventually becomes continuous with the hinder end of the median hypobranchial artery. Behind the subclavian artery there is a median coeliaco-mesenteric artery, the various branches of which are distributed to the liver, stomach, and intestine. A lieno-gastric artery supplies the pancreas and spleen, and also sends branches to the stomach. In addition, there are also arteries for the gonads, numerous segmental arteries for the myotomes, and renal arteries for the kidneys. Finally, the aorta gives off a pair of iliac arteries for the pelvic fins, and then enters the haemal canal as the caudal artery.

The more important differences in the arterial system of the Holocephali and the Teleostomi relate to (1) the absence of the posterior efferent branchial artery in each branchial arch; (2) modifications dependent on the condition of the spiracular and hyoidean hemibranchs, and the mode of origin and the course of their afferent and efferent vessels; and (3) the source from whence the air-bladder derives its blood when that organ is present.

(1) The branchial arterial system is somewhat more primitive than in the generality of Elasmobranchs. There are no complete vascular loops round the gill-clefts, and the blood from the two {334}hemibranchs of each branchial arch is conveyed to the dorsal aorta by a single efferent vessel which corresponds to the more anterior of the two in Mustelus antarcticus.

(2) In Callorhynchus among the Holocephali, where the spiracle is absent but the hyoidean hemibranchi is still a true gill, the latter organ is supplied with venous blood by a branch from the ventral aorta, the corresponding efferent vessel joining the dorsal aorta (Fig. 196). In the absence of a spiracular pseudobranch the anterior carotid may be regarded as continuous with the hyoidean artery, and as having its origin directly from the efferent artery of the hyoidean hemibranch (Fig. 196). At its origin the anterior carotid anastomoses with the mandibular artery.

The Sturgeon more closely resembles the Elasmobranchs. The hyoidean gill is supplied by an afferent branchial artery from the ventral aorta, and its efferent vessel joins the corresponding trunk from the holobranch of the first branchial arch. A hyoidean artery supplies the spiracular pseudobranch, the efferent vessel of which contributes to the blood-supply of the brain and the eye, and probably represents an anterior carotid.

Lepidosteus offers a singularly interesting transition from the {335}Elasmobranch to the Teleost. As indicated in the preceding chapter, this Fish possesses both a hyoidean gill and a spiracular pseudobranch (Figs. 197 and 198). The hyoidean gill is supplied by an afferent artery direct from the ventral aorta, but the proper efferent vessel of the gill, which primitively joined the dorsal aorta, is suppressed, and the blood is collected into a vessel, which, like the hyoidean artery in Elasmobranchs, becomes the afferent artery of the spiracular pseudobranch. The latter artery unites, however, with a second hyoidean artery derived from the efferent branchial vessel of the first branchial arch, and represents the artery termed "hyoidean" in Teleosts. The efferent vessel from the spiracular pseudobranch joins an internal branch from the carotid artery, and then distributes its blood both to the eye and the brain.

In Teleosts, as already mentioned in a preceding chapter, it is probable that the hyoidean hemibranch is suppressed, the so-called hyoidean pseudobranch being a spiracular pseudobranch. The latter is now supplied by a "hyoidean" artery, which has its origin from the ventral end of the efferent {336}branchial artery of the first branchial arch, the corresponding efferent trunk forming an ophthalmic artery, and passing to the choroid gland of the eye (Fig. 199). Both the proper afferent and efferent arteries of the hyoidean hemibranch either disappear or, as in the Cod (Gadus morrhua), the efferent artery may be represented on each side by an anastomosis between the hyoidean artery and the cephalic circle. Hence, the "hyoidean" artery of Teleosts corresponds to the one which has a similar origin in Lepidosteus.

A brief description of the remaining efferent branchial arteries and their derivatives in the Cod (Gadus morrhua) will illustrate the condition of these structures in a well-known Teleost.

In this Fish the efferent branchial vessels open dorsally into right and left suprabranchial arteries, which unite behind to form a median dorsal aorta (Fig. 199). Anteriorly, the paired suprabranchial arteries extend towards the base of the skull as the so-called "carotid" arteries. The two carotids enter the cranial cavity, and there unite in the median line, as in the Cyclostomes. By the union of these arteries in front, and of the {337}right and left suprabranchial arteries behind, the characteristic "circulus cephalicus" of Teleosts is completed. From the anterior part of the cephalic circle are derived two internal carotid arteries for the brain, and also a pair of orbito-nasal arteries for the eye-muscles and the nasal sacs, while more posteriorly an external carotid has its origin from each suprabranchial artery.

(3) In most Teleostomi the air-bladder is supplied with blood by branches of the coeliac artery, with the addition of small branches arising directly from the dorsal aorta. Polypterus and Amia are, however, exceptional, inasmuch as the arteries for the air-bladder are derived from the last or fourth pair of efferent branchial vessels, and in this respect, but not in the destination {338}of the corresponding veins, the two genera exhibit a significant resemblance to the Dipnoi.

In the Dipnoi the ventral aorta is so short that the afferent branchial arteries arise almost directly from the conus arteriosus with their roots in close contiguity to one another (Fig. 200).

In Neoceratodus (Fig. 200), there are two efferent vessels to each gill-bearing branchial arch, which unite above to form an epibranchial artery, and by the successive union of the four epibranchial arteries a short common trunk is formed on each side. Posteriorly, the two trunks unite to form a median dorsal aorta. Immediately above the gill-clefts each efferent vessel gives off a branch which, passing either forwards or backwards, unites with the corresponding branch of the efferent vessel in front or behind as the case may be. A hyoidean artery arises from the ventral extremity of the anterior efferent artery of the first branchial arch, and, after giving off a lingual artery, ascends the hyoid arch and supplies the hyoidean pseudobranch. The efferent vessel of the pseudobranch (a.c.a) or anterior {339}carotid artery, eventually enters the cranial cavity and subdivides into anterior and posterior cerebral arteries for the brain, also giving off a branch which unites with its fellow of the opposite side directly behind the infundibulum. A posterior carotid springs from the epibranchial of the first branchial arch and divides into palatine, orbital, and ocular branches; and from the ventral end of the anterior efferent vessel of the second branchial arch is derived a hypobranchial artery for the heart and pericardium. The pulmonary arteries for the lung-like air-bladder have their origin from the fourth pair of epibranchial arteries.

As in so many other details of its anatomy, Neoceratodus exhibits in its arterial system abundant evidence of the wide-spreading affinities of the group to which it belongs. In its branchial arterial system Neoceratodus presents a singular combination of features which, individually, are characteristic of Amphibia and Elasmobranchs. Special Amphibian features may be noted in the origin of the afferent branchial arteries almost simultaneously from the anterior end of the conus arteriosus; in the mode of union of the epibranchial arteries to form the dorsal aortae; in the origin of a lingual artery from the efferent vessel of the first branchial arch; and in the derivation on either side of a pulmonary artery from the fourth epibranchial artery. Agreement with Elasmobranchs is to be found in the presence of two efferent branchial vessels in each branchial arch, although the relations of these arteries are more primitive than in most adult Elasmobranchs, inasmuch as the two efferent vessels of the same arch unite to form an epibranchial artery; and also in the origin and distribution of the anterior and posterior carotids. Lastly may be mentioned the fact that Neoceratodus agrees not only with the Amphibia but also with those generalised Teleostomi, Polypterus and Amia, in the mode of origin of the great arteries for the air-bladder.

Of the two remaining Dipnoi, the arterial system of Protopterus is better known than that of Lepidosiren, but in both cases further research is needed before a satisfactory comparison can be made with Neoceratodus and other Vertebrates. It is evident, nevertheless, that both genera differ from Neoceratodus in approximating more closely to the Amphibia than to the {340}lower Fishes, in so far as the branchial part of the arterial system is concerned.

In their origin from the conus the four afferent branchial arteries of Protopterus resemble those of Neoceratodus, but their relations to the branchial clefts are somewhat different (Fig. 201). The first or hyoidean cleft is closed, and the first afferent vessel lies between the second cleft and the third, and is therefore in relation with the second branchial arch. The remaining afferent arteries are disposed between the succeeding clefts and are related to the corresponding arches. As the second and third arches, like the vestigial first arch, bear no gill-lamellae, their afferent arteries are directly continuous with the corresponding efferent vessels, as in those Teleosts in which certain arches are gill-less, as well as in the Tadpole-stage of the tailless Amphibia when the internal gills begin to degenerate; and they apparently transmit arterial blood directly to the dorsal aorta. The third and fourth afferent arteries, on the contrary, supply venous blood to the two hemibranchs which are borne by each of the two corresponding arches, viz.: the fourth and fifth, and from each pair of hemibranchs the blood is collected into two efferent vessels which unite dorsally {341}to form an epibranchial artery. From the dorsal end of the fourth afferent artery there arises a recurrent branch which curves round the upper margin of the sixth cleft and supplies the gill-lamellae on the posterior margin of that cleft, a fact which lends support to the view that these lamellae are "emigrants" from the anterior margin of the cleft; the efferent vessel from the "emigrant" lamellae joins the fourth epibranchial artery. The blood-supply of the external or cutaneous gills is derived from the dorsal extremities of the second, third, and fourth afferent arteries, while the efferent vessels from these organs join the corresponding epibranchial arteries; in this respect there is a close resemblance between Protopterus and those larval Amphibians which possess similar cutaneous gills. All four epibranchial arteries unite together at about the same point to form a short common trunk, the right or left dorsal aorta, which subsequently unites with its fellow to form the median dorsal aorta.

There is a so-called "hyoidean" artery, which, however, has its origin, not from an anterior efferent branchial vessel as in Neoceratodus, but from the first afferent branchial artery. After giving off a submaxillary or lingual artery, the "hyoidean" artery (af) becomes the afferent vessel for the "opercular gill" or "hyoidean pseudobranch," and supplies the latter with arterial blood. The efferent vessel (ef) from the pseudobranch unites with the four epibranchial arteries in forming the right or left dorsal aorta. A "carotid" artery arises from the efferent vessel of the "hyoidean pseudobranch," and a pulmonary artery has its origin from the root of the dorsal aorta of its side, and not from the fourth epibranchial artery as in Neoceratodus.

THE BLOOD.—The blood consists of a nutritive fluid plasma in which float red corpuscles and leucocytes. In the Cyclostomata (e.g. Petromyzon) the red corpuscles are circular, but in Myxine they have the usual oval shape. In Fishes the red corpuscles are almost invariably flat, oval, biconvex, and nucleated, and owe their colour to the presence of the characteristic oxygen-absorbing, iron-containing pigment, haemoglobin. They are unusually large in the Dipnoi and are only exceeded in size by those of certain Urodele Amphibians. The leucocytes are much less numerous than the red corpuscles, although their relative proportions are very variable, even in the same species under different {342}conditions. They appear to be more numerous in the Dipnoi (e.g. Protopterus) than in any other Vertebrates, except under pathological conditions.

THE LYMPHATIC SYSTEM.—In addition to blood-vessels, Fishes possess a lymphatic system, consisting of smaller vessels, lymph-capillaries or lymph-spaces, distributed in the connective tissue of different parts of the body, and by their union ultimately forming larger lymph-vessels or sinuses which communicate with certain of the principal veins, the whole forming a series of channels for the collection of the blood-plasma which has exuded from the blood-capillaries for the nutrition of the tissues, and for its conveyance to the general venous system. The fluid in the lymphatics, or lymph, consists of dilute blood-plasma containing leucocytes but devoid of red corpuscles. At the points where the larger lymphatics open into the veins, lymph-hearts may be developed. In the Eel (Anguilla vulgaris) there is a lymph-heart in the tail, which communicates by a valvular orifice with the smaller of the two caudal veins, and by its rhythmical pulsations propels the lymph into the vein. In Silurus there are two caudal lymph-hearts. Apart from the lymphoid tissue, which is so abundantly present in certain parts of the body, Fishes appear to be devoid of the special "lymphatic glands" of the higher Vertebrates.

THE DUCTLESS OR BLOOD-GLANDS.—All the important blood-glands of other Vertebrates have their representatives in Fishes. Nothing is certainly known of the function of these organs in Fishes, but from the general structural resemblance which they present to their equivalents in the higher Vertebrates, it is perhaps not unreasonable to infer that they are similar in function. If this be so, the blood-glands of Fishes are organs for leucocyte-formation and phagocytosis, involving the destruction and removal of effete red blood-corpuscles; in addition, they may also be concerned with certain obscure chemical changes in the composition of the blood, which have an important relation to general or local nutrition.

THE SPLEEN.—This lymphoid organ is the largest of all the blood-glands, and, in the form of a compact or more or less lobulated body, is present in all Fishes, and possibly in Cyclostomes. In position the spleen is usually in close proximity to the stomach, to which it is attached by an extension round it {343}of the peritoneal investment of that organ. Thus, in the Dog-Fish (Scyllium), the spleen is a large reddish body attached to the convexity of the U-shaped stomach, and, in addition, sends a long narrow lobe between the distal limb and the valvate portion of the intestine (Fig. 153, spl). In the Sturgeon (Acipenser), the organ is also large, but is attached to the left side of the commencement of the intestine. In the Cod (Gadus) among Teleosts the spleen is much elongated and is situated on the dorsal side of the stomach. In the Dipnoi (e.g. Protopterus) the organ is probably represented by a large compact lymphoid mass, closely connected with the dorsal and lateral walls of the stomach (Fig. 154, A, s).

THE THYROID GLAND.—This organ usually arises in the form of a small median evagination of the hypoblastic epithelium of the ventral wall of the pharynx, in the region of the second visceral arch. Later it becomes detached from the place of origin and converted into a solid spherical body. Eventually the component cells form the limiting epithelium of a series of follicles or vesicles embedded in a matrix of connective tissue and blood-vessels, and the characteristic adult structure is attained.

Among the Cyclostomata the evagination is relatively large in the young Lamprey (Petromyzon fluviatilis), as also is the orifice of communication with the pharynx (Fig. 202, th). The aperture soon becomes reduced to a mere pore, and finally disappears. During the larval or Ammocoetes-stage the organ consists of a median ciliated portion, communicating with a pair of laterally placed glandular sacs, but in the adult it is much smaller, and acquires the usual follicular structure. In adult Elasmobranchs the thyroid is represented by a moderately large compact organ, situated near the anterior end of the ventral aorta. In Teleostomi the organ may be paired, or, as in the Perch (Perca), more diffuse, consisting of masses of reddish lobules lying beneath the aorta, and also scattered for a variable distance along the course of the afferent branchial arteries.

In the Dipnoi (e.g. Protopterus) the thyroid is small, {344}consisting of two lateral lobes connected by a constricted median portion, and situated beneath the epithelium of the tongue, immediately above the hyoidean symphysis. A similar structure has been described by Bischoff in Lepidosiren, and was regarded by him as a salivary gland.

As in Reptiles, Birds, and Mammals, paired or accessory thyroid bodies ("supra-pericardial organs") are present in many Fishes, and appear to be similar in structure to the median thyroid. In Elasmobranchs these bodies originate as a pair of outgrowths from the epithelium of the pharynx behind the last pair of branchial arches (Fig. 202, B, a.th). Subsequently they become detached from the pharynx, and in the adult are situated on the dorsal side of the pericardium, remote from the median thyroid.

According to Dohrn the median thyroid is to be regarded as the vestige of a gill-cleft which primitively existed between the hyomandibular cartilage and the hyoidean arch. This conclusion seems, however, to be less in harmony with the facts of development than the view that the organ is derived from the characteristic hypobranchial groove or "endostyle" of Ascidians {345}and Amphioxus, which has undergone a change of function from a mucus-conveying groove to a blood-gland. On the other hand, the mode of origin of the paired thyroids certainly favours the suggestion that they represent a posterior pair of vestigial gill-clefts, a view which derives some support from the fact that in Notidanus, where additional branchial arches and clefts are present, the paired thyroids are absent.

THE THYMUS.—In the embryo Elasmobranch and Teleost the thymus has a multiple origin, being derived from a series of distinct epithelial thickenings, one of which is developed at the dorsal extremity of each of the gill-clefts except of the spiracle. These rudiments subsequently detach themselves from the epithelial surface and sink inwards, eventually fusing together on each side to form a single independent structure. Later, the epithelial mass thus formed becomes invaded by connective tissue, and by leucocytes which form lymph follicles, and the thymus gradually assumes the structure of a lymphoid organ. From its mode of development it has been suggested that the thymus owes its evolution to the metamorphosis and ingrowth of branchial filaments, but it is also noteworthy that each embryonic rudiment of the organ closely resembles, both in position and origin, one of the developing branchial tongue-bars of Amphioxus. The abundance of leucocytes which it contains has also prompted the further suggestion that the origin of the thymus may be due to the necessity of providing for the phagocytic protection of the gills themselves from the ravages of harmful micro-organisms, fungoid spores, etc., as well as to aid in the removal of such portions of the gills as may have been injured.

A thymus is probably present in all Fishes, if not in the adult at all events in the embryo, but is always relatively small in size. In Elasmobranchs the organ lies on each side above the branchial arches and beneath the dorsal musculature; and in Teleostomi at the dorsal extremity of the last branchial arch, in close proximity to the mucous membrane of the branchial cavity. In a similar position in the Dipnoi (e.g. Protopterus) there are, on each side, {346}two contiguous lobes of lymphoid tissue which apparently represent a thymus.

THE SUPRA-RENAL BODIES.—The supra-renal bodies are organs of problematic function, which are present in the Cyclostomata, and probably in all Fishes, and situated in close proximity to the kidneys.

In the Cyclostomata (Petromyzon) these bodies are represented by lobules of cells along the posterior cardinal veins, and also by masses of peculiar cells ("chromaffin cells") along the sides of the aorta and segmental arteries. In Elasmobranchs there are two distinct structures, the paired supra-renals and the inter-renals (Fig. 203, A). The former are a series of pairs of segmentally arranged bodies, situated on the successive pairs of segmental arteries given off from the dorsal aorta. The two bodies which form the first pair are much larger than any of the others, and were formerly spoken of as "axillary hearts." The inter-renal is usually a thin elongated "ochre-yellow" body, from which one or two lobes may be detached in front, and extends for a variable distance in {347}the median line between the two kidneys, or is unsymmetrically placed on the ventral surface of either kidney. Sometimes (e.g. in Raia) the inter-renals are paired, in which case they are applied to the inner and hinder margins of the kidneys. In the Sturgeon (Acipenser sturio) the "supra-renals" appear as numerous "ochre-yellow" bodies, variable in size and distribution (Fig. 203, B). Some of them are visible on the surface of the kidneys, while others are scattered about in their substance, but on the whole are more anteriorly placed than in Teleosts. In the latter group the "supra-renals" are usually two in number (Fig. 203, C), but may be as many as five or reduced to one. They are disposed either on the ventral or the dorsal surface of the kidneys, generally near their hinder extremities, or more or less deeply embedded in their substance. Besides these bodies there are also chromaffin cells in the walls of the anterior cardinal veins.

Histologically, the paired segmentally arranged bodies of Elasmobranchs differ considerably in structure from the inter-renal bodies, the former resembling the "medulla," while the inter-renals, as well as the so-called supra-renals of Acipenser, exhibit a striking resemblance to the alveolar "cortical" substance of the Mammalian supra-renals. In Cyclostomes the cortex is apparently represented by the lobules of cells along the posterior cardinal veins and the medulla by the "chromaffin" cells, while in Teleosts the cortex and the medulla have their respective counterparts in the supra-renals and the "chromaffin" cells in the walls of the anterior cardinal veins. It may be concluded, therefore, that Elasmobranchs, Cyclostomes, and Teleosts possess anatomically distinct representatives of both the "medulla" and "cortex" of Mammalia, although the Sturgeon is at present only known to possess the equivalent of the "cortex." In Amphibia, Reptilia, and Aves both "cortex" and "medulla" are present, and in the varying intimacy of their relations offer a transition to the Mammalian arrangement of a central medulla closely invested by a sheath of cortical substance. A more or less intimate connexion exists between the paired supra-renals of Elasmobranchs and the sympathetic nervous {348}system. The former are usually well supplied with sympathetic nerve fibres, and contain ganglion-cells in their substance.

The primitive origin of these organs is very obscure, and as regards their development there is much diversity of opinion. It seems certain, however, that the cortex and medulla of the higher Vertebrates, including their equivalents in the Elasmobranchs, have independent origins, and the balance of opinion seems to point to the derivation of the cortex from some portion of the germinal coelomic epithelium, while the medulla is derived from the embryonic nerve cells of the sympathetic ganglia.

LYMPHOID TISSUE.—In addition to certain of the ductless glands, and the local or diffused masses of their characteristic tissue already mentioned in connexion with the alimentary canal, lymphoid tissue is often abundantly present in other parts of the body. There is, for example, a mass of this tissue on the heart of the Sturgeon (Acipenser). The anterior enlarged portion of the mesonephros, commonly termed the "head-kidney" of the Teleostomi (Fig. 203, B, C), is almost entirely composed of lymphoid tissue, which has replaced, wholly or partially, the proper renal structure; and from the presence of free red blood-corpuscles and of crystals of oxy-haemoglobin and other derivatives of haemoglobin, it may be inferred that the "head-kidney," in common with the more orthodox blood-glands, performs a blood-destroying function. On the other hand, the example of the spleen, which is alike the seat of leucocyte-formation and of blood-destruction, renders it unnecessary to reject the view that the "head-kidney" is an organ in which leucocytes or blood-corpuscles are formed. In but few Teleostomi is a purely lymphoid "head-kidney" entirely wanting, as, for example, in the Sun-Fish (Orthagoriscus mola). As previously mentioned the Dipnoi are remarkable for the extraordinary development of lymphoid tissue, inasmuch as it forms a thick investing mass round the kidneys and gonads in addition to its exceptional abundance in the walls of the alimentary canal.

The absence of ordinary lymphatic glands in Fishes is well known, and it is at least probable that, functionally, the want of these lymphoid organs may be compensated for by the superabundance of lymphoid tissue in other parts of the body.

{349}CHAPTER XIII

MUSCULAR SYSTEM—LOCOMOTION—SOUND-PRODUCING ORGANS—ELECTRIC ORGANS

MUSCULAR SYSTEM.—The various muscles of the body may be arranged in two systems: (i.) the somatic or parietal, composed of striated or voluntary muscle-fibres; and (ii.) the splanchnic or visceral, consisting for the most part of unstriated or involuntary fibres. Somatic muscles form the great lateral longitudinal muscles of the trunk and tail, which retain the primitive embryonic metamerism to a greater extent in Fishes than in any other Vertebrates, and are the principal muscles associated with locomotion. The lateral muscles are composed of a series of transverse muscle-segments or myotomes, which are >-shaped, or S-shaped, or they even take a zigzag course from above downward. The myotomes are disposed in pairs, and they are separated from one another by fibrous septa or myocommata. Each myotome is divided into a dorsal or epiaxial portion, and a ventral or hypaxial portion, by a longitudinal, horizontal, fibrous septum extending outwards from the vertebral centra to the skin. The muscles of the pectoral and pelvic fins are derivatives from more or fewer of the adjacent myotomes. The splanchnic muscles include the musculature of the walls of the alimentary canal, as well as those specialised portions of the visceral system which are represented by the muscles of the branchial arches and the jaws, and are composed of striated fibres.

LOCOMOTION.—A Fish and a Bird are equally remarkable for the many and various ways in which they are adapted for locomotion in the particular medium in which they live. In its shape the Fish is admirably adapted for cleaving the water. Spindle-like in shape, but thicker in front than behind, a Fish resembles {350}a double wedge, the thick part of which is represented by the head and one of the thin edges by the free hinder margin of the caudal fin. The body is bounded by smooth flowing contour lines, unbroken by any sharp separation of the body regions from one another, and with no points of resistance to its forward motion through the water. The body being thicker in front than behind, and, as seen in transverse section, broader above than below, it follows that its centre of gravity will be nearer the head than the tail, and nearer the dorsal than the ventral surface. The dorsal position of the centre of gravity necessarily renders the equilibrium of the body unstable, and were it not for the balancing action of the paired fins the Fish would float belly upwards, as is always the case after death. Most Fishes are provided with a membranous gas-containing sac, the air-bladder, the principal function of which is to render the Fish, bulk for bulk, of the same weight as the water, so that in this position of equilibrium, or plane of least effort, the animal can execute its various locomotor movements with a minimum expenditure of muscular effort—an advantage which no other animal possesses. To give stability to the body, and to steady its course when swimming, the Fish has a dorsal and a ventral keel, formed by the anal and dorsal fins, which, like the sliding keel of a yacht, can be raised or lowered as occasion requires. When these fins are removed the course of the Fish becomes zigzag, and the animal wobbles.

The organs more directly concerned with swimming are the tail and the caudal fin, and the pectoral and pelvic fins, but the relative share which these structures take in the actual propulsion of the Fish differs greatly. The principal organ of locomotion in the typical Fish is the powerful muscular tail, which, in swimming, is lashed from side to side by the alternating contraction of the great longitudinal muscles on opposite sides of the vertebral column. In such movements the tail is first flexed or bent, say to the right side: this stroke has been termed the non-effective or back stroke. By a stroke in the reverse direction the tail is then extended and straightened, that is to say, the Fish makes the forward or effective stroke. By a rapid succession of such strokes to the right and left sides alternately the Fish is {351}forced through the water. It is obvious, however, that the extension or effective stroke must have a considerable surplus of power over the flexion or non-effective stroke, and how this result is achieved will now be briefly considered. Experiment, and the observation of Fishes like the Sturgeon, which habitually move with sufficient slowness to allow the phases of their swimming movements to be followed without much difficulty, show that in swimming a Fish throws its body into two opposite and complementary curves, a cephalic curve formed by the anterior half of the body and a caudal curve by the tail. The double curve enables the Fish always to present a convex, less resisting or non-biting surface to the water during the flexion of the tail to the right or left as the case may be, and a concave or biting surface during extension, that is when the tail is straightening itself during the effective stroke.

Fig. 204, which represents a Fish in two successive positions while swimming, will serve to illustrate these conclusions. A Fish in the position A has its body thrown into a cephalic concavity directed to the right and a caudal concave surface facing the left. The tail is bent to the right of the line a b, which corresponds to the axis of the Fish when at rest and to the course pursued by the animal when swimming, and is in the position which it assumes during a flexion stroke, with its convex non-biting surface directed outwards and its concave biting surface inwards. The tail is now ready for an extension stroke, and while this is in progress it is clear that the concave biting surface of the tail will meet the water, while at the conclusion of the stroke the tail will be in a line with a b. At the same time the cephalic curve has so far diminished that the long axis of the body for a momentary period will also coincide with a b, and the Fish is free to advance without impediment. The tail, {352}however, continues its movement to the left, but now as a flexion stroke, and assumes the curvature and position indicated in B, with a reversal in the direction of both the cephalic and caudal curves, but in the meantime the force of the preceding extension stroke has forced the Fish along the line a b to the new position indicated by B. By a rapid succession of alternating flexions and extensions, during which the tail describes figure-of-8 curves, the Fish travels in an undulating forward course with a maximum of propelling power and a minimum of "slip." In short, the action of the tail precisely resembles the action of the stern-oar in the operation of sculling a boat.

There are also other considerations which add to the surplus power of the extension stroke by lessening the resistance of the water to the flexion or non-effective stroke. During the flexion stroke the tail fin is less expanded and its area diminished, and by the rotation of the Fish on its long axis the surface of the tail strikes the water obliquely, and further, the tail moves with less rapidity. On the contrary, when the extension stroke is made these conditions are reversed. The caudal fin is expanded, the stroke is more rapid, and by the reverse rotation of the Fish the tail now strikes the water with its flat surface. In other words, the action of the tail during the two strokes may be compared to the "feathering" of an oar in rowing. Nor is this all. A Fish in motion through the water produces a suction current behind it. The current offers but little resistance to the flexion stroke, inasmuch as the direction of the two coincide, but during the extension stroke the tail meets the full force of the current, and consequently its grip and propelling power are greatly enhanced. There is a striking analogy between the movements of a Fish's tail in swimming and the action of the screw of a steamer, but as a propelling organ the former is far superior to the latter. As we have seen, the tail of a living Fish can so adjust its shape and surface that it alternately eludes and grips the water in accordance with the needs of particular strokes.

The curves into which the body of a Fish is thrown when swimming are never less than two, but in long-bodied Fishes, such as the Eels, the number may be increased, and in every case the curves occur in pairs and are complementary to one another.

{353}Many Fishes can jump out of the water, either in pursuit of insect food, like the Trout, or to enable them to escape the pursuit of their foes, like the Flying-Fish (Exocoetus), by means of a single forcible stroke of the tail, when the Fish is in a nearly vertical position close to the surface of the water. It is thus that the Salmon executes its remarkable leaps over weirs or up salmon-ladders when ascending rivers for spawning.

The tail is also used for steering. If kept bent to one side when the Fish is moving the tail acts like a rudder, and the course of the Fish is deflected to that side; or the direction may be altered by single strokes of the tail to the right or left, according to the course which the Fish desires to pursue.

In the majority of Fishes the paired fins are probably of little use for propulsion, and their action in this as in other functions is not always clear. In the Sharks and Dog-Fishes as well as in some Teleosts their planes are nearly horizontal when the fins are extended from the body; in others they are more oblique, so that the surfaces of the fins look upwards and backwards, and downwards and forwards; and in others again their surfaces are so nearly vertical that their strokes will be backwards and forwards. The pectoral fins also vary in their position on the sides of the body, being much more dorsal in some Fishes than in others. The paired fins may act as lateral keels in steadying the course of the Fish especially when the fins are extended and their planes are horizontal. They certainly seem to act as balancers in keeping the Fish on an even keel, and in counteracting the tendency of the Fish to turn belly upwards—a result which is attained by a slight upward and downward movement of the fins, and particularly of the pectoral fins. A Fish deprived of its pectoral members sinks downwards at the head and assumes an oblique position in the water. Removal of both the pectoral and pelvic fins of one side causes the Fish to roll over to that side; and if the fins are removed from both sides the animal turns belly upwards like a dead Fish. The pectoral fins may also be used for steering: a backward stroke of one fin while the other is kept folded back against the body will wheel the Fish round to the opposite side. From the ventral position of its mouth a Shark is forced to turn over to one side in order to seize its prey, and this movement of rotation is probably produced by the down strokes of the pectoral fin of one side. In {354}some Fishes it would seem that the pectoral fins may assist locomotion by acting as paddles. The 15-spined Stickleback (Gastrosteus spinosus) frequently progresses by their aid alone; and, as their action can be reversed at pleasure, it is not unusual to see this Fish move backwards. The fins appear to be rotated or twisted in spiral movements like the tail when used for swimming, or like the wings of Insects in flying.

It has been mentioned that the function of the median fins (dorsal and anal) is to give stability to the Fish by acting as dorsal and ventral keels. This is certainly the case in the generality of Fishes. Nevertheless, there are exceptional instances in which one, or even both, of these fins are important swimming organs, acting either as a substitute for a tail which has become adapted for other uses, or as supplementary to that organ. Thus, in some of the Syngnathidae (Pipe-Fishes and Sea-Horses) the small size or absence of the caudal fin, and its use as a prehensile organ, renders the tail of little or no value as a propelling organ: hence it is that these Fishes swim by a lateral undulating movement of the dorsal fin. To enable them to do this the supporting skeleton presents certain interesting modifications. In the majority of Teleosts the arrangement of the fin-muscles, and the nature of the articulation between the dermal fin-rays and their basal radial supports, which is generally some form of a hinge-joint, are such as to limit the motion of the rays to simple elevation or depression in the vertical plane, and no lateral motion of the fin is possible. But in the Syngnathidae, as in the Pipe-Fish (Siphonostoma typhle), there is an exceptionally mobile articulation between the dermal fin-rays and the distal radial nodules which their cleft bases embrace and the bony proximal or basal radials, so that the fin can be flexed or bent to the right or to the left. In addition to this, by a change in the insertion of their tendons, the muscles corresponding to the ordinary elevator and depressor muscles of the fin-rays in other Fishes are capable of producing extensive lateral movements of the fin, or, by contracting in orderly sequence, of bringing about the characteristic undulating motion of the fin. A similar mechanism exists in many Plectognathi (e.g. species of Balistes, Monacanthus, Diodon, Tetrodon and Orthagoriscus) in connexion with both the dorsal and anal fins, but in these Fishes the {355}action of the median fins in swimming must be regarded as supplementary to that of the tail.

Swimming is by no means the only form of locomotion in vogue amongst Fishes. A few, like the Angler-Fishes (Lophius), habitually use the pectoral fins for crawling about the sea-bottom. The East Indian Goby, Periophthalmus, uses its pectoral fins, which are bent at an angle like an elbow-joint, for hopping over sandy flats left bare by the retreating tide. The Flying-Fish (Exocoetus), when projected from the water by a stroke of its powerful tail, expands its large pectoral fins, and, using them after the fashion of a parachute, floats through the air for considerable distances before returning to its natural medium. The "Flying Gurnards" (Dactylopterus) are also capable of short aerial excursions in a similar fashion. Nor is tree-climbing beyond the province of a Fish, if credit be given to the assertion that the Indian "Climbing-Perch" (Anabas scandens) uses its opercular spines for ascending trees. Many freshwater Fishes are known to migrate across land from one pool or river to another, usually during the night. Eels do so by a serpentine or wriggling motion of their long bodies, but in others the pectoral fins seem to be the principal organs used for the purpose, aided, it may be, by a perverted use of the tail.

SOUND-PRODUCING ORGANS.—Contrary to popular belief sound-producing or vocal organs are by no means uncommon in Fishes, especially in certain families of Teleosts. It is not always easy, however, to discriminate between involuntary, abnormal, or accidental sounds, and those due to the action of special vocal organs. There are, moreover, some Fishes which observations have shown to utter highly characteristic sounds, although the precise nature of the sound-producing mechanism is at present unknown; while other Fishes appear to possess organs which, on anatomical grounds, are perhaps vocal in function, although nothing is known of the nature of the sounds they emit. Here those organs only will be considered which, either with certainty or with some degree of probability, may be regarded as vocal structures. For most of our knowledge of these interesting structures we are indebted to the researches of Sörensen and Dufossé.

{356}[Illustration: FIG. 205.—Stridulating apparatus of Callomystax gagata. is^1, The first interspinous bone, the lower part of which forms the double file and fits into the interval between the cleft neural spines ns^4 and ns^5; is^2, is^3, second and third interspinous bones; ns^3, ns^4, ns^5, neural spines of the third, fourth, and fifth vertebrae; s^1, s^2, spine-like rays of the dorsal fin; so, supra-occipital. (After Haddon.)]

(a) Stridulation.—Stridulation as a method of sound-production has been recorded in many Teleosts, and one of the most interesting examples occurs in the singular Indian Siluroid, (Callomystax gagata). In this Fish (Fig. 205) the first five vertebrae are rigidly connected with one another and with the skull, mainly through the union of the neural spines of the third, fourth, and fifth vertebrae, and their articulation with the supra-occipital bone. The united spines together form a high, laterally-compressed lamina of bone, the hinder portion of which is vertically cleft into two thin plates separated by an interval sufficiently wide to receive the first interspinous bone of the dorsal fin. The inner surface of each of the two plates is traversed by a series of about thirty parallel, close-set, vertical ridges, while the first interspinous bone is similarly ridged on both its faces like a double file. Lastly, it may be mentioned that owing to the width of the intervertebral ligament between them the fifth and sixth vertebral centra are articulated by a joint of unusual mobility. The action of the mechanism is simple. By the vertical movements of the sixth and succeeding trunk vertebrae, with the interspinous bones which they support, on the rigid structure formed by the head and first five vertebrae, the file-like first interspinous bone moves backwards and forwards, and, by scraping against the ridges on the inner surfaces of the cleft neural spines, gives rise to a harsh grating noise, which is particularly unpleasant when artificially produced. The lateral movements of the trunk in ordinary locomotion do not affect the mechanism: it is only when the trunk is alternately flexed and extended in the vertical plane that the mechanism comes into play and a noise is {357}produced. In the Bull-head (Cottus scorpius) the preoperculum is modified for stridulation, and in Dactylopterus the hyomandibular bone; in other Fishes, as in some Siluroids (e.g. species of Doras), stridulation takes place between a basal process from the great spine of the pectoral fin and the wall of a socket in the cleithrum into which the process is received, or between the small first spine of the dorsal fin and a roof-like process at the upper extremity of the first interspinous bone; also, in a somewhat similar fashion in the anterior dorsal fin of such widely different Fishes as certain Trigger-Fishes (Sclerodermi) pertaining to the genera Balistes, Monacanthus, and Triacanthus, Acanthurus chirurgus (Acanthuridae), the Boar-Fish (Capros aper), Centriscus scolopax (Centriscidae), and the Three-spined Stickleback (Gastrosteus aculeatus); and even between the spinose ray of the pelvic fin and the basipterygium in Triacanthus, Capros, and Gastrosteus.

In the "Drumming" Trigger-Fish (Balistes aculeatus), which frequents the coral-reefs off the Island of Mauritius, stridulation takes place between the postclavicles and a longitudinally grooved area on the inner surface of each cleithrum. Both the cleithra and postclavicles are in intimate relation with the air-bladder, and the sound produced by friction is apparently strengthened by the transference of the vibrations to the walls and gaseous contents of that organ. The passage of the sound-vibrations to the surrounding medium is facilitated by the fact that for a portion of their extent the lateral walls of the air-bladder are in contact with the superficial skin, which visibly shares in the vibratory movement of the bladder when the characteristic drumming sounds of Balistes are being emitted.

Stridulating sounds may also be produced by the friction of the upper and lower pharyngeal teeth, as in a species of Mackerel (Scomber brachyurus). By the grating of its teeth the Sun-Fish (Orthagoriscus mola) is said to emit sounds similar to those produced by the grinding of the teeth in Pigs and Ruminants; and Moseley has remarked of a species of Balistes that the "living Fish when held in the hand makes a curious metallic clicking noise by grating its teeth."

(b) Breathing sounds.—Characteristic breathing or murmuring sounds, or "bruits de souffle" as Dufossé terms them, are {358}produced by a few Teleosts, among which may be mentioned the Eels, certain Cyprinidae, as, for example, the Carp (Cyprinus carpio), several species of Loaches (e.g. Misgurnus fossilis and Cobitis taenia), and the European Siluroid, Silurus glanis. According to Dufossé these sounds originate in some cases from the expulsion of gas from the air-bladder through the ductus pneumaticus and mouth, and in others, as in Misgurnus fossilis, they are produced by the rapid ejection through the anus of bubbles of air previously taken in at the mouth.

(c) Sounds produced through the agency of muscles connected with the air-bladder.—In addition to its usual function as a hydrostatic organ or "float" the air-bladder is often modified in various ways in different Teleosts, and adapted for use as a sound-producing organ.

In the South American Siluroid, Auchenipterus nodosus, the transverse processes of the fourth vertebra are bent downwards and backwards, and at the same time become converted into flexible and highly elastic springs (Fig. 206, B). Their distal extremities expand into oval bony plates which are imbedded in the anterior wall of the air-bladder, and often cause the latter to bulge inwards (Fig. 206, A). From the occipital region of the skull arise two powerful muscles which pass backwards to {359}their insertion into the anterior faces of the two springs. By the contraction of these muscles the springs, and consequently also the front wall of the bladder, are drawn forwards; but directly the muscles relax, the elasticity of the springs causes them to move backwards to their former position, carrying with them the wall of the air-bladder. Hence it follows that the rapid alternating contraction and relaxation of the muscles will impart a vibratory movement to the anterior wall of the bladder and to the gaseous contents of that organ, with the result that a sound is produced. As a rule, those Fishes in which an elastic-spring-mechanism is present have the air-bladder subdivided by internal septa into a series of chambers freely communicating with one another; and no doubt the intensity of the sound is greatly increased by the vibratory movements of the gases across the free edges of the septa, and from one chamber to another. The elastic-spring type of vocal organ is apparently restricted to the Siluridae, and besides occurring in Auchenipterus is found also in the South American genera Doras, Oxydoras, Rhinodoras, and Euanemus; in the African genera Synodontis and Malopterurus; and in at least four species of the Indian genus Pangasius. There are also a few Teleosts in which the air-bladder is provided with special muscles, but, instead of being connected with elastic springs, the muscles extend from the skull, and are inserted directly into the wall of the bladder (Fig. 207); or, without being in any way attached to the skeleton, the muscles simply invest some portion of the surface of the air-bladder. In other Fishes the air-bladder, without possessing special muscles of its own, may, nevertheless, be partially invested by tendinous, or partly muscular and partly tendinous, extensions from the muscles of the body-wall (Fig. 208), or may be intimately related to certain muscles connected with the pectoral girdle.

{360}[Illustration: FIG. 207.—Ventral view of the air-bladder and its extrinsic muscle in Platystoma. a.b, Air-bladder; a.l.c, left antero-lateral caecum of the bladder; b.o, basioccipital; b.w, body-wall in contact with the lateral wall of the bladder; c^1, centrum of the first vertebra; cl, clavicle; d.p, ductus pneumaticus; m^1 and m^2, extrinsic muscles of the bladder; pt.i, post-temporal. (From Bridge and Haddon.)]

Whatever the precise relation of the air-bladder to its muscles it is probable that the physiological effect is in most cases the same. By the rapid alternating contraction and relaxation of the muscles, some part of the wall of the bladder becomes alternately compressed and relaxed in such a way as to initiate a series of vibratory movements in the gases of that organ, and so produce definite sounds. In not a few of the Fishes the cavity of the bladder is subdivided by external constrictions or by internal septa, or is complicated by the development of lateral, tubular, caecal branches; and hence the vibratory movements of the gases will be greatly strengthened by their passage across the edges of the septa, or the apertures of the caeca, and the intensity of the resultant sounds also increased. It will be readily understood that the nature and quality of the sounds emitted by different Fishes will necessarily vary with the shape of the air-bladder, the number and arrangement of the internal septa and the caeca, and the strength and disposition of the contracting muscles. In a few Teleosts (Triglidae and Zeidae) sounds are said to be produced by the rapid vibration of an annular, or centrally-perforated, muscular diaphragm, which stretches across the cavity of the air-bladder. Nevertheless, it must be strongly emphasised that, while in some Fishes the air-bladder and its muscles {361}undoubtedly constitute a vocal organ, there are many others in which the bladder can only be inferred to be sound-producing from its general agreement in anatomical structure with the same organ in Fishes where its vocal function has been clearly proved.

By one or other of these various methods the air-bladder is either known to be sound-producing, or is believed with good reason to be such, in the following Teleosts, and many others:—Certain species of the South-American genera of Siluridae, Pimelodus, Sorubim, Platystoma, Piratinga, Centromochlus, and Trachelyopterus; species of the South-American family Characinidae; Amblyopsis spelaea, the blind Fish from the Mammoth Cave of Kentucky (Amblyopsidae); among the Syngnathidae, the short-snouted Sea-Horse (Hippocampus brevirostris) of the British Coasts; certain Sclerodermi, such as the Trigger-Fishes, Batistes vetula, Triacanthus brevirostris, T. biaculeatus, and Monacanthus pardalis, and also some "Coffer Fishes" (e.g. species of Ostracion); some Gymnodontes (species of Diodon and Tetrodon); a few Serranidae (e.g. species of Therapon and Pristipoma); species of Holacanthus (Chaetodontidae) and in Holocentrum sogho (Berycidae); such Sciaenidae as the "Drum" (Pogonias chromis), the "Maigre" (Sciaena aquila), which has sometimes been taken in British waters, Umbrina cirrhosa, Otolithus regalis, and Micropogon undulatus, and, with more or less probability, many other species of the same family; one species of Zeidae, the John Dory (Zeus faber); Batrachus tau among the Batrachidae; several species of Gurnards (Triglidae) belonging to the genera Prionotus and Trigla; the so-called Flying Gurnard, Dactylopterus volitans (Dactylopteridae); the Indian species Ophiocephalus marulius and O. gachua (Ophiocephalidae); amongst the Gadidae, the Cod (Gadus morrhua) and the Haddock (G. aeglefinus); in such Zoarcidae as the blind Fish (Lucifuga subterranea) from the subterranean waters of the caves of Cuba, and also in some Ophidiidae (e.g. species of Ophidium).

In Fishes other than Teleosts, instances of normal sound-production by special vocal structures are rare. No recorded instances are known in the Cyclostomes or the Elasmobranchs, {362}but there is evidence that sounds are emitted by Polypterus among the Crossopterygii, and by the Dipnoids Neoceratodus, Protopterus, and Lepidosiren, although it is not certainly known how they are produced, or that they may not be the accidental concomitants of the inspiratory or expiratory action of the lungs in breathing.

As to the nature of the sounds produced by the air-bladder and its muscles in different Teleosts, a few examples may be given.

The sound produced by the elastic-spring-apparatus of a recently caught Doras maculatus, has been described as a "deep growling tone," which may be distinctly heard at a distance of 100 feet when the Fish is out of the water. Under like conditions the air-bladder and its muscles, in a species of Platystoma, emit a similar sound. On the other hand, the sound produced by the elastic springs of the Electric Siluroid (Malopterurus electricus) has been compared to the hissing of a cat. The Sea-Horse (Hippocampus brevirostris) utters a monotonous sound analogous to that of a tambour, which is characteristic of both sexes, but is more intense and frequent in the breeding season. The "Coffer Fish" (Ostracion trigonus) emits a growling sound, as also does the "Globe Fish" (Tetrodon honckenii) when taken out of the water. The air-bladder and its muscles in the "Drum" (Pogonias chromis), constitute the most powerful sound-producing organ yet found in any Fish. The sounds emitted by the "Drum" are better expressed by the word drumming than by any other, and have frequently been heard by persons in vessels lying at anchor on the coasts of the United States, where these Fishes abound. The "Drum" begins its drumming noise in the spawning season in April, but is rarely heard afterwards. The "Maigre" (Sciaena aquila), whose musical performances are perhaps responsible for the Homeric fable of the song of the Sirens, is remarkable among Fishes for the variety of its sounds, which have been compared to bellowing, purring, buzzing, and whistling. The sound is often so intense that it may be heard when the Fish is at a depth of 18 metres, and the {363}ear of the observer two metres above the water; and it has been recorded that by listening for these sounds, shoals of Maigres have been successfully netted. They rarely emit sounds when isolated; but in shoals, during the breeding season, they do not cease to make sounds with a vigour and a persistency which apparently must soon wear out their strength. One of the Indian Horse-Mackerels (Caranx hippos) grunts like a young Pig when captured, and the sound is repeated whenever it is moved, as long as vitality remains. A West Indian species of the same family (Argyriosus vomer) has been observed to produce a like sound, while an Egyptian Caranx (C. rhonchus) is known to the Arabs as the "Chakoura" or "Snorter." The sounds produced by the different British Gurnards, such as the Grey Gurnard (Trigla gurnardus), the Piper (T. lyra), the Elleck or Cuckoo Gurnard (T. cuculus), and the Tub-Fish (T. hirundo), have been compared to snoring, a sonorous and prolonged grunting, crooning (whence, perhaps, the term "crooner," by which the Grey Gurnard is known in Ireland), and croaking. The John Dory (Zeus faber) also utters sounds analogous to those of the Gurnards. Among the Dipnoi Lepidosiren is said to make a growling sound, and Neoceratodus a grunting noise which may be heard at night for some distance.

Whatever the nature of the vocal mechanism, it is highly probable that the sounds produced by Fishes travel to considerable distances in the water, inasmuch as the latter medium is a far better conductor of sound than air, and, moreover, the transmission of sound-vibrations from the air-bladder to the water is facilitated in many Fishes by the fact that, for a portion of its extent on each side the bladder is in direct contact with the superficial skin behind the pectoral girdle.

From the by no means exhaustive list of examples given above, it is obvious that in some form or other vocal organs are present in a considerable number of Fishes, both freshwater and marine, belonging to widely different groups; and further, that even in the same species (e.g. Doras maculatus and other Siluridae), both stridulation and the action of extrinsic muscles on the air-bladder may be utilised as a means of sound-production. Certain Teleostean families like the Siluridae, the Sciaenidae, and the Triglidae, seem to be distinguished above all others by the {364}prevalence of some form of vocal organ. According to Sörensen, the first mentioned of the three families includes no less than 68 species, which utilise the air-bladder alone as a sound-producing organ. Nevertheless, there still remain many Teleostean families, rich in genera and species, and with an almost world-wide geographical distribution, in which such organs have not yet been found.

The advantages which Fishes derive from the possession of sound-producing organs are sufficiently obvious.

A characteristic feature in the reproduction of most Fishes is the general absence of any process of conjugation between the sexes, the eggs being fertilised in the water after their extrusion from the body of the female, and, consequently, any device which will facilitate the formation of shoals during the breeding season must be of great advantage to the species by largely increasing the chances that the ova will be fertilised, and thus secure the more successful propagation of the race. Hence it may be concluded that the vocal organs of Fishes are a means to this end, and that the sounds they produce are in fact recognition-sounds which enable Fishes of the same species to congregate together at periods when reproductive activity is greatest. This view is in harmony with much that is known of the habits of these Fishes, especially with the fact that particular sounds are often characteristic of particular species, and that the sounds are produced most frequently and with greater intensity during the breeding season than at any other time. While useful to all Fishes that possess them, vocal organs are, no doubt, specially serviceable to those Fishes which, from the nature of their habitat, can make but little use of their eyes; and this fact may perhaps explain the prevalence of such organs in the Siluridae, which are frequently bottom- or ground-feeding Fishes, and often live in muddy waters.

The sounds emitted by Fishes may also, in some instances at least, be warning sounds. Many of the sound-producing Fishes are provided with exceptionally strong spines either in connexion with the median and paired fins, as in many Siluridae, or on the general surface of the body, as in Diodon hystrix. Such spines are very effective weapons for offensive or defensive purposes, and are capable of inflicting very severe wounds. The natural enemies of these Fishes learn by experience or instinct to {365}associate particular sounds with the possession of dangerous spines, and warned by the sounds, they refrain from attacking the owner of the spines, to the mutual advantage of both.

ELECTRIC ORGANS.—Electric organs capable of generating more or less powerful electric discharges are present in certain Fishes, both marine and freshwater. They occur in a few Elasmobranchs (species of Raia, Torpedo, and Hypnos), in such Teleosts as the African Silurid Malopterurus the "Electric Eel" (Gymnotus), and in species of Mormyridae (e.g. Mormyrus). With one exception electric organs are composed of metamorphosed muscular fibres, and their nerve-endings or motor end-plates. The species of Raia have two small electric organs, one on each side of the terminal portion of the tail. In Gymnotus the {366}organs are much larger, and extend the whole length of the tail, which is fully four-fifths of the total length of the Fish. The Mormyridae also have their feeble electric organs in the caudal region. In all these Fishes the electric organs are modified portions of the caudal muscles. In the Torpedo, however, these organs are two large oval masses, one on each side of the head, between the gills and the cephalic prolongation of the pectoral fin (Fig. 209). Malopterurus is exceptional in possessing an electric organ derived from the epidermis and not from the muscular system. In this Fish the organ envelops nearly the whole body like a mantle, between the skin and the subjacent muscles of the trunk and tail. An electric organ is composed of an immense number of "electric plates" (modified motor end-plates), abundantly supplied with nerves on one of their surfaces, and disposed in a series of vertical (Torpedo) or longitudinal (Gymnotus) columns, separated by septa of connective tissue. In the active state of the organ in the Torpedo the ventral surfaces of the plates, on which the nerves are distributed, become negative to the dorsal, and "the effect in all the plates of a column when summed up is, therefore, such that the dorsal end of a column becomes positive to the ventral end." Hence the current in the form of a succession of shocks passes from the ventral to the dorsal surface of the head. In Gymnotus, where the columns are longitudinally arranged, it is the anterior and posterior surfaces which become oppositely electrified, and the current passes from the tail to the head. The shock imparted by an electric discharge is most powerful in Gymnotus, Malopterurus, and Torpedo, in the order named, and relatively weak in the remaining genera. The strength of the shock increases with the number of electric plates included in the circuit. Thus in Gymnotus the maximum shock is given when the body of the Fish is so curved that the head and the tail are in contact with different points on the surface of some other Fish. The discharge may be reflex or voluntary. Repeated discharges induce fatigue and weaken the shocks. Electric organs are powerful offensive or defensive structures, enabling the Fish to repel the attacks of enemies, or to stun or kill their prey.

{367}CHAPTER XIV

NERVOUS SYSTEM AND ORGANS OF SPECIAL SENSE

The nervous system consists of the brain and the spinal cord, and of the cranial and spinal nerves. The rudiment of the future brain and spinal cord first appears in the embryos of some Cyclostomes (e.g. Bdellostoma), of Elasmobranchs, and of Chondrostei (e.g. Acipenser), and of Neoceratodus among the Dipnoi, in the form of a tubular medullary canal pinched off from the epiblast of the dorsal surface of the body. By a somewhat different method, but with the same final result, a medullary canal is formed in other Cyclostomes (e.g. Petromyzon), in the Holostei and Teleostei, and in Lepidosiren, from a solid ingrowing keel of epiblast which subsequently becomes tubular. Later, the medullary canal in the head enlarges, and becomes divided by two transverse constrictions into three vesicles, the primary fore-, mid-, and hind-brain, leaving the rest of the canal to form the spinal cord.

THE SPINAL CORD.—This portion of the medullary canal retains a simpler and more uniform cylindrical structure. Its walls thicken and their component cells become converted into nerve cells and nerve fibres, but a remnant of the original cavity remains in the adult as a minute axial canal, with a ciliated epithelial lining, the central canal of the spinal cord or myelocoele. In most Fishes the spinal cord extends the whole length of the body, but in some Teleosts, especially in certain Plectognathi, it is remarkably short. In a Sun-Fish (Orthagoriscus), 2½ metres long, and weighing about a ton and a half, the cord was only 15 mm. in length, or shorter than the brain.

THE BRAIN.—At an early stage in its embryonic history the {368}brain consists of three simple vesicles, the fore-, the mid-, and the hind-brain, the first of which lies in front of the anterior end of the notochord and is therefore pre-chordal in position. As development proceeds the walls of the vesicles undergo local thickenings, or they give rise to hollow paired or median outgrowths, and by one or other of these methods the different parts of the complex adult brain are evolved, while the original cavities of the vesicles or of their outgrowths persist as a continuous system of epithelium-lined spaces or "ventricles." The fore-brain is remarkable for the number and importance of the parts to which it gives rise. First, it bulges out in front into a hollow vesicle, the prosencephalon, leaving the rest of the fore-brain as the thalamencephalon or diencephalon (Fig. 210). The cavity of the prosencephalon is the prosocoele, and a pair of thickenings in its floor form two basal ganglia or corpora striata. In many Fishes the prosencephalon retains this simple vesicular condition, in which case the roof or pallium is usually epithelial and non-nervous; but in others two hollow lobes grow out from it in front and give rise to two cerebral hemispheres or parencephala. Both contain extensions of the prosocoele, the paracoeles or lateral ventricles, from the floor of which the corpora striata now project. The prolongation of the pallium forming the roof of the lateral ventricles either remains partially epithelial, or it may acquire a wholly nervous structure and thicken to an extent which differs greatly in different Fishes. With the formation of the hemispheres the prosencephalon and its prosocoele become of secondary importance, and may cease to be recognisable as distinct from the thalamencephalon and its ventricle. The lateral ventricles then appear to communicate directly with the third ventricle by two apertures, the foramina of Munro. The forward growth of the brain is completed by the development of two hollow lobes, the olfactory lobes or rhinencephala, each of which contains a ventricle or rhinocoele communicating behind with the prosocoele, or, if hemispheres are present, with the corresponding lateral ventricle.

{369}[Illustration: FIG. 210.—Diagram of the general structure of the brain in Craniates. A, vertical longitudinal section; B, dorsal view showing the brain cavities on the right side. c, Cerebellum; c.c, central canal of the spinal cord; c.h, cerebral hemispheres; c.s, corpus striatum; F.B, fore-brain; f.m, foramen of Munro; H.B, hind-brain; in, infundibulum; l.v, lateral ventricle; m, mesocoele; M.B, mid-brain; m.o, medulla oblongata; o.l, olfactory lobe; op.l, optic lobe; op.t, optic thalamus; p, paraphysis; pc, prosocoele; pn.o, pineal organ; p.o, parietal organ; pr, prosencephalon; pt, pituitary body; rh, rhinocoele; sp.c, spinal cord; s.v, saccus vasculosus; th, thalamencephalon; iii, iv, third and fourth ventricles. (After Parker and Haswell.)]

Scarcely less complicated, and perhaps even more interesting from a morphological standpoint, are the structures arising out of the thalamencephalon. By thickenings of its lateral walls two large ganglia, the optic thalami, are formed, and on the inner or dorsal aspect of each of these a ganglion habenulae is developed. From the sides of the thalamencephalon the primary optic vesicles are derived, which later become transformed into the retinal parts of the paired eyes and the optic nerves. Besides the optic vesicles there is a second pair of embryonic outgrowths which arise from the roof of the thalamencephalon. These outgrowths form stalked vesicles and represent a pair of degenerate visual {370}organs. Usually they become so displaced that the left one lies in front of the right, and they appear as if median. The subsequent fate of the vesicles differs greatly in different Craniates. Both persist in the Lamprey, the right vesicle to some extent retaining its primitive visual function as a parietal eye and directly overlying the left or pineal vesicle. In Elasmobranchs the two unite to form a glandular organ, the so-called pineal body of the adult, and in Teleosts the left vesicle disappears, leaving the right as a pineal body. There is also an embryonic median outgrowth from the roof of the prosencephalon, the paraphysis, which soon disappears and whose significance is not known. A median hollow downgrowth from the floor of the thalamencephalon forms the infundibulum, which becomes attached to a caecal diverticulum from the roof of the mouth. With rare exceptions the diverticulum loses all connexion with the mouth, and, as the pituitary body or hypophysis, it appears as an appendage to the extremity of the infundibulum. In the Crossopterygii the connexion is retained even in the adult by means of a slender canal extending from the pituitary body and opening into the oral cavity. Laterally, the base of the infundibulum grows out into a pair of rounded lobes, the lobi inferiores, and distally into a thin-walled glandular sac, the saccus vasculosus, which lies just behind the pituitary body. The cavity of the thalamencephalon persists as the third ventricle or diacoele. The parts of the brain developed from the mid-brain and the hind-brain are much less complicated, and, except for variations in size, they present a fairly uniform character in most Fishes.

In the mid-brain the roof bulges out into a pair of optic lobes, and by the growth of lateral thickenings in its floor two thick strands of longitudinally disposed nerve fibres, the crura cerebri, are formed. The cavity of the mid-brain remains as the mesocoele, and from it an extension may be prolonged into each optic lobe.

From the hind-brain are formed the cerebellum or epencephalon and the medulla oblongata or metencephalon, the former as a dorsal bulging, the latter as a ventral thickening. Except where the cerebellum is developed the dorsal wall remains epithelial, and forms the roof of the persistent cavity of the {371}hind-brain, the fourth ventricle or metacoele, which retains its primitive continuity with the central canal of the spinal cord. Lateral lobe-like outgrowths from the dorsal columns of the medulla are conspicuous structures in some Fishes, and are known as corpora restiformia. The paired portions of the brain are connected across the middle line by a series of transverse commissures. The more important modifications of the brain in Cyclostomes and Fishes will now be briefly dealt with.

{372}[Illustration: FIG. 212.—Dorsal view of the brain of Myxine. c.r, Corpora restiformia; m.o, medulla oblongata; n.p, naso-pituitary canal; ol.o, olfactory organ enclosed in its fenestrated cartilaginous capsule; op.l, optic lobes; pr, prosencephalon; s, s, dorsal roots of spinal nerves; sp.c, spinal cord; th, thalamencephalon. (From Wiedersheim, after Retzius.)]

In the Cyclostome Petromyzon there is a small prosencephalon with an undivided prosocoele, and on each side of it a small cerebral hemisphere which appears as a mere appendage to the much larger olfactory lobe (Fig. 211). The prosocoele divides in front into two outwardly directed branches, and of the two diverticula into which each branch divides one extends as a lateral ventricle into the hemisphere of its side, and the other as a rhinocoele into the corresponding olfactory lobe. The ganglia habenulae are unusually large, the right one being larger than the left. The optic lobes are large, but not obviously double. So small is the cerebellum that it seems to be little more than a narrow transverse band crossing the fore-part of the fourth ventricle. The roof of the brain is largely epithelial, especially in the prosencephalon, the thalamencephalon, and the hind-brain. Over these epithelial areas the pia mater is unusually vascular and forms a series of "choroid plexuses." The ventricular system is complete and continuous. By contrast with the Lamprey the brain of Myxine is very primitive, more so perhaps than in any other Craniate (Fig. 212). In a dorsal view the brain is divided into four pairs of laterally expanded and longitudinally compressed lobes by a median longitudinal fissure and three transverse fissures. The two anterior lobes are little more than the thickened anterior wall of the thalamencephalon, although, judging from their histological structure, they represent a very imperfectly differentiated prosencephalon and olfactory lobes. The second and largest pair constitute the thalamencephalon. The last two pairs of lobes represent a transversely divided pair of optic lobes, or "corpora quadrigemina." There is a large medulla oblongata with a pair of corpora restiformia, but the {373}cerebellum is entirely absent. The ventricles are subject to some individual variation. Third and fourth ventricles are generally recognisable, either as isolated cavities or connected by a remnant of the mesocoele. In the feeble development of the prosencephalon, in the striking preponderance of the mid-brain over the rest of the brain, and in the absence of a cerebellum, Myxine is unique amongst Craniates.

In Elasmobranchs among Fishes the brain attains a much higher grade of structure. In Scyllium (Fig. 213) there is a large prosencephalon, and directly in front of it a pair of imperfectly differentiated cerebral hemispheres, while from its antero-lateral regions the large olfactory lobes arise. The prosocoele divides in front into four diverticula, of which the two {374}inner ones extend into the hemispheres as lateral ventricles, and the two outer as rhinocoeles into the olfactory lobes (Fig. 214). In connexion with the infundibulum there is a pair of sacci vasculosi, consisting mainly of gland-tubules, opening into the infundibular cavity. The cerebellum is exceptionally large, but it does not form a "valvula cerebelli." Large ear-like corpora restiformia are present. The third and fourth ventricles alone retain an epithelial roof in relation with choroid plexuses.

In all essentials the brain of the Holocephali is a repetition of the Elasmobranch type, more especially of the elongated form seen in Notidanus. Indications of a higher grade of structure are, however, to be seen in the reduction of the prosencephalon which, with its prosocoele, is now scarcely distinguishable from the thalamencephalon and its ventricle; and in the more complete differentiation of the cerebral hemispheres from one another and from the rest of the brain. Large frilled corpora restiformia are conspicuous structures on each side of the medulla oblongata. Besides the usual intra-cranial pituitary body, there is also a separate extra-cranial portion lodged in a pit on the ventral surface of the basis cranii: in the embryo the two are continuous.

In the Teleostomi the brain is distinctly of a more primitive type than in any other Fishes (Fig. 215). The most striking feature is the absence of cerebral hemispheres, the evolution of the primary fore-brain proceeding no farther than the formation of an undivided prosencephalon with a non-nervous roof, and a prosocoele which forms a continuous cavity with the third ventricle, or at the most is only separated from it by an infolding of the epithelial roof or velum transversum.

{375}[Illustration: FIG. 215.—A, dorsal view of the brain of a Trout (Salmo fario); B, a vertical longitudinal section. c.il, Commissura interlobularis; g.h, ganglion habenulae; h.c, habenular commissure; i.c, inferior commissure; l.i, lobus inferior; myc, myelocoele; p.c, posterior commissure; v.o, valvula cerebelli; v.t, velum transversum; ii., optic nerve; v.iii., v.iv., third and fourth ventricles; v, vii, viii, ix, x, fifth, seventh, eighth, ninth, and tenth cranial nerves; remaining reference letters as in Fig. 210. (A, From Wiedersheim; B, after Haller.)]

Amongst other diagnostic characters may be mentioned the predominance of the mid-brain over the other divisions, the anterior extension of the large cerebellum into the mesocoele as a "valvula cerebelli," {376}and the absence of corpora restiformia. This type of brain is most strongly marked in the Teleostei, but in other Teleostomes some, like Acipenser, are typically Teleostean in this respect (Fig. 216), while others, such as Lepidosteus, have small cerebral hemispheres with lateral ventricles as well as a prosencephalon.

The most obvious feature in the brain of the Dipnoi is the great development of the cerebral hemispheres. In this respect these Fishes approach the Amphibia, but in other features of brain-structure they present points of agreement with most other groups of Fishes without being closely related to any one of them. In Protopterus (Fig. 217) the hemispheres are quite distinct except behind, and the walls of their spacious lateral ventricles are entirely nervous. Olfactory lobes are sessile on their anterior extremities, and behind and below they enlarge into ventral lobes which probably represent the hippocampal lobes of the higher Vertebrates. A vesicular pineal body at the end of a slender stalk overlies a singular conical projection from the roof of the thalamencephalon or "pineal pillow."

{377}[Illustration: FIG. 217.—Dorsal (A), ventral (B), and lateral (C) views of the brain of Protopterus annectens. C, Cerebellum; C.H, cerebral hemisphere; D.S.E, branches of the sinus endolymphaticus; In, infundibulum; L.I,, lobi inferiores; M.O, medulla oblongata; O.L, olfactory lobe; Op.L, optic lobe; P, pituitary body; P.B, "pineal pillow"; S.E, sinus endolymphaticus; Sp.c, spinal cord; Sp.n, spinal nerve; Vel, velum transversum; Z, pineal body; IV.V., fourth ventricle; ii., iii., iv., v., vi., vii., viii.1, viii.2, viii.3.4, ix., and x., roots of the cranial nerves. (From Burckhardt.)]

The optic lobes form a single oval body, and, as in Petromyzon and the Amphibia, the cerebellum is very small. A posterior choroid plexus covers the roof of the fourth ventricle, and an anterior plexus in connexion with the roof of the thalamencephalon projects downwards into the third ventricle, and is also prolonged forwards into each lateral ventricle. In Neoceratodus the brain is certainly more primitive and distinctly less Amphibian. As compared with Protopterus the olfactory lobes and the cerebellum are larger, and the optic lobes are paired. The smaller hemispheres are {378}non-nervous dorsally and medianly, the roof and inner wall of each being formed by an extension of the thick, glandular choroid plexus which forms the roof of the thalamencephalon.

THE SPINAL NERVES.—The spinal nerves of Cyclostomes (e.g. Petromyzon) consist of a series of dorsal nerves arising on each side from the dorsal surface of the spinal cord, and of a similar double series arising from the ventral surface, the dorsal nerves regularly alternating with the ventral nerves. Each myotome is supplied by a dorsal and a ventral nerve which pass separately to their peripheral distribution in the skin and muscles. In Fishes, as in the higher Vertebrates, each dorsal nerve, now termed a dorsal root, enlarges into a ganglion and then unites, either before or directly after issuing from the neural canal, with the next ventral nerve or ventral root in front to form a main spinal nerve. At the same time the spinal nerves of opposite sides tend to form pairs in the same transverse plane. After the union of the two roots the spinal nerve divides into three typical branches: a dorsal nerve (ramus dorsalis), and a ventral nerve (ramus ventralis), both of which include somatic sensory or afferent fibres, and somatic motor or efferent fibres, for the innervation of the skin and muscles of the dorsal and lateral portions of a myotome; and a visceral branch (ramus visceralis), composed of afferent and efferent visceral fibres, which supplies the adjacent viscera (alimentary canal and its glands and blood-vessels), and helps to form the sympathetic nervous system. The somatic afferent and the visceral afferent fibres enter the spinal cord by the dorsal roots, the somatic efferent leaving the cord through the ventral roots, although the visceral efferent fibres traverse both roots. In the region of the paired fins more or fewer of the rami ventrales unite to form a plexus, the brachial or the pelvic plexus, from which the nerves to the fins take their origin.

THE CRANIAL NERVES.—It is usual to describe the cranial nerves of Cyclostomes and Fishes as consisting of ten serially disposed pairs, viz.: the olfactory (i.), optic (ii.), oculomotor (iii.), trochlear (iv.), trigeminal (v.), abducens (vi.), facial (vii.), auditory (viii.), glossopharyngeal (ix.), and the vagus (x.) Like the spinal nerves, the cranial nerves collectively include somatic sensory (general cutaneous) and motor fibres, and also visceral sensory {379}and motor fibres, all of which have their own special centres in the brain, but the proportions of these nerve components differ greatly in different nerves. Certain preoral nerves (iii., iv., and vi.) are exclusively somatic motor; others (i. and ii.) are special sensory nerves for the olfactory and visual organs; but most of the other cranial nerves include several components, and are therefore "mixed" nerves. Besides these components some cranial nerves include also a quasi-independent system of nerve-fibres, which converge from certain cutaneous sense-organs to an independent centre in the medulla oblongata, the tuber acusticum, and is probably derived from the general cutaneous system of nerve components. Such nerve fibres, including also the auditory nerve, which has its origin from the same centre, constitute the lateralis system. Perhaps the most striking feature in the postoral cranial nerves is the predominance of the visceralis or sympathetic system over the somatic. Omitting the lateralis fibres and a relatively few somatic sensory fibres, visceral fibres, sensory and motor, are the principal components of all these nerves, including v. but excluding viii. The reason for this is to be found in the fact that splanchnic or visceral muscles in relation with the jaws and branchial arches have usurped the place of somatic muscles in the muscular system of the head. For developmental and other reasons the olfactory and optic nerves stand in a category of their own, and the same may be said of the third, fourth, and sixth nerves, which innervate the muscles of the eyeball. The remaining nerves, all of which have their origin in the medulla oblongata, possess certain features in common, and as they are related to the gill-clefts in such a way that each forks over a cleft, they may be conveniently distinguished as "branchial" or "branchiomeric nerves." A typical branchial nerve consists of (1) a principal ganglion near the origin of the nerve from the brain; (2) a main trunk which gives off (3) a somatic sensory branch or dorsal nerve to the skin; (4) a palatine nerve (visceral sensory) to the oral or pharyngeal mucous membrane; (5) an epibranchial ganglion which is associated with a transitory embryonic epibranchial sensory organ at the dorsal border of a branchial cleft; (6) a pre-branchial nerve (visceral sensory), skirting the anterior margin of a cleft in its ventral course; and (7) a post-branchial branch (visceral motor) similarly related to the hinder margin.

{380}[Illustration: FIG. 218.—Diagram showing the principal branches of the cranial nerves in a Fish, mk.c, Meckel's cartilage; ol.o, olfactory organ; p.q, palato-quadrate; s, spiracle; i-v, branchial clefts; i, ii, iii, iv, vi, the first, second, third, fourth, and sixth cranial nerves. The remaining nerves are differently shaded. Black.—The trigeminal nerve: g.g, Gasserian ganglion; md, mandibularis; mx, maxillaris; op.p, ophthalmicus profundus. Oblique shading.—The lateralis system and its centre (t.a), the tuber acusticum: bucc.vii, buccalis branch of vii; md.ex, external mandibular branches of vii; l.n.x, lateralis nerve, with its supra-temporal branch (s.t), and its commissural connexion (c) with op.s.vii, the ophthalmicus superficialis of vii; viii, auditory nerve. Dotted.—The facialis proper, including c.t, chorda tympani; gn.g, geniculate ganglion; hy, hyomandibularis, with its motor branches m, m, m; p.n.vii., palatine. Dark grey.—The glossopharyngeal (ix), with its pre- and post-branchial branches and its palatine nerve, p.n.ix; anastomosing with the palatine branch of vii (Jacobson's anastomosis). White.—The vagus: x^{1-4}, the branchial nerves, ganglionated and forking over clefts ii-v; v.n.x, visceral nerve; oc, occipito-spinal nerves; d.r and v.r, the dorsal and ventral roots of the first two spinal nerves. (Slightly modified after Wiedersheim.)]

{381}The first six cranial nerves resemble those of the higher Craniates in their mode of origin from the brain, in the physiological nature of their component fibres, and in their peripheral distribution, and therefore they need not be specially referred to here. The principal branches of the fifth or trigeminal nerve are shown in Fig. 218. Comparing this nerve with a typical branchial nerve it would seem that the profundus and superficialis ophthalmic nerves are dorsal nerves; the maxillaris and mandibularis, pre- and post-branchial branches, respectively, in relation with the modified gill-cleft which forms the mouth, while the branch to the oral surface represents a palatine nerve. The most important of the distinctive features in the cranial nerves of Fishes are to be found in the relations of nerves vii., ix., and x. to branchial clefts, and in the lateralis system of nerve components and its association with the lateral line sensory organs. The seventh or facial nerve is an exceptionally interesting nerve. Besides the usual components of a typical branchial nerve certain of its so-called branches are wholly or largely derived from the lateralis system. For this reason the nerve may be said to consist of two portions, the facial proper, or those fibres which constitute the facial nerve in air-breathing Craniates, and the lateralis branches which solely innervate lateral line sense-organs, and are therefore peculiar to aquatic forms. The facial proper has a ganglion (the facial or geniculate ganglion) on its root, and on entering the orbit after traversing the cranial wall it gives off a palatine nerve. Just over the spiracle a pre-branchial nerve, the representative of the chorda tympani of Mammals, leaves the main trunk, and passes ventrally in relation with the anterior wall of the spiracle to its ultimate distribution in the walls of the mouth cavity. The main trunk, now called the ramus hyomandibularis, then pursues a ventral course behind the spiracle as a post-branchial nerve, and certain of its mainly motor branches which pass downwards in connexion with the hyoid arch supply the muscles of that arch, and, if an operculum is present, the opercular muscles as well. The lateralis portion of the facial includes the following principal branches, {382}each of which may have a ganglion on its root: (1) an ophthalmicus superficialis; (2) a buccalis nerve with its ramus oticus; and (3) external mandibular nerves which course in the ramus hyomandibularis. The addition of the great lateralis nerve, which is usually described as the lateral branch of the tenth nerve, and of the eighth or auditory nerve which supplies the auditory organ, completes the enumeration of the main factors of the lateralis system. The ninth or glosso-pharyngeal nerve, perhaps the most typical of all the branchial nerves, has pre- and post-branchial branches which enclose the hyo-branchial cleft. Its palatine nerve usually extends forwards and anastomoses with the corresponding branch of the seventh, thus forming a connexion (Jacobson's anastomosis) between the two cranial nerves. In some Elasmobranchs and Teleosts fibres derived from the dorsal branch of the ninth nerve innervate a few sense-organs of the lateral sensory canal of the head, and hence that nerve sometimes contains lateralis fibres. The tenth or vagus is a compound nerve. Besides the great lateralis nerve generally associated with it, the vagus includes as many typical branchial nerves as there are branchial clefts behind the hyo-branchial cleft, and in Elasmobranchs and in Chimaera these nerves have independent origins from the medulla oblongata. Each nerve has the typical structure, a ganglionated trunk which forks over a gill-cleft into the usual pre- and post-branchial branches, and palatine branches to the pharyngeal walls. In the Dipnoi the lateralis nerve is connected with the superficial ophthalmic branch of the seventh nerve by a commisural nerve which curves across the outer face of the auditory capsule. A somewhat similar anastomosis is also present in Petromyzon. The vagus also includes a large ramus intestinalis, which in Elasmobranchs, at all events, has a distinct ganglionated root. The nerve forms characteristic plexuses on the oesophagus and stomach, and in Cyclostomes its branches may extend nearly the whole length of the intestine. In Ganoids and Teleosts there is an interesting nerve known as the "lateralis accessorius." It is a compound nerve, and owes its formation to the union of somatic sensory fibres derived in succession from dorsal branches of the v., vii., ix., and x. nerves, and also from the corresponding branches of a variable number of spinal nerves. The finer branches of the nerve are distributed to the skin of one or more of the fins, or {383}even, as in Gadus, to all the fins, especially to the numerous "end-buds" which are present on those organs. In many Fishes a variable number of the anterior spinal nerves (spino-occipital) perforate the occipital region of the skull. They probably represent the ventral roots only of the ordinary spinal nerves of this region.

SENSE-ORGANS

THE CUTANEOUS SENSE-ORGANS.—These organs, the most remarkable and certainly the most characteristic of the sense-organs of Cyclostomes and Fishes, are bud-like groups of epidermic cells in relation with the ends of sensory nerve fibres. Each consists of a central core of sensory cells, provided with terminal cuticular sensory hairs, and surrounded by a zone of supporting and mucus-secreting cells which leave the hairs exposed at the apex of the bud. Two kinds of these organs can be distinguished, which differ in their innervation and in their position in the skin. Of the two, the so-called end-buds are the more primitive. They occupy a superficial position in the epidermis, and their sense-cells are as long as the supporting cells. They are present in Cyclostomes and Elasmobranchs, and especially in Teleosts, where they are irregularly distributed over the surface of the body, on the fins, lips, and barbels, and also in the epithelium of the mouth and pharynx. In the Dipnoi they are limited to the oral cavity, and in the higher Craniates they become taste-buds. Their somatic sensory nerves are derived from the vii., ix., and x. cranial nerves, and the lateralis accessorius. In the second type, usually called "nerve-eminences," the sensory cells are shorter than the supporting cells, and they are always innervated by the lateralis system. When first developed in the embryo they are quite superficial, like end-buds, but later the epidermis in which they lie sinks inwards so as to line a series of pits, closed sacs, tubes, open grooves, or closed canals. Pit-organs, so abundant on the head and trunk of Teleosts (Fig. 220), are simple epidermic pits with insunken nerve-eminences, disposed in groups or in {384}lines (accessory lateral lines) or irregularly distributed. The "Spalt-papillen" of Elasmobranchs are pit-organs in which the orifice of the pit is reduced to a slit. The more deeply-seated Savi's vesicles on the ventral surface of the Torpedo, and the nerve-sacs of Ganoids, are similar organs converted into closed sacs and pinched off from the rest of the epidermis. Lorenzini's ampullae or mucus canals, which are found in definitely located groups on the lateral and upper surfaces of the head in Elasmobranchs, may perhaps be compared to pit-organs prolonged inwards to form subcutaneous tubes, each of which terminates in a radially-septate, chambered dilatation or ampulla, containing groups of sensory cells.

Besides the more diffusely scattered sense-organs there are others which become disposed in definite lines along the sides of the body and on the head, and, enclosed in grooves or closed canals, constitute the highly characteristic lateral line system of Cyclostomes and Fishes. The auditory organ must also be included as a specialised portion of this system. Both organs are innervated by the lateralis system, and both arise from a common rudiment in the embryonic epidermis in the position of the future auditory organ. This rudiment grows backwards along the side of the body in the form of a cord of cells differentiated from the epidermis, and also forwards, where it soon divides into the rudiments of future supra-orbital and infra-orbital canals. Sense-organs are differentiated at intervals along the line of the cord; and in the body, but not on the head, they frequently exhibit a segmental disposition. Each sensory organ then sinks down into a short epidermic groove, which by the subsequent meeting of its lips becomes a canal detached from the epidermis. The short canals then become continuous, leaving, however, an externally opening primary pore between every two consecutive canals, and the result is a continuous canal having sense-organs imbedded in its epidermic lining and connected with the exterior by pores at intervals (Fig. 219). The enclosure of the canals in the scales of the lateral line of the trunk or in special drain-pipe ossicles on the head, and the dichotomous subdivision of the primary pores into groups of surface-pores, complete the evolution of the system in its more advanced condition.

{385}[Illustration: FIG. 219.—Vertical longitudinal section through the lateral canal of Amia calva. l.n, Lateralis nerve with its branches, n, n, to the sensory organs, s.o, s.o; p, p, p, external pores; s.c, sensory canal; s, s, scales of the lateral line. (From Wiedersheim, after Allis.)]

Typically, the lateral line system consists of certain canals or grooves, usually but not invariably continuous, and defined by their innervation, (i.) a lateral canal extending along the side of the body and the hinder part of the head, and having its sensory organs supplied by the great lateralis nerve (Fig. 220); (ii.) a supra-orbital canal passing forwards over the eye and innervated by the superficial ophthalmic branch of the facial nerve; (iii.) an infra-orbital canal supplied by the buccalis and otic branches of the same nerve; and (iv.) a hyo-mandibular or operculo-mandibular canal, situated on the outer side of the hyoid region, and thence prolonged downward and forward in relation with the lower jaw, and innervated by the external mandibular branches of the facial nerve. The hyo-mandibular canal is sometimes distinct from the other canals, as in Elasmobranchs and some Teleosts (Fig. 220); and in certain North American Siluroids the same may be said of the supra-orbital. But, as a rule, the infra-orbital is continuous behind both with the lateral and the supra-orbital canals, while the hyo-mandibular canal joins the infra-orbital, or, exceptionally, {386}the supra-orbital canal. Transverse commissural canals often connect the lateral and supra-orbital canals of opposite sides across the dorsal surface of the head, and the corresponding infra-orbital and hyo-mandibular canals may also be continuous at the extremity of the snout or at the mandibular symphysis.

Throughout their extent the canals communicate with the exterior by pores, or short canals terminating in pores, or by branched canals ending in groups of pores. In Cyclostomes the lateral line system is represented by pit-organs disposed as in Fishes, and innervated by a true lateralis nerve. Some Elasmobranchs have the lateral canal of the trunk represented by an open groove protected by marginal denticles. Chimaera is more primitive still in this respect, for on the head as well as on the body the sensory organs are in open grooves. Amongst Fishes these organs are most primitive in the Dipnoi, where they retain their superficial position in the epidermis. In Teleostomes the lateral canals perforate the scales of the lateral line, and at intervals they open externally by simple or multiple pores which perforate the scales. On the head they are more or less completely enclosed in special ossicles which either remain distinct or fuse with certain of the adjacent dermal or cartilage bones of the skull. The use of the lateral line organs is not certainly known. They occur only in Fishes and Amphibia, and as blind Fishes are able to avoid obstacles with the greatest ease when swimming, it is possible that these organs enable their possessors {387}to appreciate undulatory movements in water in the shape of reflex waves from contiguous surfaces or objects. Their great antiquity is shown by their existence in most of the Heterostraci, and in the Antiarchi and Arthrodira, although they have not yet been discovered in the Osteostraci.

THE AUDITORY ORGANS.—In its more typical condition each auditory organ consists of a membranous sac or vestibule, partially constricted into an upper portion or utriculus and a lower or sacculus (Fig. 221, A). Three semicircular canals are connected with the utriculus, of which two are vertical and at right angles to one another, and the third is horizontal. One end of each canal is dilated into an ampulla. A slender tube, the ductus endolymphaticus, leaves the sacculus, and ends in a sac-like swelling, the sinus endolymphaticus, which apparently represents a portion of the embryonic epidermic involution from which the auditory organ is formed. A smaller sac-like outgrowth from the sacculus, the lagena, corresponds to the cochlea of the higher Vertebrates. The epidermic lining of this system of cavities is differentiated into patches or ridges of sense-cells (maculae or cristae), separated by supporting cells and innervated by the terminal branches of the auditory nerve. There is a crista acustica in each ampulla; and maculae acusticae are present in the utriculus, sacculus, and lagena. A fluid, the endolymph, fills all the cavities, and a similar fluid or perilymph occupies the spaces in the periotic capsule in which the various chambers are lodged. Among the more notable deviations from this type of auditory organ the Cyclostome Myxine, apparently, has but a single semicircular canal with an ampulla at each end, and the vestibule is a simple sac (Fig. 221, B). Petromyzon has two canals, but lacks the horizontal canal. In Elasmobranchs, including Chimaera (C), the ductus endolymphaticus retains its primitive connexion with the exterior by means of a pore on the dorsal surface of the head. In the Dipnoi (e.g. Protopterus) the paired endolymphatic sinuses divide into a number of caecal branches containing otoliths, which meet and interlace over the fourth ventricle (Fig. 217). Otoliths, either in the form of fine, {388}mucus-connected, calcareous particles, as in Elasmobranchs, or as massive solid concretions in Teleosts, are present in relation with the sensory areas of the utriculus, sacculus, and lagena.

In a few marine and in a large number of freshwater Teleosts the auditory organ enters into a more or less intimate connexion with the air-bladder by one of three different methods.

{389}[Illustration: FIG. 222.—Cavity of the air-bladder of a Siluroid (Macrones nemurus) exposed by the removal of its ventral wall. a.c, Anterior chamber; b.o, basioccipital; b.w, body wall, here reduced to the external skin; cl, clavicle; l.c, lateral chamber; l.s, longitudinal septum; pt, post-temporal; tr.a, anterior portion of the tripus; tr.c, crescentic portion of the tripus; t.s, transverse septum; t.s', shorter transverse septum. (From Bridge and Haddon.)]

The first and simplest is by the apposition of the extremities of a pair of caecal tubular prolongations from the air-bladder to the outer surfaces of the fibrous membranes which close a pair of vacuities in the outer bony walls of the periotic capsules, the inner surfaces being bathed by the perilymph surrounding the auditory organs. This method is characteristic of certain Serranidae, Berycidae, Sparidae, Gadidae, and Notopteridae, and probably in the Hyodontidae. In the second method, of which several Clupeidae (e.g. Herring, Pilchard, etc.) furnish examples, the periotic vacuities are open instead of closed, and the sac-like ends of the tubular extensions from the air-bladder are in actual contact with protruding outgrowths from the utriculus. The third method, by far the most elaborate, is by the intervention of a series of movably connected "Weberian" ossicles, of which the most posterior on each side (the tripus) is inserted into the dorsal wall of the air-bladder (Fig. 223), while the anterior one (scaphium) forms the outer wall of a median backward prolongation (sinus impar) of the perilymph-containing spaces surrounding the two auditory organs. This in turn encloses a similar median prolongation (sinus endolymphaticus) from the two sub-cerebrally united endolymphatic ducts (Fig. 223). This complex mechanism is present in the Cyprinidae, Siluridae, Characinidae, and Gymnotidae; and hence the term "Ostariophysi" as a collective name for these families. {390}The physiological raison d'être of the connexion between the air-bladder and the auditory organ cannot yet be regarded as satisfactorily determined. It is possible, as Weber thought, that it may be an auxiliary to the function of hearing by transmitting to the ear sound-waves impinging on the surface of the body and affecting the gases in the air-bladder. On the other hand, it may be urged with perhaps greater probability that the connexion exists for the purpose of conveying to the ear stimuli due to the varying degrees of distension of the air-bladder, such as, it may be presumed, are naturally brought about by the variations of hydrostatic pressure which a Fish encounters in the course of its ascent or descent in the water. Whether regarded as an accessory to hearing, or as a means of regulating the distension of the air-bladder, the physiological value of the connexion must be considerable, and on this point it is at least significant that the Weberian mechanism is characteristic of the dominant families of freshwater Teleosts at the present day.

THE OLFACTORY ORGANS.—These organs are essentially a pair of pit-like inpushings of the skin of the ventral side of the head in front of the mouth, with their lining epidermis differentiated into sensory cells separated by supporting cells, and connected with the olfactory lobes of the brain by olfactory nerves.

{391}[Illustration: FIG. 224.—Two stages in the development of the olfactory organ and the pituitary involution in Petromyzon. A is the earlier, B a much later stage. br, Brain; in, infundibulum; l.lp, lower lip; ms, mesenteron; n, notochord; ol.o, olfactory organ; pn, pineal body; pt.s, pituitary sac; st, stomodaeum; u.lp, upper lip. (From Parker and Haswell, after Dohrn.)]

The Cyclostomata are unique amongst Craniates in the apparently unpaired condition of the olfactory organ, and in its remarkable relation to the pituitary involution. In the embryo Lamprey the median and ventral olfactory pit is carried inwards with the pituitary invagination, so that the former appears as a dorsal outgrowth from the latter, and the two have a common external opening, the naso-pituitary aperture (Fig. 224). Later the extraordinary forward growth of the upper lip to form the roof of the buccal funnel has the effect of shifting the naso-pituitary involution and its aperture to a final position on the dorsal side of the head. It is due to this dorsal displacement that, as we shall see, the pituitary caecum reaches the ventral surface of the brain by perforating the basis cranii from above, instead of from below as in all other Craniates. The pituitary body is pinched off from the dorsal side of the naso-pituitary involution. In the adult Lamprey the olfactory organ appears as a round sac divided by a median septum into two lateral chambers (Fig. 225), the lining epithelium of which is raised into prominent ridges. Behind the sac the pituitary involution is prolonged backwards beneath the brain, and, after traversing the basi-cranial fontanelle, it widens out into a spacious cul-de-sac and terminates on the dorsal side of the pharynx, beneath the anterior end of the notochord. In Myxine the pituitary involution ends by opening into the pharynx.

{392}[Illustration: FIG. 225.—Side view of the brain of Petromyzon, with the olfactory organ and the pituitary caecum in section. cblm, Cerebellum; crb.h, cerebral hemisphere; dien, thalamencephalon; f, fold in the nasal tube; gl, nasal glands; inf, infundibulum; l.gn.hb, left ganglion habenulae; med.obl, medulla oblongata; na.ap, naso-pituitary aperture; n.ch, notochord; Nv^1-nv^{10}, cranial nerves; Nv^{12}, first ventral spinal nerve; olf.cp, olfactory capsule; olf.l, olfactory lobe; olf.m.m, olfactory mucous membrane; opt.l, optic lobe; pn, pineal body; pn′, inferior pineal body; pn.e, parietal eye; pty.b, pituitary body; pty.p, pituitary cul-de-sac; sp, median septum of the olfactory sac; sp^1, first dorsal spinal nerve. (From Parker and Haswell, after Ahlborn and Kaenische.)]

The apparently monorhinal condition of the Cyclostomes is probably a secondary acquisition. At the earliest embryonic stage at which any trace of an olfactory organ is apparent, there is a median thickening of the epidermis, possibly a vestige of some older sensory organ comparable, it may be, to the so-called olfactory organ of Amphioxus; on each side of it there is a lateral thickening, the rudiments of the paired organs. The three thickenings, or "plakodes," then sink inwards to form an olfactory pit. The partial subdivision of the adult organ by a vertical septum, and the presence of two olfactory nerves, point to the same conclusion. All Fishes possess olfactory organs which are obviously paired. In Elasmobranchs and Dipnoi they retain their primitive ventral position. Many Sharks and Dog-Fishes possess an oro-nasal groove leading from each olfactory organ to the corresponding angle of the mouth. The Dipnoi proceed a stage farther, and, by the conversion of the grooves into short canals, the olfactory pits communicate with the mouth by true internal nostrils, as in the higher {393}Vertebrates. In the adults of existing Teleostomi the orifice of each organ is usually divided into two by the growth of a fold of skin across it, and the two apertures rotate outwards and upwards on to the lateral or the upper surface of the snout. Of the two nostrils the posterior one probably corresponds to an external nostril, and the anterior one to the internal nostril. Occasionally each olfactory organ has only a single orifice. In the Crossopterygii and in some Teleostei the nostrils become tubular. The lining epithelium of the olfactory pits is usually produced into ridges, disposed longitudinally or transversely, or in the form of radii from a central point in the roof. Many Teleosts have each olfactory organ prolonged backwards into one or two sacs, the nasal sacs, which are either simple reservoirs, or glandular and mucus-secreting. In a species of Chinese Sole (Cynoglossus semilaevis) the two sacs, one from each olfactory organ, unite over the roof of the mouth in a common median sac, and in one unique specimen the latter communicated with the mouth by a large naso-pharyngeal aperture.

THE EYES.—In essential structure the eyes of Cyclostomes and Fishes resemble those of the higher Craniates. As a rule, in Fishes they are relatively larger, however, and the lens is globular and the cornea somewhat flatter. Ciliary processes and ciliary muscles are absent. As the eyes are nearly always lateral in position it is probable that monocular vision is the rule. In Teleosts and in Amia a "choroid gland," consisting of a mass of capillary blood-vessels, surrounds the optic nerve externally to the retina, and derives its blood from the efferent artery of the pseudobranch (Fig. 226). In most Teleostomi, but not in Cyclostomes, Elasmobranchs, and Dipnoi, there is a singular prolongation of the choroid coat, known as the "processus falciformis," which extends across the vitreous humour to the inner face of the lens, where it ends in an expansion, the "campanula Halleri" (Fig. 226). Accommodation to vision at different distances is not effected by alterations in the convexity of the lens, but by a change in its position with regard to the retina, apparently brought about by the contraction of a special retractor muscle. Some oceanic pelagic Teleosts are remarkable for their curious telescopic eyes in the shape of short protruding {394}cylinders, each terminating in a strongly convex cornea (Fig. 227). The eyes are directed either upwards or forwards, and, as their long axes are parallel in either position, it is probable that these Fishes are capable of binocular vision. In the young of certain Teleosts occurring in the Antarctic and Indian Oceans the large eyes are situated at the extremities of extraordinary long stalks extending from the sides of the head.

In the quasi-parasitic Cyclostome, Myxine, and in many Teleosts belonging to widely different families, which live at great depths in the sea or inhabit subterranean waters, the eyes suffer from disuse and degenerate in structure. The influence of a deep-sea habitat on the eyes of Fishes is somewhat varied. The eyes are often small. A few abyssal Fishes are totally blind, and no external trace of eyes can be seen (Fig. 430). In such Fishes compensation is often afforded by an extraordinary development of tactile organs in the form of long barbels, or of trailing filaments derived from the median or the paired fins (Fig. 371, B). Many deep-sea forms possess eyes of the normal size, or even exceptionally large eyes, probably because either they occasionally migrate towards the surface, or else they possess phosphorescent organs and are able to see by the aid of the light they themselves emit. A blind Siluroid (Amiurus nigrilabris) frequents the cave streams of Pennsylvania, and many members of the same family which live in muddy waters have very small or even minute eyes. One of the Gobies (Typhlogobius), which buries itself in the sand, or is found under stones in the holes of a burrowing Crab on the coast of California, is also {395}blind. Amongst other blind Fishes Amblyopsis and Typhlichthys (Amblyopsidae) and Lucifuga (Zoarcidae) may be mentioned, the first two inhabiting the cave streams of North America, while the third has a similar habitat in Cuba. When the eyes degenerate they dwindle in size and recede from the surface. The lens and the iris wholly or partially disappear, and although it is generally recognisable the retina loses certain of its characteristic layers, or the latter are but imperfectly formed. In Myxine even the eye-muscles are absent.

The eyelids of Fishes are little more than marginal folds of skin, capable of little if any movement, and leave the eyes largely uncovered. Some Sharks have a third eyelid or "membrana nictitans" at the anterior corner of the eye. Lachrymal glands are unknown.

THE PARIETAL EYE.—It is only in the Cyclostomes that this structure can have any claim to be regarded as a visual organ. In the Lamprey (Fig. 228) the parietal eye is a slightly flattened vesicle lying directly over the pineal vesicle, and connected by a slender stalk or nerve with the right ganglion habenulae. The dorsal or more external half of the vesicle is bi-convex, and forms the "pellucida," while the inner half or retina is said to consist of supporting cells with interspersed deeply pigmented sense-cells and ganglion cells. The external skin over the parietal eye is partially transparent in the living animal.

In many of the oldest known Fishes, such as the Ostracodermi, the Antiarchi, and the Crossopterygian Osteolepida, there are indications of the existence either of one or of two median sense-organs on the upper surface of the skull, in the shape of one or two foramina, or hollow protuberances, or pit-like grooves or {396}depressions, but, as a rule, when one of them is present the other is absent. It is probable that both these structures were associated with sensory organs, of which one may have been a parietal eye and the other a pineal eye. Some Teleosts (e.g. many deep-sea Scopelidae) have a transparent, convex, cornea-like prominence on the upper surface of the head which may be related to one of these singular organs.

{397}CHAPTER XV

THE KIDNEYS AND THE REPRODUCTIVE ORGANS—BREEDING

The kidneys and the reproductive organs are so intimately connected that it is necessary to deal with them together. Both organs are specialised portions of the coelom and its epithelial lining. The KIDNEYS are essentially a series of tubular and at first segmentally-disposed outgrowths from the coelom (urocoeles) which acquire a connexion with the exterior, while the gonads have their origin from local modifications of the coelomic epithelium. At a very early embryonic stage each lateral half of the coelom presents three well-marked divisions: (1) a series of dorsal portions ("myocoeles"), the cavities of the myotomes or muscle-segments; (2) a longitudinally continuous unsegmented portion extending round the alimentary canal, the "ventral coelom"; and (3) a series of intermediate tubular portions or "nephrotomes," each of which leads from a myocoele to the ventral coelom (Fig. 229, A). The essential components of the kidneys, the urocoeles or renal tubules, are derived from the nephrotomes. In its typical condition each kidney consists of three portions, which, in accordance with their embryological and evolutionary sequence, are termed the "pronephros," the "mesonephros," and the "metanephros." The pronephros, the larval or provisional kidney, is formed from a limited number of the nephrotomes immediately behind the head. From each nephrotome a hollow tubular outgrowth is formed, which grows towards the lateral surface of the body, and then unites with its fellows of the same side to form a main longitudinal duct—the "archinephric" or "pronephric duct" (Fig. 229, A, Fig. 230, A). This duct grows backwards until it opens into the cloaca.

{398}[Illustration: FIG. 229.—Diagrammatic transverse sections through an embryo Craniate to show the mode of development of the pronephros (A) and of the mesonephros (B). The right side of each figure shows an earlier stage than the left. In B (left side) the connexion of a vas efferens with a mesonephric tubule, and the division of the archinephric duct into Müllerian and mesonephric ducts are shown, a, Aorta; a.c, alimentary canal; a.d, archinephric duct; g, glomus; gl, glomerulus; i.n, inner nephrostome; mb, Malpighian body; md, Müllerian duct; mnd, mesonephric duct; mnt, mesonephric tubule; myc, myocoele; myt, myotome; n, notochord; np, nephrotome; nt, nephrostome; o.n, outer nephrostome; pn.t, pronephric tubule; s.c, spinal cord; t, testes; v.c, ventral coelom; v.ef, vas efferens. (After Kingsley and Semon.)]

At the same time the nephrotomes lose their connexion with the myocoeles, although they still retain their "nephrostomes" or apertures through which they communicate with the ventral coelom. When fully developed the pronephros consists of a few tubules, more or less convoluted, opening at their inner extremities into the coelom by means of their ciliated nephrostomes, and at their outer ends communicating with the exterior through the archinephric duct. In relation with the pronephros a branch from the dorsal aorta forms a tuft of capillary blood-vessels or "glomus," opposite the nephrostomes, which projects into the ventral coelom on each side. Later, a second series of much more numerous tubules is formed behind the pronephros, which constitute the mesonephros. In forming mesonephric tubules the nephrotomes become disconnected from the myotomes and their myocoeles, and curving outwards they {399}come to open into the archinephric duct, although they do not in any way contribute to its formation (Fig. 229, B). Segmentally-arranged twigs from the dorsal aorta end in tufts of capillaries or glomeruli, each of which projects into a small sac-like enlargement of a mesonephric tubule, pushing before it the wall of the sac. In this way a double-walled "Malpighian body," containing a "glomerulus," is formed in connexion with each tubule. Subsequently, the mesonephric tubules increase in number by budding. New nephrostomes and Malpighian bodies are developed on the secondary branches, and the original segmental arrangement of the tubules becomes obscured. With the growth of new tubules, and the formation of blood-vessels and of connective and lymphoid tissues between them, each mesonephros finally assumes the condition of a compact gland imbedded in the dorsal wall of the coelom, with its ventral surface invested by the peritoneum. A "metanephros," which in the higher Vertebrates replaces the mesonephros as the functional kidney, is perhaps not represented in Fishes.

A more or less well-developed pronephros is present in the embryos or larvae of the Cyclostomes and of all Fishes, but as a rule it completely disappears at an early period and is replaced by the mesonephros. It is retained throughout life, however, in the Myxinoid Cyclostomes (Fig. 230, B), and has its persistent nephrostomes opening into the pericardial cavity. In a few Teleosts the pronephros is also persistent, as in Fierasfer and Dactylopterus, and in others the organ may not completely disappear until the approach of sexual maturity. But with these exceptions the mesonephros is the sole functional kidney in the adults of the Cyclostomes and of all Fishes. As regards the nature of the duct by which the excretion of the mesonephros is conveyed outwards, there are notable differences in different Craniates. The Cyclostomes and the Teleostomi retain that part of the archinephric duct into which the mesonephric tubules open, and which remains after the atrophy of the pronephros (Fig. 230, B, E, F). In Elasmobranchs, and probably also in the Dipnoi, a special mesonephric duct is developed in a way which will be described later (Fig. 230, C, D).

{400}[Illustration: FIG. 230.—Showing the principal modifications of the kidneys and reproductive organs in Cyclostomes and Fishes. A, The pronephros and its duct in the embryo; B, the kidneys and genital pores in Petromyzon, the vestigial pronephros represented as in Myxine; C and D, the urinogenital organs of a male and female Elasmobranch; E, of a male or female Teleost, or a male Lepidosteus; F, of a female Polypterus, Acipenser, Amia, or Osmerus. a, Anus; a.d, archinephric duct; c, cloaca; c.a, the coelomic aperture of the Müllerian duct; c.p, cutaneous pit; g, gonad; gd, gonoduct; g.p, genital pore; i, intestine; m, Malpighian body; m.d, Müllerian duct; mn, mesonephros; mn^1, vestigial mesonephros; mn^2, excretory portion of the mesonephros ("metanephros"); mn^3, genital portion of mesonephros; mn.d, mesonephric duct; mtn.d, metanephric duct; n, nephrostome; ov, ovary; p.a, abdominal pore; p.f, peritoneal funnel; pn, pronephros; pn′, vestigial pronephros; s.g, shell gland; s.s, sperm sac; t, testis; ug.s, u.s, urinogenital sinus; v.ef, vasa efferentia; v.s, vesicula seminalis.]

{401}In the males of Elasmobranchs some of the hinder mesonephric tubules unite to form a single main duct opening into the terminal part of the mesonephric duct, and these tubules and their separate duct are sometimes regarded as a metanephros and a metanephric duct. The mesonephric nephrostomes are persistent throughout life in a few Elasmobranchs (e.g. Notidanidae, Heterodontidae, Rhinidae, and some Scylliidae), and also in Amia: in all other Fishes as well as in the Cyclostomes they become closed in early life.

In many Fishes the hinder extremity of the coelom communicates directly with the exterior through "abdominal pores," of which there is usually a pair, rarely a single pore, situated close to the cloacal or the anal aperture. Elasmobranchs usually have a pair, often at the extremities of a pair of cloacal papillae (Fig. 231), but they are absent in some families (e.g. Heterodontidae and Rhinidae); and in some Scylliidae (e.g. Scyllium canicula) they are very variable, being either present or absent on both sides, or an open pore is present on one side only. Pores are present and paired in the Crossopterygii, the Chondrostei, and the Holostei. Amongst the Dipnoi Neoceratodus has a pair of pores. Protopterus sometimes has two pores opening into the cloaca, but as a rule the two become confluent and have a single external aperture. In Lepidosiren pores are wanting. Abdominal pores are rarely present in Teleostei. They exist, however, in the Mormyridae (Gymnarchus and several species of Mormyrus), and also in the {402}Salmonidae, where they are as singularly variable in different species and individuals as in the Elasmobranch Scylliidae. The use of abdominal pores is not certainly known, unless the coelom of those Fishes which possess them continues to retain some measure of its primitive excretory function, and the pores act as excretory ducts. That the nephrostomes are excretory organs has been shown by experiment, and it is worthy of note that there exists a reciprocal relation between these structures and abdominal pores, to the extent that while there are a few Fishes (e.g. certain Elasmobranchs and Amia) in which both coexist, there are many others in which the presence of nephrostomes is correlated with the absence of pores and vice versâ.

The male and female GONADS, testes and ovaries, are derived from the coelomic epithelium near the inner or median aspect of the nephrotomes (Fig. 229, B). Here the epithelium remains columnar, and soon projects into the ventral coelom as a continuous longitudinal ridge. It is probable that at first the modified epithelium is segmented as a series of "gonotomes," but if so, the latter must soon coalesce into a continuous ridge. Some of the epithelial cells enlarge to form the primitive sex-cells. In the development of an ovary, portions of the epithelium sink inwards, carrying with them the primitive ova. Certain of the cells form the epithelial walls of a number of ovisacs, each of which encloses an ovum. As the ovisacs increase in number and size the germinal ridges project more and more into the coelom until, as ripe ovaries, they become suspended from its dorsal wall by a double peritoneal fold, the "mesovarium" (Fig. 156). The testes develop in a similar fashion except that the primitive sex-cells, which later give rise to spermatozoa, form the lining of a number of simple or ampulla-like tubules, the seminiferous tubules, and the suspensory fold is termed the "mesorchium."

The Cyclostomes have gonads in the shape of unpaired organs extending nearly the whole length of the coelom, but in all Fishes the organs are primarily paired, although by fusion, or by the absorption of one gonad, the ovaries or the testes sometimes appear as if single. The ovaries may either be naked, as in Elasmobranchs, Dipnoi, Crossopterygii, and Chondrostei, and in Amia amongst the Holostei; or, as in Lepidosteus and most Teleosts, they become enclosed in coelomic sacs. The {403}former, or "gymnoarian," condition is primitive; the latter, or "cystoarian," is secondary, and is brought about by the growth of two peritoneal folds round the ovary and the union of their margins. Into these coelomic sacs the egg-bearing or real ovarian tissue projects either in the form of processes or of transversely- or longitudinally-arranged plates or folds (Fig. 232, B). The testes are composed of seminal ampullae, as in Elasmobranchs, or of radially-arranged and sometimes plexiform tubules opening into the gonoduct, as in nearly all other Fishes (Fig. 232, A).

In the Cyclostomes (e.g. Petromyzon) the eggs and spermatozoa are discharged from the gonads into the coelom, whence they reach the exterior through a pair of "genital pores" leading from the hinder end of the coelom into a urinogenital sinus formed by the united extremities of the two archinephric ducts. Myxine has, however, but a single median pore, opening into an integumentary cloaca, which also receives the rectal and urinary orifices. Bdellostoma has two such pores communicating with a similar cloaca.

{404}The nature and homologies of the genital ducts in the different groups of Fishes are amongst the most puzzling of the many problems which vex the soul of the Vertebrate morphologist, and although there is a fairly general agreement on some points, there are others of great importance of which it may be said quot homines, tot sententiae.

Broadly speaking, there are two types of genital ducts in Fishes: (1) those which are obviously derived from some part of the kidney system; and (2) those which are special ducts and appear to have no connexion with kidney-ducts.

The Elasmobranchs offer a typical example of gonoducts of the first kind. At an early embryonic period in both sexes each archinephric duct becomes longitudinally split into two ducts, of which one continues to receive the openings of the mesonephric tubules and remains as a mesonephric duct (Fig. 229, B). The other, which has no connexion with the mesonephros, opens anteriorly into the coelom by means of the united nephrostomes of the pronephros, and is known as the "Müllerian duct" (Fig. 230, C and D). In the adult male the Müllerian ducts are useless vestiges, but in the female they persist and act as oviducts, receiving the eggs set free from the ovarian ovisacs through their coelomic apertures, and thence conveying them to the cloaca. In the male, certain of the anterior mesonephric tubules become connected with the testicular ampullae by means of a network of slender tubules, the "vasa efferentia" or testicular network, and through the latter the spermatozoa pass from the testes to the mesonephric duct (Fig. 230, C). Consequently, the mesonephric duct conveys both spermatozoa and the kidney excretion to the cloaca. It is obvious, therefore, that both the male and female gonoducts are derived from kidney-ducts.

The Teleostei afford an equally typical illustration of the second type. Each female gonoduct (oviduct) is formed by a backward growth of the same two peritoneal folds which enclose the ovary; these are converted into a "peritoneal tube" or canal by the union of their margins. The male gonoducts are also formed in continuity with the testes, that is, as backward prolongations from the latter. Each duct, male or {405}female, seems to be a duct sui generis and to have no connexion whatever with the kidney system (Fig. 230, E). In the Salmonidae, Anguillidae, Galaxiidae, Hyodontidae, Notopteridae, and Osteoglossidae, and also in Misgurnus, the oviducts lose their continuity with ovaries and degenerate to an extent which differs greatly in different families. Thus in some Salmonidae, as in the Smelt (Osmerus eperlanus), the oviducts end anteriorly in wide funnel-like coelomic apertures after the fashion of Müllerian ducts, and do not embrace the ovaries: hence the ovaries are naked and not cystoarian, and their ducts are not peritoneal tubes but "peritoneal funnels" (Fig. 230, F). In other Salmonidae and in the Anguillidae the oviducts appear to have so far degenerated that they are represented either by a pair of very short funnels or by a pair of genital pores, which, as in the Salmon, have a common external aperture behind the anus and in front of the single orifice of the united archinephric ducts (Fig. 233, A). In all such instances the eggs are set free from the ovaries into the coelom, from whence they escape through the peritoneal funnels or genital pores. In the Eels the male gonoducts also degenerate, and, losing all connexion with the testes, they become reduced to genital pores as in the female.

The Holocephali and probably the Dipnoi conform to the Elasmobranch type in the nature of their male and female gonoducts. In the Crossopterygii each testis has its own proper duct, which has no connexion with the kidney system and apparently belongs to the Teleostean type, while the oviduct, which is almost certainly not a Müllerian duct, is probably a peritoneal funnel. On the other hand, the Chondrostei and the Holostei are in the interesting transitional condition of possessing male ducts of the Elasmobranch type and female ducts of the Teleostean type, the latter being either ducts directly continuous with the ovaries, as in Lepidosteus, or of the nature of peritoneal funnels, as in Acipenser, Polyodon, and Amia (Fig. 230, E and F).

How far the distinction between the two types of gonoduct holds good in the case of the male is not quite clear, and it has recently been argued that the Dipnoi offer a connecting link between the two.

{406}[Illustration: FIG. 233.—Diagram to show the kidneys and gonoducts of a female Salmon (A), and of a male Protopterus (B). md^1 and md^2, Anterior and posterior vestiges of the Müllerian duct; t.t, tubular posterior portion of the testis (t). Other reference letters as in Fig. 230. (B, after Graham Kerr.)]

In Protopterus each testis is divided into an anterior sperm-producing part and a posterior tubular portion which has lost the capacity of producing sex-cells. The testicular network is greatly reduced, and forms but a limited connexion between the tubular portion of the testes and the mesonephric duct (Fig. 233, B). If it be supposed that the testicular network became still further reduced so that the connexion between the testes and the kidney-duct took place directly through a single channel instead of through several, the result would be a gonoduct essentially similar to the male duct of an ordinary Teleost. Should this view prove to be correct, it will follow that the male gonoducts of all Fishes are differently-modified examples of the Elasmobranch type. But there will still remain the female gonoducts of Ganoids and Teleosts, which must be regarded as distinct from Müllerian ducts unless it can be shown that their different methods of development are not necessarily fatal to their homology with Müllerian ducts, or that both types of gonoduct can be derived from some intermediate type. Assuming that some Fishes do possess male or female ducts which have not been derived from the kidney system, but have been independently acquired, there is still the question, which of the two types is the more primitive, or, in other words, has the Elasmobranch type superseded the Teleostean, or vice {407}versa? To this question no decisive answer can at present be given.

The terminal relations of the kidney-ducts and the gonoducts, and the presence of accessory or of vestigial organs in connexion with them, will now be briefly dealt with. In the males of the Elasmobranchs the mesonephric ducts which, as already pointed out, act both as kidney-ducts and gonoducts, dilate posteriorly to form a pair of vesiculae seminales, and then unite to form a urinogenital sinus, opening into the cloaca at the extremity of a median papilla (Fig. 230, C). The sinus also receives ducts from the hinder part of the mesonephros, either separately, as in the female, or by a common duct on each side—the so-called metanephric duct—as in the male. Two tubular caecal outgrowths from the sinus form two sperm sacs. Only the anterior portions of the Müllerian ducts with their coelomic apertures are retained in the adult. In the female the mesonephric ducts are purely excretory, but otherwise they are similar, and the oviducts (Müllerian ducts) open into the cloaca separately or by a common orifice (Fig. 230, D). A glandular dilatation of each oviduct forms the oviducal or shell gland by which the horny egg-cases are secreted. In the males of the Holocephali the gonoducts open into a urinogenital sinus with an external orifice distinct from and behind the anus; but the female has separate apertures for the rectum, the conjoined oviducts, and the united mesonephric ducts. Both sexes have complete Müllerian ducts communicating with the coelom in front, and behind with the exterior. The Dipnoi of both sexes essentially resemble the Elasmobranchs in the general relations of their ducts, but the Müllerian ducts of the male exhibit marked differences in the three genera. In Neoceratodus the ducts are as complete as their functional representatives in the female. Protopterus retains anterior vestiges and the coelomic apertures, and also vestiges of the hinder portions which unite and end blindly in the urinogenital papilla, but the middle sections of the two ducts are suppressed (Fig. 233, B). In the Teleostomi there is a general similarity in the terminal relations of the gonoducts and kidney-ducts. In the Ganoids the archinephric ducts unite and then expand into a urinary sinus or bladder, and the gonoducts of the female, or of both sexes in Lepidosteus, open either into the archinephric {408}ducts or into the common sinus, and therefore both ducts communicate with the exterior by a urinogenital orifice behind the anus. Peritoneal funnels, similar to the functional oviducts of the female, are present in the males of the Chrondrostei and of Amia. In Teleosts the terminal connexions of the ducts tend to become less intimate. The archinephric ducts often dilate into a urinary bladder either before or after their union, and the common duct joins the united gonoducts to form a short urinogenital sinus which opens externally, or the confluent gonoducts have an independent genital orifice between the anus and the urinary aperture. Not rarely the genital or the urinogenital orifice is prolonged into a tubular papilla, which in the male acts as an intromittent organ, or, as in the females of the Cyprinoid Rhodeus amarus, the long oviducal tube serves the purpose of an ovipositor. The males and females of the Siluroid Plotosus have a remarkable vascular and glandular arborescent appendage just behind the urinogenital papilla, the use of which is unknown.

The EGGS of different Fishes exhibit considerable diversity in size and shape as well as in the nature of their external coverings and their mode of deposition. The size of the eggs largely depends on the quantity of food-yolk stored up in their substance for the nutrition of the embryo: hence the eggs of Elasmobranchs, which resemble Fowls' eggs in the superabundance of their yolk, are by far the largest. Teleostomi have much smaller eggs. The largest Teleostean ova are those which are heavy and sink (demersal ova); the smallest, those which are buoyant and float (pelagic ova). Of the former, the eggs of Gymnarchus are about 10 mm. in diameter; those of the Salmon about 5 mm.; and those of some species of Arius, 5 to 10 mm. The eggs of the Wolf-Fish (Anarrhichas lupus) are about 6 mm. Smaller demersal ova are those of the Lump-sucker (Cyclopterus) and Heterotis, which are 2.6 and 2.5 mm. respectively. Pelagic {409}eggs are very small, those of the Plaice, which are exceptionally large, varying from 1.65 to 1.95 mm.

An egg-cell consists of living protoplasm and a nucleus, a variable quantity of non-living food-yolk, and of certain enveloping and protective egg-membranes. The ova of Fishes differ principally in the amount and disposition of the food-yolk, in the character of the egg-membranes, and in the presence or absence of special perforations in the egg-membranes for the entrance of spermatozoa into the eggs. In the small ova of some of the lower Chordata (e.g. Amphioxus), where the very small quantity of food-yolk is uniformly distributed, and its presence affects all parts of the egg alike, the process of segmentation which follows fertilisation results in the transformation of the entire egg into a mass of approximately equal-sized cells or blastomeres (Fig. 82). {410}The eggs are therefore described as "alecithal," and the segmentation as being "holoblastic" and "equal." On the other hand, all Fishes possess "telolecithal" eggs, that is, ova in which the food-yolk is more or less abundant, and tends to accumulate at one pole of the egg ("vegetative pole"), while the opposite or "animal pole" consists of protoplasm, comparatively free from yolk granules and containing the nucleus (Fig. 234, A). The term telolecithal is, however, a somewhat comprehensive one, and covers important variations in the relations of the inert food-yolk and the living protoplasm in different Fishes, which greatly modify the process of segmentation. Thus there are some Fishes in which the amount of food-yolk at the vegetative pole is sufficient to retard segmentation in that part of the egg without actually preventing it, and consequently segmentation begins in the animal pole, and takes place more rapidly there than it does when it extends into the vegetative pole. Hence it follows that although the entire egg is segmented the blastomeres are of unequal size, the animal pole giving rise to a large number of small cells or micromeres, and the vegetative pole to a smaller number of much larger cells or macromeres. The segmentation of such an egg is said to be holoblastic but unequal (Fig. 234, B and C). This type of egg is characteristic of the Chondrostei, the Holostei, and the Dipnoi. In other Fishes, like the Elasmobranchs and the Teleostei, the food-yolk so greatly preponderates that it entirely prevents segmentation in the vegetative part of the egg, and segmentation is restricted to the small mass of protoplasm (germinal disc) at the animal pole, in which the nucleus is situated (Fig. 234, D). Eggs undergoing partial segmentation in this way are termed "meroblastic." No hard and fast line can be drawn between the two types, and in the Chondrostei and Holostei an interesting transition between the holoblastic and meroblastic ova may be observed. The egg-membranes are formed either by the egg itself or by the epithelium of the ovarian ovisacs, and, as will shortly be seen, the character of the outer egg-membrane greatly influences the mode of deposition of the eggs and their location afterwards. In Elasmobranchs the egg is enclosed in a stout horny egg-shell, secreted by the oviducal shell gland. In many Fishes, as in the Chondrostei, Holostei, and Teleostei, the egg-membranes {411}are perforated at the animal pole of the egg by a small aperture or "micropyle," which is only large enough to admit of the entrance of a single spermatozoon at a time (Fig. 235). Generally, there is only a single micropyle, but, according to Salensky, the Sturgeon (A. sturio) has from 3 to 9, and the Sterlet (A. ruthenus) from 5 to 13.

An important distinction may be made between the ova of different Teleostomi as regards their location after extrusion from the female. From this point of view two types of ova can be distinguished, demersal and pelagic ova. Demersal eggs are characterised by their larger size and greater weight, so that they always sink after extrusion; and by their opacity. They may either have an outer egg-membrane which is viscid and adhesive, so that the eggs readily adhere to one another or to foreign objects, or the membrane is smooth and non-adhesive. The Salmonidae, for example, produce non-adhesive demersal eggs, which remain separate after being deposited on the gravelly bed of a stream. Most freshwater and many marine shore Fishes have adhesive demersal eggs, which are deposited at the bottom of the water, generally adhering to one another in larger or smaller clumps, masses, or sheets, and attached to rocks, stones, or empty shells, like the eggs of many shore Fishes, or to aquatic plants after the fashion of the eggs of the Carp, Perch, and Pike, or even to branching zoophytes, as is the case with the eggs of the Sea-snail (Liparis). In some adhesive eggs the external egg-membrane forms threads for their attachment. The eggs of the Gar-Fish (Belone), and those of the Saury Pike (Scombresox) and of the Flying Fishes (Exocoetus), have viscid threads developed from opposite points on the surface, which are either attached to foreign objects or they become entangled with those of other eggs of the same species. The oval eggs of some of the Gobies have a bunch of fibres at one pole which serves to attach them. In the Smelt (Osmerus eperlanus) a portion of the outer egg-membrane breaks away from the rest and becomes turned back, inside out, but remains attached to the egg at one point. By means of this membrane the egg is attached to rocks or stones. Pelagic eggs are distinguished by their lightness and buoyancy, so that they always float near the surface of the water, and by their smaller size and remarkable transparency (Fig. 235). A conspicuous feature in many of them is the presence of a single {412}large oil globule on the surface of the yolk, and not infrequently the yolk becomes partially or completely broken up into small masses. Pelagic eggs are always non-adhesive and free, and they invariably belong to marine Fishes. Amongst the British food Fishes which produce pelagic ova may be mentioned the Gadidae (e.g. Cod, Whiting, Hake, Ling), the Pleuronectidae (e.g. Turbot, Brill, Sole, Plaice), Scombridae (e.g. Mackerel), Triglidae (e.g. the Gurnards), Percidae (e.g. the Bass), and Clupeidae like the Pilchard and Sprat, but not the Herring, whose adhesive demersal eggs are deposited in clumps on shingly banks in the sea at varying distances from the shore.

The eggs of Elasmobranchs are deposited singly or in pairs at considerable intervals, and the period of egg-laying is prolonged over a considerable part of the year. In most other Fishes, as in Teleosts, the period of spawning is limited to a few months, usually in the spring and summer in temperate latitudes; and in the case of a single Fish it may last only a few days or weeks, but the number of eggs produced is often enormous. Thus, in a Ling 61 inches long and weighing 54 pounds the ovaries contained 28,361,000 eggs. A Turbot, 17 pounds in weight, had 9,161,000 eggs; and a Cod of 21½ pounds 6,652,000. The least prolific of the British food Fishes is the Herring, in which the number of ovarian eggs varied from 21,000 to 47,000 in four specimens examined. The extraordinary fecundity of many Fishes seems to bear no relation to the relative abundance of the Fishes themselves, but rather it is to be associated with certain {413}disadvantages attendant on the sexual relations of Fishes, involving a considerable waste of the sex-cells, while in many Fishes it no doubt helps to compensate for any subsequent mortality among the larvae, which may result from an uncertain and precarious food supply and from the attacks of enemies. Whenever internal fertilisation is the rule, or when, as in nest-building and marsupial Fishes, the propinquity of the sexes in the breeding season ensures the fertilisation of a larger proportion of the eggs and the protection of the young, the number of eggs produced is small.

The male sex-cells or spermatozoa are essentially similar to those of other Vertebrates, although in different Fishes they may vary in such details as length, and the shape and size of the head, which may be rod-like and wavy, elliptical or globular.

As a rule, in Fishes females are more numerous than males, and generally they are larger, but to both statements there are notable exceptions. The relations of the sexes in the breeding season are usually very promiscuous, especially in those Teleosts which discharge their sex-cells while swimming together in shoals. A female may, however, consort with several males (polyandry), or a male with several females (polygamy); or, as in some of the nest-building Fishes (e.g. Gastrosteus), there are not wanting examples of the pairing of one male with one female (monogamy).

Fishes often migrate at the commencement of the breeding-season to localities most suitable for the deposition of the eggs. Many marine species seek banks or shallower water near the shore, and some, like the Salmon and the Sturgeon, are anadromous, and ascend rivers for long distances to deposit their spawn.

In all Fishes except the Elasmobranchs and a few Teleosts the fertilisation of the eggs takes place in the water after their extrusion, the male depositing its seminal fluid over the eggs or in their neighbourhood. The waste of the sex-cells is often, no doubt, very considerable, especially when the eggs are adhesive and fixed, and the seminal fluid is liable to drift at the mercy of tides and currents. With pelagic ova the waste is perhaps not so great, inasmuch as the eggs as well as the spermatozoa would probably drift at the same rate and in the same direction. Liability to waste must also be greatly diminished in many Fishes by their habit of living in shoals, or of congregating {414}together in the breeding season, in which they are sometimes aided by their power of emitting characteristic sounds, and in the case of nest-building Fishes by the still more intimate relations of the sexes. Even when the liability to waste is very great, compensation may be afforded by exceptional fecundity. The copulation of the sexes and the internal fertilisation of the eggs occur only in Elasmobranchs and some Teleosts. The copulatory organs of Elasmobranchs are the so-called "claspers" with which the males are provided. Some form of copulation is probably the rule in the viviparous Teleosts, where the eggs are fertilised in the oviducts, or even while they are still in the ovaries, and the young are born alive. As mentioned above, an intromittent organ is often formed by the prolongation of the genital or the urinogenital orifice into a papilla, or a longer or shorter tube. Some Cyprinodontidae (e.g. Anableps) have the anterior part of the anal fin modified in the male to form an intromittent organ along which the urinogenital canal runs (Fig. 374). In the females the genital aperture is covered by a special scale, which is free on one side and not on the other. "The male organ in some individuals is turned to the right, in others to the left, and in some females the opening beneath the special scale is to the right, in others to the left. Copulation thus takes place sideways, a left-sided male pairing with a right-sided female, and vice versa." The anal fin also forms an intromittent organ in the "Half-beak" (Hemirhamphus). In a genus (Girardinus) of the same family the anal fin is modified to form an apparatus for holding the female during sexual congress. The singular method of fertilisation practised by the males and females of Callichthys paleatus is referred to elsewhere.

With the exception of the pelagic Antennarius, which builds its nest in the Sargasso weed in mid-ocean, nest-building and parental solicitude for the young are confined to freshwater Fishes and to marine forms with demersal ova. Pelagic ova must necessarily be beyond the scope of parental care. As a rule it is the male which acts as guardian nurse, the female troubling herself but little about the fate of her eggs or her offspring.

{415}[Illustration: FIG. 236.—The Butter-Fish (Pholis gunnellus) coiling round a mass of eggs. (From Cunningham, after Holt.)]

Perhaps the more primitive form of parental foresight is exhibited by those Fishes which, like the females of the Salmonidae, make a furrow in the gravelly bottom of a running stream for the reception of the eggs, and then cover them over with a layer of gravel, or like the Siluroid Arius australis, of the Burnett river in Queensland, which deposits its eggs in circular excavations in the sandy bed of the river and covers them with layers of large stones. But in neither case does it appear that either the male or the female takes any further interest in the eggs or in the young when hatched. Without actual nest-building, or even the preparation of a place for their reception, the eggs may be protected in various ways by the male. The common British Gunnel or Butter-Fish (Pholis gunnellus) rolls its eggs into a rounded mass by coiling its body round them, the male and female taking possession of them alternately. The little clumps of eggs are then deposited in holes made by the boring Mollusc, Pholas. Some British Blennies attach their eggs in a single layer to the sides of cavities in rocks, or between stones, where they are watched over by the male parent. The eggs of the Lump-Sucker (Cyclopterus lumpus) are attached in masses to rocks or to piles and guarded by the male, who aerates them by keeping up a flow of water over the spawn through the action of his pectoral fins. When hatched, the young fry cling to the body of their watchful parent by their suckers. A more decided approach to nest-building is exhibited by the Sand Goby (Gobius minutus). In this species the male scoops out the sand from beneath an empty shell, generally that of a Pecten, and the female deposits her adhesive eggs on the under surface of the shell.

{416}[Illustration: FIG. 237.—Showing the embryos of Rhodeus amarus in the gill-cavities of Unio. e, Embryos; g, inter-lamellar cavities; i.l.j, an inter-lamellar junction. (From Olt.)]

The male remains on guard, and by the movements of its pectoral fins promotes the aeration of this rude form of nest. References to some of the more striking examples of true nest-building in Fishes will be found in the systematic part of this volume, especially in those chapters treating of the Dipnoi and Amiidae, and such Teleosts as the Mormyridae, Osteoglossidae, Siluridae, Gastrosteidae, Centrarchidae, Osphromenidae, Labridae, and Antennariidae. Other illustrations of parental care are to be found in the development of marsupial pouches or grooves for the reception of the eggs in the males of the Syngnathidae (Fig. 387) and the females of the Solenostomidae, and the use of the oral cavity for a similar purpose in the males, rarely in the females, of some Siluridae, and the males or females, according to the species, of the Cichlidae. The singular method by which the female Aspredo safeguards both her eggs and her progeny is referred to on p. 596. The Cyprinoid, Rhodeus amarus (the "Bitterling" of Central Europe), is unique in the means which it adopts to {417}secure the same result. By means of its long ovipositor the female Fish deposits its eggs in the mantle cavity of a Unio, or of an Anodon. Here they are fertilised by spermatozoa carried in through the inhalent siphon of the Mollusc with the inspiratory water current, and they complete their development in the gill-cavities (Fig. 237).

The time which elapses between the fertilisation of the egg and the hatching out of the young Fish varies greatly in different Teleosts. The eggs of some Clupeidae hatch in a very short time, two to three days in the Anchovy, and three to four days in the Sprat. In most of the British marine food Fishes the period rarely exceeds twelve to fourteen days. The larger demersal eggs with much food-yolk are longer in hatching; in the Salmon the time ranging from thirty-five to one hundred and forty-eight days. A low temperature lengthens the time. The eggs of the Herring which hatched in eight to nine days at a temperature of 52° to 58° F. took forty-seven days in water at 32° F.

The extent to which the development of the embryo proceeds while it is still enclosed in the egg-membranes, and consequently the condition of the embryo when hatched, depends largely but not exclusively on the quantity of food-yolk which is present in the egg and available for the nutrition of the embryo during its earlier stages. Embryos hatched from pelagic ova are very small and imperfectly developed. The mouth is usually not yet formed. The median fins, which later become isolated, are continuous, and the caudal fin is diphycercal, although it subsequently becomes homocercal after passing through a heterocercal stage. The blood is colourless, and even the gill-clefts may at first be lacking. In this condition the newly-hatched Fish is nourished at the expense of the residual food-yolk, which is enclosed in a yolk-sac projecting from the ventral surface of the body (Fig. 238). As the yolk is gradually used up the mouth is formed, and the young Fish feeds on the minute organisms of various kinds living in the sea, and by degrees the form, proportions, and structure of the more mature Fish are acquired. In the case of the larger demersal eggs the young are not only longer in hatching, but when hatched they are larger and more advanced in development. The young of many Fishes are {418}provided with larval or provisional organs, and they may be so unlike the adult in other respects that their subsequent development assumes the form of a more or less striking metamorphosis. As examples of larval organs, mention may be made of the adhesive or cement organs of the larval Chondrostei and Holostei, and of the Dipnoi (e.g. Protopterus and Lepidosiren), and also of a Teleost, probably the Mormyrid (Hyperopisus bebe, Lacép); the cutaneous gills of the Crossopterygii and some Dipnoi; the so-called external gills of such Teleosts as Cobitis, Gymnarchus (Fig. 239), and Heterotis, which are singularly like those of Elasmobranchs; and the defensive spines which are developed on the scales or scutes of the trunk, and the dermal bones of the skull, in the young of some Plectognathi. The most striking metamorphosis to be found in Fishes occurs in the Flat-Fishes and in the Eels, an account of which will be found in other parts of this volume (pp. 685, 602).

The only examples of viviparous Fishes occur in certain families of Elasmobranchs, and in five families of Teleosts, viz. the Blenniidae, the Cyprinodontidae, the Scorpaenidae, the Comephoridae, and the Embiotocidae. In the Teleosts mentioned the eggs are fertilised while they are still either in the ovarian ovisacs or in the cavity of the ovary, and their development may take place in either position. In such Cyprinodonts as Gambusia and Anableps the embryos are developed in the ovisacs, but as a rule both fertilisation and development occur in the ovarian cavity. During a prolonged gestation the young are nourished partly by the {419}food-yolk present in the eggs, and partly by a nutritive secretion derived from the ovarian walls or from the epithelial wall of the ovisacs as the case may be. In Anableps the secretion of the walls of the ovisacs is absorbed by papillae developed on the surface of the yolk-sac of the embryo along the course of its blood-vessels. The eggs of the Embiotocidae have little food-yolk, and the embryos are mainly nourished by the secretion of the ovarian walls, which is swallowed by the embryo and absorbed by villi on the inner surface of the intestine. The number of young produced varies considerably. In the Embiotocidae the ovarian cavity contains 40 to 50 young. The viviparous Scorpaenid, Sebastes norvegicus of Northern Europe, produces, it has been estimated, about 1000 young, while the Blenny (Zoarces viviparus), the only other European viviparous Teleost, produces from 20 to 300 or more, according to the size of the female. In the Blenny the eggs are hatched in about twenty days, but the young are not born until about four months after fertilisation, when they are about an inch and a half long, and in every outward respect similar to the adult Fish.

Besides the distinction between the sexes resulting from the different nature of their gonads and sex-cells, the males and females are often distinguished by secondary sexual characters {420}("sexual dimorphism"). As mentioned above, females are usually larger as well as more numerous than the males, although in one or both respects the reverse may be the case. Secondary sexual characters are best marked in Teleosts, where they are generally related to the special rôle which each sex takes in the deposition and fertilisation of the eggs, and the nurture and protection of the young, of which examples have already been given. To a more limited extent they may be associated with the struggle of the males for the females, and in at least a few Teleosts the exuberant coloration of the males in the breeding season suggests that instances of courtship and sexual selection are not altogether wanting.

Although the vast majority of Fishes are dioecious, instances of functional hermaphroditism are not unknown in a few Teleosts. Species of the Percoid genus Serranus (e.g. S. cabrilla, S. hepatus, and S. scriba) are invariably hermaphrodite and self-fertilising. Chrysophrys auratus is an example of successive hermaphroditism, the male and female sex-cells ripening alternately. As an occasional variation hermaphroditism has been recorded in several other Teleosts, including amongst others such well-known Fishes as the Cod, the Mackerel, and the Herring. The relations of the gonads in hermaphrodites is subject to much variation. In the Cod, for example, the testes may be double, each being continuous with the hinder end of the ovary of its side, or there may be only a single testis confluent with the anterior or the posterior portion, or with some other part of the surface, of either the right or left ovary. In other Teleosts individuals occasionally present themselves with a testis and an ovary on opposite sides.

{421}CHAPTER XVI

CYCLOSTOMATA (SYSTEMATIC)

CLASS I. CYCLOSTOMATA

The Cyclostomata, or, as they are sometimes called, the Marsipobranchii, from the pouch-like, or rather sac-like, shape of their branchial clefts, are divided into two orders, the first comprising the "Hag-Fishes" or "Borers," and the second the Lampreys.

ORDER I. MYXINOIDES.

The Hag-Fishes are probably the most primitive of all existing Craniates. The mouth is nearly terminal, and there is no buccal funnel. The naso-pituitary involution communicates behind with the oral cavity and functions as a channel for the in-streaming water-current to the gills. Four pairs of short tentacles, supported by a special tentacular skeleton, are present in relation with the mouth and the terminally-placed naso-pituitary orifice. The gill-sacs open directly into the pharynx. The branchial basket is but feebly developed, and at the most it is only represented by small isolated cartilages in relation with the external branchial apertures. The lingual apparatus is remarkably developed. Besides the lingual teeth there is only a single dorsal tooth in the roof of the mouth. The dorsal arcualia are restricted to the tail, or they extend for a short distance only into the trunk. A spiral valve is absent. There is a row of mucus-secreting sacs along each side of the body. The brain has no obvious cerebral hemispheres, nor a cerebellum. Only one semicircular canal is present in the auditory organ. The eyes are degenerate, and the usual eye-muscles with the cranial nerves {422}supplying them have atrophied. The embryonic pronephros is retained in the adult. The eggs are large; segmentation is meroblastic; and development is direct, without a larval metamorphosis. Two families can be distinguished.

FAM. 1. MYXINIDAE.—Gill-sacs not exceeding six pairs, with a common external aperture on each side of the body.

The family includes a single genus, Myxine, of which the common Hag (M. glutinosa) from the North Atlantic is the best known species (Figs. 92, A, and 240). This Hag-Fish occurs off the coasts of Northern Europe, including the British Isles, as well as on the Atlantic sea-board of North America, southwards to Cape Cod. Other species are found off the coasts of Chili and Japan. Myxine is quasi-parasitic in its habits, boring its way into the bodies of large Fishes. By means of its rasping "tongue" it devours all the soft parts of its prey, leaving little more than a mere shell of skin and bones. The Fishes usually attacked are the Cod and other Gadoids, but the Sturgeon is not immune, and the presence of a Hag in the abdominal cavity of a Shark (Lamna cornubica) has been recorded. Myxine has the reputation of being very destructive to Fishes caught on lines, and it is said that whole "catches" have been destroyed by its depredations, so that North Sea fishermen have been forced to change their fishing-ground. To what extent the Hags attack Fishes which are living and free is somewhat uncertain, but the little evidence obtainable seems to point to the conclusion that, as a rule, they only prey on Fishes when the latter are hooked or netted, or injured or dead. When not seeking food the Hag lives {423}in the mud of the sea-bottom at depths ranging to nearly 350 fathoms. They are able to swim very rapidly in an undulatory eel-like fashion. M. glutinosa may grow to a length of nearly two feet. The Hag has been described as a protandrous hermaphrodite, that is, it is first a male and then a female, the gonad of the young first producing spermatozoa, and at a later period becoming an ovary and giving rise to eggs. This view has hitherto met with general acceptance, but it has recently been urged with some force that the presence of the two kinds of sex-cells in a young animal is no proof of functional hermaphroditism, since it is not uncommon "to find immature eggs in the testis of many Vertebrates (Teleosts, Petromyzon, Amphibia), where the assumption of hermaphroditism, to say nothing of its protandric form, is entirely unwarranted." Myxine produces eggs similar to those of Bdellostoma. Nothing is known of its breeding habits, or of its embryology.

FAM. 2. BDELLOSTOMATIDAE.—Gill-sacs 6-14 pairs, all with separate external orifices. Bdellostoma (Fig. 92, B) is found on the Pacific sea-board of both North and South America, at the Cape of Good Hope, and on the coasts of New Zealand. The numerical variation of the gill-sacs in different species, and in different individuals of the same species, and even on opposite sides of the same individual, is very remarkable. Out of 354 examples of the Californian species (B. stouti) examined by Dr. Ayres, 101 had 11 gill-sacs on each side; 26 had 11 on one side and 12 on the other; 208 had 12 on each side; 11 had 12 on one side and 13 on the other; and 8 had 13 on each side. Occasional specimens may have 14 gill-sacs on each side. The variations are apparently quite independent of size, age, or sex; and when the gill-sacs are asymmetrically developed, the additional sac may be either on the right side or on the left. In the Chilian species there are 10 gill-sacs on each side, but in the species from the Cape of Good Hope the number is reduced to 6 or 7. Bdellostoma closely resembles Myxine in its habits and mode of feeding. The Californian species attaches itself to the gills or to the isthmus of large Fishes, and then rapidly bores its way into the body, devouring the viscera and muscles but leaving the skin intact. It usually attacks large {424}Flounders and species of Sebastodes, and it is especially destructive to Fishes taken in gill-nets. At Monterey every net in the summer contains the empty shells of eviscerated Fishes, and when these are taken out of the water the Hag scrambles out with great alacrity. Large fishes of even 30 pounds weight are often captured without either flesh or viscera, and it cannot be supposed that they entered the net in this condition. The species lives on the sea-bottom most abundantly at a depth of 10-20 fathoms, but becomes rarer as the water deepens or becomes shallower.

The eggs of the Californian Bdellostoma are large, varying in size from 14.3-29 mm. in length, and from 6.8-10.5 mm. in width, and each egg is enclosed in a horny egg-case secreted by the epithelium of its ovarian ovisac (Fig. 241). At each pole of the egg-case there is a tuft of numerous horny filaments which end in 2- 3- or 4-hooked, anchor-like extremities. In the centre of the tuft of filaments at the animal pole of the egg the egg-case is perforated by a micropyle, and a little below this {425}point the case is encircled by an opercular groove, which enables the polar portion to be thrown off like a cap at the time of hatching, so as to allow the young Bdellostoma to make its escape. The large size of the egg, which almost completely fills the cavity of the egg-case, is due to the fact that it consists mainly of food yolk, the germinal protoplasm containing the nucleus forming only a small hillock near the inner extremity of the micropyle. Bdellostoma spawns during the greater part of the year, but chiefly in the early summer, and probably about 20 eggs are deposited at one time, generally on a shelly or rocky bottom. After deposition the eggs become connected together in long chains or clusters by the interlocking of their polar hooks. Fertilisation takes place after extrusion, and the segmentation is meroblastic and discoidal, much as in Teleosts. The embryo completes its development within the egg, and when hatched it is a miniature of the adult.

ORDER II. PETROMYZONTES.

In the Lampreys there is a large suctorial buccal funnel leading behind and above into the mouth, which is supported by special cartilages, and furnished with a marginal fringe of small cirri. Numerous horny teeth are present on the inner surface of the funnel as well as on the tongue. The naso-pituitary involution forms a caecum and does not communicate with the mouth. The gill-sacs, seven in number, open externally by separate orifices, but internally they open into a median branchial canal, situated below the oesophagus and opening into the mouth in front. There is a well-developed branchial basket. Dorsal arcualia are present throughout the precaudal as well as in the caudal region. A rudimentary spiral valve is present. The brain consists of parts usually present in other Craniates, including cerebral hemispheres and a cerebellum. The auditory organ {426}has two semicircular canals, and the eyes are not degenerate. The pronephros is suppressed in the adult. The eggs are small; the segmentation is holoblastic; and there is a larval metamorphosis. There is but one family.

FAM. 1. PETROMYZONTIDAE.—The family has a nearly world-wide distribution. Most Lampreys are marine, although to a greater extent in some species than in others, but all of them seem to ascend rivers for spawning. The genus Petromyzon is characteristic of the northern hemisphere, where it is represented by various species on the coasts and in the rivers of Europe, West Africa, Japan, and North America. Three species, widely distributed in Europe, occur in the British Isles, viz.:—the Sea-Lamprey (Petromyzon marinus), which may reach or even exceed three feet in length, and is also found on the west coast of Africa and on the Atlantic coast of North America; the "Lampern" or fresh-water Lamprey (P. fluviatilis), about 18 inches long; and the Sand-Pride, Sand-Piper, or lesser freshwater Lamprey (P. planeri), usually less than a foot in length. Ichthyomyzon, Bathymyzon, Entersphenus, and Lampetra are also northern forms, collectively distributed along the Atlantic and Pacific coasts and in the rivers and great lakes of North America. Other Lampreys occur only in the southern hemisphere. Geotria is common in the rivers of Chili, Australia, and New Zealand; and another genus, Mordacia, has a parallel distribution, being found on the coasts of Chili and Tasmania. A new genus and species from Chili has been recently described under the name of Macrophthalmia chilensis. This Lamprey, which is only 107 mm. in length, has remarkably large eyes (2.5 mm. in diameter), vertically compressed gill-clefts, and a simple dentition resembling that of Myxine. All Lampreys are carnivorous. They feed by attaching themselves to the bodies of Fishes by their suctoral buccal funnels, and then rasping off the flesh with their lingual teeth. While thus engaged they are carried about by their victims. Salmon have been captured in the Rhone with the marine Lamprey attached to them. The Lamprey usually keeps near the bottom, either swimming with a graceful serpentine movement, or attached to stones by the buccal funnel.

{427}[Illustration: FIG. 243.—Spawning of the Brook-Lamprey (P. wilderi). On the right side of the figure a male is attached to the head of a female. (From Bashford Dean and F. B. Sumner.)]

In the spring the Sea-Lamprey ascends the rivers to spawn, and, after depositing its eggs in furrows which it excavates in the river-bottom, it returns to the sea. The river-Lampreys spawn in the smaller streams and brooks. The North American Brook-Lamprey, Petromyzon (Lampetra) wilderi, which is found in the neighbourhood of New York, deposits its eggs on the gravelly bottom of a brook, in a small gravel-filled hole lying between a number of large rounded stones (Fig. 243). In the vicinity of the "nest" some ten to twelve Lampreys congregate, the males, however, being much more numerous (five to one) than the females.

{428}[Illustration: FIG. 244.—Head of the Ammocoetes of P. fluviatilis. A, ventral view; B, side view. br.1, First branchial aperture; eye, eye; l.l, lower lip; na.ap, naso-pituitary aperture; u.l, upper lip. (From Parker and Haswell, after W. K. Parker.)]

Much energy is spent by both sexes in moving stones by lifting them with the buccal funnel, but it is not always clear that this is done to circumscribe the nest, or to remove impeding obstacles. Eventually, a male attaches himself to the back of the head of a female, who at the same time is holding fast to a stone. The male then rotates its body so that the urino-genital papilla is brought near the genital orifice of the female, and the simultaneous extrusion of eggs and spermatozoa at once follows. Owing to the small amount of food-yolk which they contain the eggs of the Lamprey (e.g. P. planeri) are small, measuring about 1.1-1.2 mm. in length, and from 0.9-1.0 mm. in width. There is a micropyle at the animal pole of the egg, but the characteristic horny egg-case and the polar hooks of the Myxinoids are both wanting. The embryo hatches out as a larva known as the "Ammocoetes." At this stage of its development the larva lacks several of the most striking features which characterise the adult, and it is highly probable that the Ammocoetes represents a stage in the evolution of Vertebrates in some respects intermediate between Amphioxus and a very primitive Craniate. The mouth of Ammocoetes is bounded laterally and in front by a curious hood-like upper lip, and behind by a short transverse lower lip (Fig. 244). The eyes are deeply seated and rudimentary, and as visual organs they are useless, but the parietal eye is well developed. As in the adult, there are seven pairs of gill-sacs, but they open internally into a pharynx, directly continuous behind with the rest of the alimentary canal, and there is no dorsal oesophagus. Like the skull, the branchial basket is still very rudimentary. The dorsal and caudal fins are {429}continuous. A gall-bladder is present, and also a bile duct opening into the gut. In its mode of life, and especially in the manner in which it obtains its food, Ammocoetes presents a most remarkable resemblance to Amphioxus and the Ascidians. In the median line of the pharyngeal floor there is an open groove, the hypopharyngeal groove or endostyle, and a tract of ciliated cells along the dorsal wall represents a hyperpharyngeal groove: connecting the two in front there is a peripharyngeal ciliated groove. The Ammocoetes feeds on small food particles carried through the mouth into the pharynx by currents of water produced by ciliary action. The food becomes entangled in strings of mucus probably secreted by the cells lining the endostylar groove. The mucus is then swept upwards in the pharyngeal groove, and finally wafted backwards to the stomach and intestine by the cilia of the hyperpharyngeal band. The skin exhibits the remarkable peculiarity of containing a peptic ferment capable of digesting proteids in a .2 per cent solution of hydrochloric acid. As the larva lives buried in the mud, the epidermic secretion probably helps to keep the skin free from bacteria, microscopic spores, and fungoid, or other parasitic growths. The young Lamprey lives as an Ammocoetes from 3-4 years, and then in the course of a few weeks in the winter it undergoes a metamorphosis, losing its larval characters and acquiring the structure and habits of the adult. During this period the buccal funnel is completed and teeth are developed. The eyes approach the surface and become functional. The continuity of the median fins becomes interrupted. The endostylar groove becomes transformed into a thyroid gland, the gall-bladder disappears, and the bile duct becomes obliterated and changed into a mass of small follicles. The skull and branchial basket complete their development. At the same time the pharynx loses its connection with the rest of the alimentary canal and remains as the branchial canal. The so-called oesophagus of the adult is apparently a new formation which grows forwards and acquires a connection with the mouth. It is probable that it represents a hyperpharyngeal groove constricted off from the dorsal wall of the pharynx.

Both the marine Lamprey and the "Lampern" are captured {430}for food, either by nets or wicker traps. Formerly the Lampern was taken in enormous numbers in several British and Irish rivers, especially in the Severn from February to May, and in the Thames during May and June, but for various reasons the supply has much diminished in recent years. The Lampern makes excellent bait for Cod and Turbot, and for this purpose large numbers used to be taken in the Trent and Thames for despatch to Grimsby and other fishing ports.

{431}CHAPTER XVII

ELASMOBRANCHII: GENERAL CHARACTERS—PLEUROPTERYGII—ICHTHYOTOMI—ACANTHODEI— PLAGIOSTOMI—SELACHII—BATOIDEI—HOLOCEPHALI

CLASS II. PISCES.

SUB-CLASS I. ELASMOBRANCHII.

In both the ancient and the modern Sharks, Dog-Fishes, and Rays, the exoskeleton takes the form of a more or less uniform investment of dermal denticles or "shagreen." The endoskeleton is wholly cartilaginous or partially calcified, and there are neither cartilage- nor membrane-bones. The vertebral column is acentrous or chordacentrous, generally with alternating basi- and inter-dorsal elements, and terminating in a heterocercal tail. The skull is usually hyostylic, very rarely amphistylic or autostylic, and the lateral halves of the primary upper jaw (palato-quadrate cartilages) usually meet in a highly characteristic median symphysis beneath the base of the skull. Branchial arches and clefts are five to seven in number, and the clefts are separated by complete interbranchial septa, which, as a rule, are continuous externally with the skin. An operculum is developed only in the Holocephali. A pelvic girdle is present. With rare exceptions the pectoral fin is uniserial. The pelvic fin is invariably uniserial. The exoskeletal supports of all the fins consist of ceratotrichia, and, when present, the fin-spines are invested by enamel. Claspers are generally present in the males.

In the surviving members of the group the nostrils retain their primitive ventral position. There is a conus arteriosus with several rows of valves. A spiracle, often furnished with a spiracular pseudobranch, is generally present, and, as a rule, {432}there is a hyoidean hemibranch supplied with venous blood from the ventral aorta. The gill-filaments are attached throughout their length to the interbranchial septa. There is an optic chiasma. An air-bladder is not developed. The intestine has a spiral valve, and there is a cloaca. The gonoducts in both sexes are derived from the kidney system. The ova are large, few in number, and enclosed in horny egg-cases, and they are fertilised before extrusion. The segmentation is meroblastic, and the embryo is furnished with long external gills.

The Elasmobranchs are for the most part active predaceous Fishes, living at different depths in the sea, from the surface to nearly a thousand fathoms, and ranging from mid-ocean to the shallower waters round the coasts in almost every part of the world. Although typically marine, they sometimes ascend rivers beyond the reach of tides, and a few are permanent inhabitants of fresh water. They are most abundant in tropical and subtropical areas, where they also attain their greatest size, and are numerous in temperate regions, but there are some species which are typically Arctic. None of them are small, and some of the Sharks are the largest of living Fishes. All are carnivorous, but so diversified is their food that in different species it may range from other Fishes of no mean size to Molluscs, Crustaceans and other Invertebrates, or even to plankton. In their breeding habits the Sharks and Dog-Fishes present many interesting features. Unlike the generality of Fishes, the eggs are fertilised internally as a sequel to the copulation of the sexes. For this purpose the males are furnished with special intromittent organs, the myxopterygia or so-called claspers, which are developed as modifications of the hinder portions of the pelvic fins. Each clasper is supported by an internal skeleton, consisting of several cartilages derived from the radialia of the fins, and is traversed along its inner aspect by a groove. When sexual congress takes place the claspers are thrust through the cloaca of the female into the oviducal orifices, and in some instances it is probable that they are retained in this position by hook-like denticles developed at their free extremities. The seminal fluid then flows along these conduits into the oviducts, in the upper portions of which it meets and impregnates the eggs. After fertilisation the egg is enclosed in a dark brown horny egg-case, secreted by the oviducal gland.

{433}[Illustration: FIG. 245.—Egg-case of Heterodontus (Cestracion) galeatus. (From Parker and Haswell, after Waite.)]

As a rule each egg-case has but a single egg, but in Rhinobatus and Trygonorhina (Batoidei), both of which are viviparous, each case contains three to four eggs. Generally the egg-cases are somewhat quadrangular in shape, with the four angles, two at each end, prolonged either into short horns, or into long tapering tendrils (Fig. 246). The oval egg-cases of the Heterodontidae are remarkable not only for their size, but also for the presence of a broad spiral lamina winding round the exterior of the case from one pole to the other (Fig. 245). The majority of the Sharks, Dog-Fishes, and Rays are viviparous, that is, the young are born alive; the rest, like the Scylliidae (e.g. the common British Dog-Fishes, Scyllium canicula and S. catulus), the Heterodontidae, and the Raiidae are oviparous, that is, the young are hatched out after the extrusion of the eggs. In the oviparous species the eggs are extruded either singly or in pairs, and generally deposited on the sea-bottom. When, however, the egg-cases are provided with tendrils, as, for example, in the two British Dog-Fishes just mentioned, these organs act as anchoring filaments. When extruding an egg, the female swims round and round some piece of upright seaweed, and the curling tendrils become entwined round it in such a way that the egg becomes securely attached thereto (Fig. 246). The embryos are long in developing, and in Scyllium it may be several months after fertilisation (200 to 275 days) before they are hatched, the young Fish finally escaping through a rupture in the egg-case.

{434}[Illustration: FIG. 246.—Egg of the Spotted Dog-Fish (Scyllium canicula), showing its mode of attachment after extrusion. (From Hertwig, after Kopsch.)]

In the oviparous species the nutritive food-yolk stored up, first in the egg and subsequently in the yolk-sac (Fig. 248), suffices for the nourishment of the embryo until the period of hatching, but in viviparous forms, whose embryonic development is completed within special uterine dilatations of the oviducts, additional means of nutrition are provided for the young. Such Elasmobranchs as Spinax, Acanthias, Centrina, Scymnus, Trygon, Torpedo, and Myliobatis have long filaments (villi or trophonemata) developed from the inner surface of the uterus, which secrete a nutritive fluid, and this fluid is either absorbed by the blood-vessels of the embryonic yolk-sac, or it is taken up by the embryo in some more direct manner. In some of the Trygonidae and Myliobatidae of the Indian Ocean it seems probable that the secretion is taken into the alimentary canal of the embryo either through the mouth or through the open spiracles. One species, Pteroplatea micrura, has its long and highly vascular and glandular trophonemata gathered into two bundles, which are thrust through the huge spiracles into the pharynx of the embryos, of which there may be from one to three, and the nutritive secretion is apparently digested in the alimentary canal of the embryo and absorbed by the foetal blood-vessels (Fig. 247). A few Sharks, like most species of Mustelus, develop a placenta when the food-yolk in the yolk-sac of the embryo is nearly used up. Folds or projections from the highly vascular wall of the yolk-sac interlock with similar vascular folds of the lining membrane of the uterus, and a diffusion of nutrient material takes place from the maternal blood in the uterine blood-vessels to the foetal blood in the {435}vessels of the yolk-sac. Each embryo has its own placenta, and in Mustelus antarcticus the uterine portion of the oviduct is divided by septa into several chambers, each containing a single embryo. It is worthy of note that in the viviparous species a distinct but very thin, delicate egg-case is formed, occasionally even with the rudiments of tendrils, which may either be retained or thrown off in the uterus. The Greenland Shark (Laemargus borealis) is unique amongst Elasmobranchs. Its eggs are small and unprotected by egg-cases, and their fertilisation is said to be effected in the water after deposition, as in the generality of Fishes.

Fossil remains of Elasmobranchs in the shape of fin-spines (ichthyodorulites) and dermal denticles, associated with various Ostracodermi (Coelolepidae, Pteraspidae, and Cephalaspidae), are amongst the earliest undoubted indications of Vertebrate life. They first appear in the Upper Ludlow Bone Bed and in Silurian rocks in other parts of Europe, and in North America; and in greater or less abundance the group is represented in almost every subsequent geological period. It cannot be said that the group shows signs of decadence, for Elasmobranchs still survive in apparently undiminished numbers and variety in the marine fauna of the present day.

The Elasmobranchs are certainly a very primitive race of Fishes. Their earliest representatives of whose structure we have any precise knowledge (e.g. Cladoselache and Pleuracanthus) are in many respects the most archaic of known gnathostomatous {436}Craniates, and from such types as these, amongst others, we may reasonably look for the ancestors of all or most of the remaining groups of Fishes. It has been well said of Pleuracanthus that "it is a form of Fish which might with little modification become either a Selachian, Dipnoan, or Crossopterygian," while the condition of the primary upper jaw in the Chondrostean Polyodon suggests that even the more primitive Actinopterygii had an Elasmobranch origin. The important researches of Dr. Traquair render it also highly probable that the ancient Ostracodermi may claim kinship through their Coelolepid ancestors with some primitive type of Elasmobranch; and within the limits of the group there is ample evidence that differentiation has taken place on many divergent lines, of which we have notable examples in such specialised offshoots as the Acanthodei and the Holocephali, to say nothing of several highly specialised families which became extinct at successive periods in the history of the group.

ORDER I. PLEUROPTERYGII.

The only certain representative of this group is the Palaeozoic form Cladoselache, probably the most primitive Elasmobranch at present known (Fig. 249). Elongated and somewhat cylindrical in shape, Cladoselache has a terminal mouth, five or possibly seven pairs of branchial clefts, and a pair of olfactory organs, lateral in position near the extremity of the snout. Wide-based, triangular pectoral and pelvic fins, a low anterior and a posterior dorsal fin, devoid of spines, and a heterocercal caudal fin with homocercal tendencies, are present, but no anal fin has yet been detected.

{437}[Illustration: FIG. 249.—Restoration of Cladoselache fyleri. Lateral and ventral views. (From Parker and Haswell, after Dean.)]

The exoskeleton consists of minute lozenge-shaped denticles, which invest the body and extend on to the surfaces of the fins, and there is also a circumorbital ring of several concentric rows of small square plates. A lateral line, in the form of a groove between two rows of denticles, extends along each side of the body. The notochord is persistent. Calcified neural and haemal arches (basidorsals and basiventrals) have been observed in the caudal region, where they correspond numerically with the remains of the myotomes, but interdorsal or intercalary arcualia seem to be absent. The upper and lower jaws, similar in size and shape, are apparently supported by a hyomandibular cartilage; hence the skull is hyostylic. The endoskeletal supports of the pectoral, and especially those of the pelvic fins, exhibit a more primitive disposition than in any other Fishes. They extend nearly to the distal margins of the fins, where they seem to interdigitate with the proximal ends of feebly-developed ceratotrichia (Fig. 145). The extension of the fins in the horizontal plane, the gradual shading off of their broad bases into the sides of the body, and the resemblance between their radialia and those supporting median fins, are very suggestive of the origin of the paired fins from continuous lateral fin-folds. Claspers are absent. The dentition is well developed, and several rows of teeth seem to be functional at the same time. {438}Each tooth consists of a broad base, supporting a long pointed central cusp and a variable number of similarly shaped but much shorter lateral cusps. The teeth in the various transverse rows from without inwards are closely wedged together by the interlocking or overlapping of their bases.

FAM. 1. CLADOSELACHIDAE.—Several species of Cladoselache, varying from 2 to 5 feet in length, have been found in the Cleveland Shale (Upper Devonian) of Ohio. Isolated teeth similar to those of Cladoselache occur in the Lower Carboniferous of Europe, India, and North America, and have been referred to various species of the genus Cladodus, but with one exception nothing more is known of the structure of these Fishes, and consequently their relationship to Cladoselache is doubtful. C. neilsoni, from the Lower Carboniferous (Calciferous Sandstones) of Kilbride in Scotland, has a very different type of pectoral fin, which appears to be distinctly uniserial, but intermediate in structure between the biserial fin of Pleuracanthus and that of the modern sharks. There are several other genera from the Devonian and Lower Carboniferous whose claims to inclusion in this group rest on no better foundation.

ORDER II. ICHTHYOTOMI.

While more specialised than the Pleuropterygii the Fishes included in this group represent an extremely generalised type of Elasmobranch, which, as already indicated, may easily have been the ancestor of more than one group of Fishes. In the typical genus Pleuracanthus (Fig. 250) the body is elongate, but slightly depressed, with a terminal mouth, and a tapering diphycercal tail fringed above and below by a continuous caudal fin. A long dorsal fin, two small anal fins, and well-developed paired fins with contracted bases, are present. The head is armed with a prominent, serrated, dorsal spine, but it is doubtful if dermal denticles (shagreen) are present. The vertebral column {439}is acentrous, and the persistent notochord supports a series of basidorsal cartilages, which alternate with small interdorsals, a series of basiventrals supporting small ribs, and in the caudal region well-developed haemal arches. The dorsal fin is supported by slender, tri-segmented radialia, which appear to be twice as numerous as the neural arches in the trunk; but in the dorsal lobe of the caudal fin the two structures agree in number. Ventrally-prolonged haemal spines are the sole endoskeletal supports of the inferior lobe of the caudal. The coraco-scapular cartilages of opposite sides remain distinct, and each supports a biserial fin. The pelvic girdle is represented by a pair of small cartilages supporting the basipterygia. The pelvic fins are uniserial, with post-axial skeletal supports for claspers in the males. Both the median and the paired fins are provided with marginal ceratotrichia. The skull is probably amphistylic. Five, possibly six or seven, branchial arches, bearing clusters of minute denticles, are present. Circumorbital plates are wanting. All the endoskeletal structures are partially calcified. The teeth are tricuspid, each with two long divergent lateral cusps and a minute median cusp; the broad bases of the teeth overlap and articulate with one another by means of facets.

{440}FAM. 1. PLEURACANTHIDAE.—The single family included in the group ranges from the Lower Carboniferous to the Lower Permian. Within these limits the family is widely distributed in different formations in Great Britain, Continental Europe, New South Wales (Lower Hawkesbury Formation), and North America. Pleuracanthus, of which complete skeletons and skulls have been found, is the best known genus.

ORDER III. ACANTHODEI.

The Fishes comprising the Acanthodei may be regarded as a highly specialised and terminal offshoot from some primitive race of early Elasmobranchs. The Elasmobranch kinship of the Acanthodei is indicated by their exoskeleton of shagreen tubercles; the completely heterocercal tail; the absence of an operculum, the external gill-clefts apparently being exposed; the position of the lateral line of the trunk between two rows of shagreen denticles; the nature of the powerful spines in connexion with the dorsal and anal, and the pectoral and pelvic fins; and the formation of the hard parts of the skeleton, not by ossification involving the presence of bone-cells, but by the calcification of cartilage, or of more superficial membranous or fibrous tracts. On the other hand, it may be noted that the Acanthodei appear to have undergone much specialisation on lines in some respects parallel to those which have marked the evolution of the Teleostomi, but by methods which are simply an exaggeration of features normally characteristic of Elasmobranchs. Perhaps the most striking illustration of this is to be seen in the development of a species of secondary skull by an extension of a process of calcification as distinguished from ossification. Hence the presence of membrane-calcifications in relation with the upper and lower jaws, whose development is proportional to the size of the teeth they support, and of smaller investing plates of the cranial roof. Similar exoskeletal calcifications, when most completely developed (e.g. Diplacanthus), form a dorsally incomplete arch, apparently corresponding to a secondary pectoral girdle for the support of the stout pectoral spines, in which elements {441}analogous to clavicles or cleithra and infra-clavicles can be recognised. Each pectoral spine forms the preaxial margin of the fin, and behind it there is a series of ceratotrichia. Nothing is known of the endoskeletal supports, but having regard to the nature and proportions of the pectoral spines it may be inferred that the exoskeletal elements of the fins predominate over the former to an extent which is only paralleled elsewhere in the Teleostei.

Apparently the notochord is persistent, and there are long and slender neural and haemal arches, but no ribs. The dermal denticles are uniform in size, and so small as to give a granular appearance to the skin. In structure they are thick, with a flat, enamelled, often sculptured, external surface, quadrate or rhombic in shape, and fitting closely together. Teeth are either absent or very minute, but sometimes (e.g. Acanthodopsis and Ischnacanthus) they are few in number and large, conical in shape, occasionally with minute cusps between the larger teeth. Claspers are absent. The Acanthodei are small Fishes, most of them being less than .3 m. in length, and ranging from the Upper Silurian to the Lower Permian inclusive. Two families are recognised.

FAM. 1. DIPLACANTHIDAE.—Two dorsal fins are present. Usually there is a row of lateral spines extending along each side of the body between the pectoral and pelvic fins. Exclusively Upper Silurian and Devonian.

The genera Diplacanthus, Climatius, Parexus, Euthacanthus, and Ischnacanthus are all found in the Lower Old Red Sandstone of Scotland. Climatius and Diplacanthus are also represented in the Devonian of Canada.

FAM. 2. ACANTHODIDAE.—A single dorsal fin; lateral spines vestigial or absent. Lower Devonian to the Lower Permian.

{442}The widely-distributed genus Acanthodes (Fig. 251) is represented in the Lower Old Red of Scotland, the Devonian of Siberia and Canada, the Carboniferous of England and Scotland, and the Lower Permian of France, Germany, and Bohemia. Acanthodopsis (Coal Measures), and Mesacanthus and Cheiracanthus (Lower Old Red) are the remaining genera.

ORDER IV. PLAGIOSTOMI.

Head prolonged in front of the ventrally-situated mouth as a more or less prominent preoral rostrum, vertebral column consisting of alternating basi- and inter-dorsal cartilages, generally supported by more or less well-developed chorda-centra. Pectoral and pelvic fins uniserial. Pelvic girdle and claspers present. Except in two families the branchial arches and clefts are invariably five in number. An operculum is not developed.

SUB-ORDER 1. SELACHII.

Body elongate or fusiform, shading imperceptibly into a powerful swimming tail. Pectoral fins of moderate size, with contracted bases; not confluent with the sides of the head. Branchial clefts lateral in position. Vertebral centra generally asterospondylic or cyclospondylic.

This sub-order includes such typical Elasmobranchs as the modern Sharks and Dog-Fishes as well as numerous fossil representatives ranging from the Carboniferous, and probably from still earlier periods, to the present day.

FAM. 1. NOTIDANIDAE.—Body moderately elongate, the spineless dorsal fin opposite the anal. Mouth ventral; nostrils ventral, near the extremity of the snout, without oro-nasal grooves. Branchial arches and clefts six or seven. Interbranchial septa devoid of marginal frills. Notochord persistent and continuous, partially constricted by simple chorda-centra, each consisting of two distinct rings, without either concentric or radial lamellae, except {443}in one species (Notidanus cinereus), which exhibits a feeble asterospondylism in the caudal vertebrae. Skull amphistylic. Teeth unlike in the two jaws; those in the upper jaw usually with a large central cusp and smaller lateral cusps; those in the lower jaw comb-like, each consisting of numerous graduated pointed cusps inclining in the same direction, and supported on a long basal plate.

The very few species included in this family are widely distributed in the tropical and subtropical regions of the Atlantic and Pacific Oceans. Notidanus (Heptanchus) cinereus, which has seven branchial arches and clefts, inhabits the Mediterranean and Atlantic. N. (Hexanchus) griseus, with six branchial arches and clefts, has a similar distribution, but besides being an occasional visitant to the British coasts, it is not uncommon at Cuba in the West Indies. It is said to grow to a length of 26 feet.

Fossil remains of Notidanus, principally teeth, occur in the Middle and Upper Jurassic, in the Cretaceous, and in the Eocene and Pliocene of England and the Continent.

FAM. 2. CHLAMYDOSELACHIDAE (Frilled Sharks).—Body much elongate. Median fins as in Notidanus. Mouth nearly terminal. Nostrils lateral, nearly terminal, and without oro-nasal grooves. Branchial arches and clefts six. The outer margins of the interbranchial septa are produced into overlapping cutaneous frills, the first of which is developed from the hyoid arch and overlaps the hyobranchial cleft, like a rudimentary operculum. Vertebral column as in the preceding family, but in the hinder part of the trunk the notochord is unconstricted and uniform in diameter, centra being absent. Skull hyostylic. Lateral line an open groove. Teeth alike in both jaws, each consisting of a broad basal plate supporting three slender curved cusps, separated by a pair of much smaller cusps.

The only living species known is Chlamydoselachus anguineus (Fig. 252), which occurs in the Pacific near Japan, in deep water off Madeira, and also off the Azores and the coast of Norway. It reaches a length of 4 to 5 feet. Teeth from the Pliocene deposits of Tuscany have been referred to an extinct species, C. lawleyi.

{444}Scarcely anything is known of the habits of the Notidanidae and the Chlamydoselachidae. It is evident that they are closely-related forms, and from the unusual number of their gill-clefts and branchial arches, and the condition of the vertebral column, it is also obvious that they are the most archaic of modern Selachians.

FAM. 3. HETERODONTIDAE (Bullhead Sharks).—Head large and high, with a blunt snout projecting but little in front of the small and almost terminal mouth, and with prominent supraorbital crests. Trunk thick-set and somewhat trihedral, covered with fine shagreen. Nostrils ventral but nearly terminal, with oro-nasal grooves. Spiracles small, beneath the eyes. Two dorsal fins, each with a spine in front, the first opposite the interval between the pectorals and pelvics, the second in front of the anal. Vertebral centra asterospondylic when fully developed. Palato-quadrate cartilages with an extensive articulation with the sides of the preorbital regions of the cranium, the normal suspensoria of a hyostylic skull (hyomandibular cartilages) taking little share in their support. Dentition similar in both jaws. Teeth at the symphyses numerous, small, and conical, furnished with three to five cusps in the young; those behind broad and pad-like, arranged in oblique rows, the teeth forming the two middle rows being much larger than those in the front or behind. Living species, oviparous. Egg-cases large, with an external spiral lamina (Fig. 245).

About four species belonging to one genus, Heterodontus (= Cestracion) (Fig. 253), or possibly to two, represent this dwindling family. All are inhabitants of the Pacific Ocean (Japan, Amboyna, Australia, the Galapagos, and the Californian coast of North America). Little is known of their habits. They feed {445}principally on Molluscs, the shells of which are crushed by their massive grinding teeth. The different species vary in size from 2 to 5 feet.

The Heterodontidae were the most characteristic and abundant Sharks of the Mesozoic period. Amongst extinct genera Hybodus ranges from the Middle Trias to the Lower Cretaceous (Wealden); an allied genus, Acrodus, from the Middle Trias to the Upper Cretaceous (Gault). Palaeospinax occurs in the Lias and possibly in the Upper Trias. Synechodus is a Cretaceous genus, and Asteracanthus, which has large hooked spines on the head, is characteristic of the Middle and Upper Jurassic. An even greater antiquity may be claimed for the Heterodontidae if, as is not improbable, such Palaeozoic Sharks as Orodus, Sphenacanthus, Tristychius (Carboniferous), and Wodnika (Permian) belong to this family. Many ichthyodorulites are probably the spines of various extinct Heterodontidae.

FAM. 4. COCHLIODONTIDAE.—This Palaeozoic family includes a number of Sharks probably related to the Heterodontidae, but of which little is known except their dentition. The teeth are in some respects similar to those of Heterodontus, except that those which appear to correspond to one or both of the middle rows of the latter genus tend to fuse and form a few large, convex, and often scroll-like plates. The typical Cochliodonts are exclusively Carboniferous (Europe and North America). Psephodus, Pleuroplax, Deltodus, Poecilodus, Cochliodus, Deltoptychius, Helodus, and Menaspis (Permian) are characteristic genera. {446}Probably some ichthyodorulites described under various generic names belong to this family.

FAM. 5. PSAMMODONTIDAE.—Teeth large, flat or slightly arched, oblong or quadrate, and arranged in one, two, or more longitudinal rows. Only the teeth are known, and from differences in their shape, size, and surface markings, the genera Psammodus, Archeobatis, and Copodus have been recognised. The family is confined to the Lower Carboniferous of Great Britain and Ireland, Russia, Belgium, and North America.

FAM. 6. PETALODONTIDAE.—Teeth transversely elongated, with a blunt or a sharply-ridged crown, separated from a single or multiple root by a constricted neck, and disposed in transverse and longitudinal pavement-like rows; exoskeleton of smooth, oval, rounded or quadrate shagreen denticles. Only the teeth, and in some genera the dermal denticles, are known, except in Janessa, which has a Ray-shaped body, with large pectoral fins prolonged towards the head. The family is mainly confined to the Carboniferous formations of Great Britain, Europe, and North America. Petalodus, Janessa (also represented in the Permian), Glossodus, Polyrhizodus, and Callopristodus are characteristic genera.

FAM. 7. SCYLLIIDAE (Dog-Fishes).—Dorsal fins two in number, small, and without spines, the first above or behind the pelvic fins, the second usually behind the anal. Tail not bent upwards or but slightly so, without lateral keels. Spiracles present. Nictitating membranes absent. Vertebrae asterospondylic. Teeth small, each with a median cusp, and one to four small cusps on each side. Oviparous. Egg-cases (Fig. 246) large, quadrate, with long twining tendrils at the angles for attachment.

The genus Scyllium includes the true Dog-Fishes (Fig. 254). The species are coast Fishes of small or moderate size, and are widely distributed in temperate and tropical seas, at depths not as a rule exceeding 400 fathoms. Two species, S. canicula and S. catulus, are common on the British coasts, living near the bottom and feeding on Crustaceans and Molluscs. An allied form, Pristiurus, is also common in European and British waters. Chiloscyllium is a widely-distributed genus ranging from the Cape of Good Hope through the Indian Ocean to the coasts of Australia, China, and Japan. Stegostoma tigrinum of the {447}Indian Ocean attains a length of 10 to 15 feet, and is remarkable for its handsome coloration of dark bands on a yellow ground, which has suggested the name of Tiger- or Zebra-Shark. The pelagic genus Ginglymostoma has the terminal portion of the tail bent upwards, and grows to a length of 6 to 12 feet. It is represented by species in the Indian Ocean and the tropical parts of the Atlantic (West Indies and the west coast of Mexico). Crossorhinus includes species of large size, some of which are 10 feet long. They are ground-sharks, frequenting the coasts of Australia and Japan, which lie on the bottom watching for their prey, and in accordance with this habit their coloration closely resembles that of their surroundings. A large North Atlantic Shark (Pseudotriakis microdon), of which only two specimens are known, one taken on the Portuguese coast, and the other, 10 feet in length, off Long Island, on the Atlantic coast of North America, has the general characters of the Scylliidae, except that the first dorsal fin is opposite the interval between the pectoral and pelvic fins. Some Scylliidae live at great depths, Scyllium (Scylliorhinus) profundorum having been obtained from a depth of 816 fathoms in the North Atlantic.

Most of the fossil Scylliidae belong to existing genera. The earliest known representatives of the family occur in the Upper Jurassic (Lithographic Stone of Bavaria), where the extinct genus Palaeoscyllium, a near ally of the existing Scyllium, and Pristiurus, are found, nearly complete. Scyllium itself ranges from the Cretaceous through the different Tertiary formations. A species of Chiloscyllium has been recorded from the Miocene Tertiaries, and detached teeth of Ginglymostoma from the Eocene of Belgium and North America. An extinct genus (Mesiteia), which is found in the Upper Chalk of Mount Lebanon and the Upper Eocene of Monte Bolca, is remarkable for the enclosure of {448}its lateral sensory canals in a series of incomplete calcified rings, as in the Holocephali.

FAM. 8. CARCHARIIDAE.—Sharks with two dorsal fins, the first in front of the pelvic fins and the second opposite the anal fin, both devoid of spines. Tail without lateral keels. Preoral rostrum elongated. Mouth crescentic. Eyes with nictitating membranes. Spiracles small or absent. Vertebrae asterospondylic. Teeth usually consisting of a single triangular cusp, with smooth, trenchant, or serrated margins, rarely with basal cusps; generally with an axial cavity when fully developed. Viviparous. The family comprises about twenty genera, and approximately sixty species; found in all seas, often in mid-ocean. Amongst the more important genera may be mentioned Carcharias (Carcharhinus), Galeocerdo, Triakis, Thalassorhinus, Galeus, Mustelus and Scylliogaleus.

Species of Carcharias are found in nearly all tropical and subtropical seas. The genus is a somewhat comprehensive one, and groups of its species have been distinguished as sub-genera under the names of Prionodon, Hypoprion, Scoliodon, Aprionodon, etc. One of the most widely distributed of the thirty to forty species is the Blue Shark, C. (Prionodon) glaucus (Fig. 255), of the Atlantic and Pacific Oceans, which may grow to a length of 25 feet, although the young forms not infrequently captured in British waters do not exceed 6 to 8 feet. It is a slender, swift, pelagic Shark, of a slaty-blue colour above and white underneath, and a voracious hunter of other Fishes. C. nicaraguensis, a Shark about 7 feet long, is confined to Lake Nicaragua and its outlet the Rio San Juan, and is one of the very rare strictly freshwater Sharks. Galeocerdo is a large Shark found in temperate and tropical waters, but one species, G. arcticus, is confined to Arctic seas. The variegated G. tigrinus, or West Indian Tiger-Shark, is said {449}to reach a length of 15 to 20 feet. The genus Galeus includes the small Sharks commonly known as "Topes," which are common in nearly all tropical and temperate seas. The British species, G. canis, which ranges from 4 to 6 feet in length, is a bottom-feeding Fish, preying on Molluscs, Crustacea, Star-Fish, and small Fishes. The various species of Mustelus, or "Hounds," resemble the Topes in their habits and distribution. Living principally on Molluscs and Crustaceans, the dentition has lost the trenchant, unicuspidate type characteristic of most other Carchariidae, and is adapted for crushing and grinding, the teeth being flat, without cusps, and arranged in pavement-like rows. Two species, M. vulgaris and M. laevis, are abundant on the coasts of Europe and the British Isles. Scylliogaleus, which combines the general characters of Mustelus with nostrils similar to those of a Scyllium, is known only from a single specimen from the coast of Natal.

The Carchariidae are comparatively modern Sharks. No undoubted remains are known earlier than the Eocene, in which, as in the succeeding Miocene and Pliocene deposits, they are represented principally by their characteristic teeth. The extinct fossil genera are few in number, and so far as their dentition is concerned they differ but little from their living allies.

FAM. 9. SPHYRNIDAE (Hammer-head Sharks).—In their general characters the Hammer-head Sharks agree with the Carchariidae. They are distinguished, however, by the remarkable shape of the head, which is prolonged into two conspicuous lateral lobes, supported internally by corresponding cartilaginous outgrowths from the post-orbital and the lateral ethmoidal or nasal regions of the skull, with the eyes at their distal extremities, and the nostrils in relation with their anterior margins. One genus and five species.

The Sphyrnidae are denizens of nearly all tropical and subtropical seas. Sphyrna (Zygaena) tudes occurs in the Mediterranean, and S. zygaena is a very rare visitant to the British coasts. A specimen over 13 feet in length was captured at Ilfracombe in 1865, and other examples have been taken off Banffshire, at Newlyn in Cornwall, at Yarmouth, and in Carmarthen Bay. The shape of the head differs in different species, and in young {450}forms the peculiarities of the adult are less marked. In the Bonnet Shark (S. tiburo) (Fig. 256, A), the head is crescentic or kidney-shaped, with prominent postero-lateral angles, and between this type of head and the more pronounced "hammer" of S. zygaena (Fig. 256, B) an almost perfect gradation is supplied by other species. The Hammer-heads are voracious Sharks, usually living in deep water, and they may grow to a length of 15 feet. As many as thirty-seven embryos have been taken from the oviducts of a female nearly 11 feet in length.

Teeth assigned with more or less probability to Sphyrna are found in the Miocene of Europe and North America.

FAM. 10. LAMNIDAE (Porbeagle Sharks).—Large, stout-bodied Sharks with two dorsal fins, the first just behind the pectoral fins, the second, which is small, opposite the small anal fin; both {451}without spines. Tail with a prominent lateral keel on each side. Nictitating membranes absent. Spiracles minute or wanting. Branchial clefts very wide. No oro-nasal grooves. Vertebrae asterospondylic. When fully developed the teeth are solid.

In the genus Lamna, which includes the Porbeagle Sharks, the teeth are large, each consisting of a long narrow central cusp, usually with smaller cusps at the base. The common Porbeagle (L. cornubica), a fierce pelagic Shark, which may reach a length of 10 feet, frequents the North Atlantic and the North Pacific (Fig. 257). It has often been captured off the coasts of Great Britain and Ireland in Mackerel or Salmon nets, or by lines laid for food Fishes. An allied genus, Isurus, is represented by species on the Atlantic coast of North America, in the Mediterranean and the neighbouring parts of the Atlantic, and also in Asiatic seas. Carcharodon rondeletii is a pelagic Shark with large, triangular, finely-serrated teeth, without basal cusps, and is found in all tropical and subtropical seas from the Mediterranean to Australia and New Zealand. It is one of the largest and most formidable of Sharks, and it is said to grow to a length of 40 feet. Nothing is known of its breeding habits. Odontaspis, which has minute pore-like spiracles, but no lateral caudal keels, is a Shark of moderate size, chiefly inhabiting the Atlantic, but found also in the Mediterranean and the Southern Pacific. Its teeth are long and awl-like, with small basal cusps.

The Thresher or Fox Shark (Alopecias vulpes) is remarkable for the extraordinary length of the upper lobe of the caudal fin, {452}which is as long as the rest of the body (Fig. 258). Its teeth are of moderate size, triangular in shape, and without serrations. The "Thresher" has a wide distribution, being abundant in the Atlantic and Pacific Oceans, besides being the commonest of the larger Sharks frequenting the British coasts. It grows to a length of 15 feet, of which the tail forms at least one-half. Quite inoffensive to man, the Thresher feeds on the shoals of smaller Teleosts, such as Pilchards, Herrings, and Sprats. When feeding it swims in gradually diminishing circles round the shoal, splashing the water with its long tail, and keeping its victims so crowded together that they become an easy prey. A remarkable Lamnoid Shark (Mitsukurina owstoni), which has the snout produced into a "long, flat, flexible, leaf-like blade," somewhat resembling that of Polyodon, but narrower and more pointed, and has protractile jaws and large spiracles, is found in deep water near Yokohama, and may prove to be generically identical with the Cretaceous Shark Scapanorhynchus.

Lamnoid Sharks are not certainly known to have existed until the Upper Cretaceous formations, in which, as well as in different Tertiary deposits, teeth indistinguishable from those of the existing genera Lamna, Odontaspis, and Carcharodon are found. The interesting genus Carcharodon has one extinct species in the Cretaceous and several others distributed in Tertiary formations in nearly every part of the world. The teeth of some of the Tertiary species measure 5 inches along the margin and 4 inches across the base, and it is evident that they belonged to Sharks so gigantic as completely to dwarf the existing species. That these giant Lamnidae have only recently {453}become extinct is proved by the fact that similar teeth have been dredged from the bottom of the Pacific. Teeth and detached vertebrae from various Tertiary deposits have been referred to species of Alopecias. Entire Fishes, with an elongated rostrum and an extensive anal fin, from the Cretaceous of Mount Lebanon, have been assigned to an extinct genus, Scapanorhynchus.

FAM. 11. CETORHINIDAE (Basking Sharks).—Two dorsal fins, without spines, the anterior midway between the pectoral and pelvic fins. Tail without lateral keels. Nictitating membranes absent. Spiracles small, situated just above the angles of the mouth. Branchial clefts wide and of great vertical extent, extending from the dorsal to the ventral surface. Teeth small, very numerous, conical in shape, without serrations. Claspers of the male provided with horn-like denticles.

The single species included in this family, the Basking Shark, (Cetorhinus (Selache) maximus), is one of the largest of living Fishes, reaching a length of 40 feet (Fig. 259). It is a pelagic Shark, inhabiting the Arctic seas, but wandering as far south on opposite sides of the Atlantic as the Mediterranean, the coasts of Portugal and Virginia, and in the Pacific to the Californian coast. Although generally described as a northern form, Cetorhinus is known to occur in Australian waters. It is fairly common off the coasts of Scotland, and it has been seen or captured at various points on the western coast of Ireland, and {454}the eastern and southern coasts of England. The Fish is gregarious in its habits, often swimming in shoals near the surface. The name "Basking Shark" has been suggested by its habit of lying motionless on the surface in warm or calm weather, as if basking in the sun, with its dorsal fin protruding from the water. Unless attacked, this Shark is quiet and inoffensive. It derives its food-supply from small pelagic Fishes, and also from marine Invertebrates, which are strained from the water by the fringes of long, slender gill-rakers with which the branchial arches are provided. At one time harpooned and caught off the Irish, Scotch, and Norwegian coasts for the sake of the oil obtained from its liver, the Fish is now of little economic importance. Nothing is known of its mode of reproduction.

Extinct species of Cetorhinus have been founded on detached vertebrae and isolated teeth from deposits of Pliocene age in Belgium and Italy, and possibly from still earlier Tertiary formations. Dermal spines similar to those found on the claspers of the males in the existing species occur in the Antwerp Crag, and in the Red Crag of Suffolk.

FAM. 12. RHINODONTIDAE.—Two dorsal fins, without spines, the anterior a little in front of the pelvic fins, the second opposite the anal. Tail with lateral keels and a pit at its root. Spiracles small. Nictitating membranes absent. Mouth and nostrils nearly terminal. Teeth very minute, numerous, and conical in shape.

One genus, Rhinodon, with one or two species, is known. These Sharks are very widely distributed, specimens having been seen or captured in the neighbourhood of Ceylon, at the Seychelles, the Cape of Good Hope, Callao on the Peruvian coast, in the Gulf of California, and off the coast of Florida. Rhinodon is probably the largest known Shark. It is stated to exceed 50 feet in length, but to be quite harmless. Scarcely anything is known of its habits, but the small size of the teeth, and the length of the gill-rakers, which resemble those of the Basking Shark, suggest a similar kind of food.

FAM. 13. SPINACIDAE.—Two dorsal fins, the first in advance of the pelvic fins. Anal fin absent. Nictitating membrane absent. Spiracles rather large. Vertebrae cyclospondylic. Teeth variously modified in different genera.

{455}The more typical representatives of this family are the Spiny Dog-Fishes, which are distinguished by the presence of a strong spine in front of each dorsal fin. They are more abundant in temperate regions than in the intervening tropics. The more important genera are Acanthias, Centrina, Centrophorus, Spinax, and Centroscyllium. Acanthias vulgaris, the Picked or Piked Dog-Fish, is a gregarious, voracious Shark, about 3 to 4 feet in length, and is frequently seen in huge shoals all round the British coasts, especially during the summer months. It is very destructive to food Fishes, and its ravages result in serious loss to fishermen. Acanthias is viviparous. Centrina salviani is a much smaller Shark, which frequents the Mediterranean and the Bay of Biscay; on rare occasions it has been taken off the southern coast of England. Centrophorus occurs in deep water in the Mediterranean and adjacent portions of the Atlantic, and off the coasts of Japan. Centroscyllium is found on opposite sides of the North Atlantic (Greenland and Massachusetts), and in the opposite hemisphere at the Falkland Isles. A deep-water form, Paracentroscyllium, has been obtained in the Bay of Bengal at depths from 285 to 405 fathoms.

Three remaining genera (Scymnus, Laemargus, and Echinorhinus) differ from the preceding in the absence of dorsal spines.

Scymnus lichia is common in the Mediterranean and the neighbouring parts of the Atlantic. The Greenland Shark (Laemargus borealis) (Fig. 260) is an inhabitant of the Arctic regions, wandering as far southwards on opposite sides of the Atlantic as the French coast and Cape Cod. It is a huge, clumsy shark, reaching a length of 26 feet. Numerous instances are recorded of its capture off the coasts of Great Britain, especially in northern waters. The Greenland Shark is said to be a determined foe to {456}the Right Whale, which it attacks, biting pieces out of its body. Scymnus is viviparous, Laemargus oviparous, and the latter is unique among Sharks in producing eggs devoid of a horny shell, which are deposited on the sea-bottom. Echinorhinus has dermal denticles in the form of relatively large rounded tubercles, each surmounted by a tuft of fine spines. One species only is known, E. spinosus, a large Shark attaining a length of 10 feet, and frequenting deep water off the Atlantic coasts of Europe and Africa from the North Sea to the Cape of Good Hope. A single specimen has been taken at Cape Cod on the eastern coast of the United States, and another off Dunedin, New Zealand. The capture of thirty examples in British waters since 1828 has been recorded, the largest a female 9 feet in length.

Most of the existing genera of Spinacidae are represented by teeth or detached spines in the later Tertiary deposits, but none are certainly known to occur earlier than the Pliocene.

FAM. 14. RHINIDAE (Angel-Sharks).—Ray-like Sharks with a flattened head and body, and nearly terminal mouth and nostrils. Pectoral fins very large, horizontally expanded, but constricted at the base and not adherent to the sides of the head or trunk. Two dorsal fins, both small, without spines, and situated on the tail behind the pelvic fins. Anal fin absent. Spiracles large {457}and crescentic. Vertebrae tectospondylic. Teeth conical and pointed. A single species only is known.

Rhina squatina, the Angel-Shark or Monk-Fish (Fig. 261), is intermediate between the ordinary Sharks and the Skates and Rays, both in external appearance and internal structure, but is more Ray-like than Shark-like in its habits. Within the temperate and tropical regions of both hemispheres it is almost cosmopolitan in its distribution, frequenting the coasts of Europe, including the British Isles, the Atlantic and Pacific coasts of North America, and the shores of South Australia and Japan. The Angel-Shark is viviparous, producing about twenty young at a time. Not rarely it grows to a length of 5 feet.

The family ranges from the Upper Jurassic to the present time. Species of Rhina are represented by more or less complete skeletons in the Lithographic Stone of Bavaria, and in the Upper Cretaceous of Westphalia and Mount Lebanon, and by teeth and vertebrae in the English Chalk, as well as in different European Tertiary formations.

FAM. 15. PRISTIOPHORIDAE.—Prenasal portion of the head and cranium produced into a long flattened rostrum, furnished with a pair of long tentacles on its under surface, and, as in Saw-Fishes, with a series of large, tooth-like, dermal denticles, of equal or unequal size, along each of its lateral margins. Two dorsal fins, without spines, the first in front of the pelvics. No anal fin. Pectoral fins large, distinct from the head and trunk, with a contracted base. Spiracles large and crescentic. Teeth small, with a conical cusp and a broad base.

These singular Sharks closely resemble the true Saw-Fishes (Pristidae), but they differ in the lateral position of their gill-clefts, the presence of rostral tentacles, and their smaller size. The few species known belong to the genus Pristiophorus, and are confined to the Australian and Japanese seas.

Pristiophorus is represented in the Upper Cretaceous of Mount Lebanon, and in the Miocene deposits.

SUB-ORDER 2. BATOIDEI.

Body generally discoidal or rhombic in shape, the axial portion being formed by the flattened head and trunk, and the lateral portions by the enormously expanded pectoral fins, which {458}are usually confluent with the sides of the head. Tail slender, sharply marked off from the trunk, to which it usually appears as a mere appendage. Dorsal fins, when present, on the tail. Anal fin absent. Branchial clefts ventral in position. Spiracles large, usually crescentic. Vertebrae tectospondylic.

For the most part the Batoidei are sluggish ground-Fishes, slowly moving over the sea-bottom by the gentle undulatory vibrations of the margins of their huge pectoral fins, the tail being of little use in locomotion. They feed principally on Crustacea, Molluscs, and the smaller Teleosts. As with other Fishes of similar habits, the coloration of the dorsal surface harmonises with that of the sea-bottom, while the ventral surface is either deficient in pigment or white. The majority of them are coast Fishes, rarely descending to a greater depth than 500 fathoms, but some are pelagic. The Batoidei are a relatively modern race, first appearing towards the middle of the Mesozoic period, and evidently representing an assemblage of specialised Elasmobranchs adapted for a bottom-living existence. As remarked by Smith Woodward, the three families, Rhinobatidae, Raiidae, and Trygonidae, are not so clearly differentiated before the close of the Cretaceous period as they subsequently become.

The first two families, the Pristidae and the Rhinobatidae, are interesting connecting-links between such Selachii as the Rhinidae and the Pristiophoridae and the more specialised Batoidei like the Skates, Rays, and Trygons. While they agree with the latter in the ventral position of the gill-clefts, the absence of an anal fin, and the caudal position of the dorsal fins, the body still retains an elongated and somewhat Shark-like shape, and shades off imperceptibly into a powerful swimming tail, and in the Pristidae at all events the pectoral fins are of moderate size and free from any fusion with the sides of the head. It must be admitted that the institution of the two sub-orders introduces a somewhat arbitrary distinction between certain families of Plagiostomes which has little to recommend it except custom and some measure of convenience. The two series of Fishes shade almost imperceptibly into one another, and the importance of the ventral position of the gill-clefts has probably been overestimated. Primitively, the gill-clefts are lateral, and lie wholly in front of the pectoral fins, a position which is retained in many {459}Selachii. In others, however, the hinder gill-clefts tend to extend backwards above the base of the pectoral fins, while in some the clefts assume a more ventral position, and extend beneath the pectoral fin; hence, even within the limits of the Selachii the position of the gill-clefts varies to the extent that these structures may be lateral, or they may tend to become either dorsal or ventral. On the score of convenience the customary usage is adopted here.

FAM. 1. PRISTIDAE (True Saw-Fishes).—Although somewhat depressed, the body is still elongate and Shark-like, with a well-developed tail terminating in a heterocercal caudal fin. Dorsal fins large, the first opposite the pelvic fins. Head and skull prolonged into a long flattened rostrum, the lateral margins of which are armed with a series of strong tooth-like denticles, firmly implanted in sockets in the calcified rostral cartilage. No rostral tentacles. Teeth in the jaws minute and obtuse. One genus and about four or five species are known, all inhabitants of tropical and subtropical seas.

Some of the true Saw-Fishes attain a considerable size, 10 to 20 feet or even longer, and "saws" 6 feet long and a foot in width across the base are not uncommon. By means of powerful lateral strokes of its saw the Fish is capable of lacerating the bodies of other animals and tearing off pieces of flesh, which it then devours. Indian species are known to ascend rivers beyond tidal influence, and an American species, ranging northwards to the West Indies and the Gulf of Mexico, where it is abundant, enters the lower Mississippi. P. antiquorum occurs in the Mediterranean and the Atlantic, but does not extend so far northward as the British coasts.

The earliest known representative of the family is the {460}extinct genus Sclerorhynchus from the Upper Chalk of Mount Lebanon, in which the smaller size and more superficial position of the rostral "teeth," and the absence of sockets in the rostral cartilage, prove that the "teeth" approximate more to ordinary dermal spines in this genus than in any of the more recent Saw-Fishes. An extinct genus Propristis, from the Upper Eocene of Egypt, with non-socketed teeth, and species of the existing genus Pristis from the English Middle Eocene, are also known.

FAM. 2. RHINOBATIDAE.—Owing to the increased expansion of the pectoral fins and the forward growth of their anterior cutaneous portions along the sides of the head, as well as backwards along the trunk, the body now assumes a sub-rhombic shape, and approximates to the disc of the more typical Batoidei, but the tail with its dorsal and caudal fins is still strongly developed, and blends imperceptibly with the trunk in front. Teeth very obtuse. No electric organs. About five genera and twenty species are known, distributed in most tropical and subtropical seas.

The cosmopolitan Rhinobatus is represented by species from the Mediterranean, the Red Sea, the west coast of Africa, the Indian Ocean, Australia and China, as well as from the Atlantic and Pacific coasts of America, and the Galapagos. Rhynchobatus ranges from the Red Sea through the Indian Ocean to China, Zapteryx occurs at San Diego and Panama, and Platyrhinoidis on the Californian coast. Trygonorhina is an Australian genus.

The family dates from the Upper Jurassic. Rhinobatus is {461}represented by complete skeletons in the Lithographic Stone of Bavaria, the Upper Cretaceous of Mount Lebanon, and the Upper Eocene of Monte Bolca. Trygonorhina occurs in the Eocene.

FAM. 3. RAIIDAE (Skates or Rays).—The endoskeletally supported portions of the large pectoral fins extend along the lateral margins of the trunk and head from the pelvic fins to the snout, and are confluent therewith, forming the lateral portions of a large rhombic disc. The tail is slender, and sharply marked off from the trunk. Usually two small dorsal fins on the tail. Caudal fin small or absent. No serrated spine on the tail. Caudal electric organs are often present. Larger or smaller denticles or spines are generally present on the skin. Oviparous. Egg-cases four-horned, without tendrils. Four genera and from thirty to forty species. Found in all temperate seas, a few ranging into deep water.

The great majority of the species belong to the genus Raia (Fig. 264), which chiefly inhabits temperate seas, but is more abundant in the northern than in the southern hemisphere, and approaches nearer to the Arctic and Antarctic regions than any other Batoidei. The colour of the upper surface of the body is closely assimilated to that of the sandy or gravelly bottom on {462}which they live, and thus concealed, small Fishes, Crustaceans, and other organisms are lured unsuspectingly within the reach of the comparatively inactive and sluggish Ray. From the ventral position of the mouth the Ray cannot at once seize its prey, but the Fish darts over its victim and covers it with its body, and then readily devours it. The sexes are usually distinguished by secondary sexual characters, which take the form of differences in size and coloration, in the dentition, and also in the presence and position of patches or rows of specially modified dermal spines on the dorsal surface (Fig. 264). Some of the larger species reach a great size, the disc measuring 7 to 8 feet in width. A few species range into deep water. R. mamillidens, a uniformly jet-black species, has been obtained from a depth of 597 fathoms in the Bay of Bengal, and R. abyssicola from 1588 fathoms off Queen Charlotte Islands, British Columbia. The following are British species: the Thornback (R. clavata); the Spotted Ray (R. maculata); the Painted Ray (R. microcellata); the Starry Ray (R. radiata); the Cuckoo or Sandy Ray (R. circularis); the Skate (R. batis); the Flapper Skate (R. macrorhynchus); the White Skate (R. alba); the Long-nosed Skate (R. oxyrhynchus); and the Shagreen Ray (R. fullonica). Most of the species are of some economic value as food Fishes. Psammobatis, with a circular disc, frequents the southern coasts of South America, and Platyrhina the coasts of India, China, and Japan.

The family ranges from the Upper Cretaceous, in which, as well as in different Tertiary deposits, it is represented by species of Raia. An extinct genus, Cyclobatis, with a circular or oval disc, occurs in the Upper Cretaceous of Mount Lebanon.

FAM. 4. TAMIOBATIDAE.—The systematic position of the only representative of this family, Tamiobatis vetustus, from the Devonian or Lower Carboniferous of Kentucky, is very uncertain, but in some respects this unique type seems to be intermediate between the modern Sharks and the Rays.

FAM. 5. TORPEDINIDAE (Electric Rays).—A disc is formed as in the Raiidae, but it is sub-circular in shape rather than rhombic, and in the nature of its endoskeletal supports it is in some respects unique. Its semicircular anterior margin is supported {463}in the centre by a branched prenasal rostrum, and laterally by the curiously branched preorbital cartilages, each of which radiates outwards and forwards from a common basal articulation with the lateral ethmoid regions of the skull. Tail relatively short and thick, with two dorsal fins, a caudal fin, and two lateral longitudinal folds. Skin smooth, without denticles. Mouth transverse and ventral. A characteristic quadrangular naso-frontal lobe, with a free hinder margin, which forms the anterior lip, is enclosed by the two nasal organs and the oro-nasal grooves leading from them to the corresponding angles of the mouth. A pair of large electric organs between the pectoral fins and the head. Seven genera and about fifteen species. Inhabitants of most warm seas.

The well-known genus Torpedo (Fig. 265) is represented by species in the Mediterranean (T. marmorata, T. narce, T. hebetans), the Red Sea, and the Atlantic and Pacific Oceans. T. hebetans {464}has been taken at several places in British waters. An American Torpedo (Tetronarce) is represented by species on the Atlantic and Pacific coasts. Narcine is a very widely distributed genus, species having been recorded from the East Indies, Tasmania, China, Japan, South Africa, and the Atlantic coasts of North and South America. Discopyge is an eastern Pacific genus (Peru and Panama). Hypnos frequents the Australian seas.

The family seems to be exclusively Tertiary, and its earliest fossil representatives are from the Upper Eocene of Monte Bolca.

FAM. 6. TRYGONIDAE (Sting- or Whip-tailed Rays).—Disc sub-rhombic, broader than long. Pectoral fins confluent with the sides of the head, their preaxial endoskeletal radialia meeting in front of the skull along the lateral margins of a slender prenasal rostral cartilage. Tail usually whip-like, terminating in a small caudal fin, and generally armed with a sharp, serrated spine, which takes the place of a dorsal fin. Skin smooth or spinose. A rectangular naso-frontal flap in front of the mouth. About ten genera and fifty species. Found in nearly all tropical and subtropical seas.

Of the more important genera, Trygon (Dasyatis) is represented by numerous species in the tropical parts of the Atlantic and Pacific Oceans, including the Pacific coasts of North and South America. Two species occur in the Mediterranean, and one of them (T. pastinaca), ranges from the coasts of Norway and the British Isles through the Atlantic and Indian Oceans to Japan. Urogymnus frequents the Red Sea and the Indian Ocean. Urolophus includes a few species of small size, distributed along the Atlantic and Pacific coasts of Central and North America, and in Australian seas. Pteroplatea comprises rather large species, and is almost cosmopolitan in its distribution, being represented by species on the Atlantic and Pacific coasts of North and South America, in the Mediterranean and the Red Sea, the Indian Ocean, the Malay Archipelago, and on the coasts of China and Japan. The caudal spines, which may be 8 to 9 inches long in some of the larger species, are capable of inflicting very severe wounds, the danger of which is greatly increased by the apparently poisonous cutaneous mucus introduced into the wound. As the spines become lost they are replaced by others developed from behind. Some Trygonidae live in fresh waters. Trygon (Dasyatis) sabina frequents the streams and estuaries of Florida {465}as well as on the adjacent coasts, and specimens have been obtained from Lake Munroe at some distance from salt water. Ellipesurus and Paratrygon are freshwater genera, found in Colombia, Venezuela, and Guiana.

Fossil remains of undoubted Trygonidae appear to be confined to the Tertiary period.

FAM. 7. MYLIOBATIDAE (Eagle-Rays).—Disc much broader than long, and rhombic in shape. The huge pectoral fins are not continued to the extremity of the snout, but cease on the sides of the head, and reappear in front of the snout as a pair of distinct folds, the so-called cephalic fins. The head projects above the level of the disc, and consequently the eyes and spiracles are lateral in position. Tail long, slender, and whip-like, with a single dorsal fin near the root, and usually one or two serrated spines behind the fin. A rectangular naso-frontal fold is present. The dentition consists of flat, hexagonal, pavement-like crushing teeth arranged from before backward in arched rows in both jaws, and there is either a single median row of large teeth, with (e.g. Myliobatis) or without (e.g. Aëtobatis) the addition of several rows of much smaller teeth on each side, or there are numerous rows, the teeth then decreasing in size from the middle line laterally (e.g. Rhinoptera). Skin smooth. Sexes similar. Five genera and about twenty-seven species are known; all inhabitants of tropical and subtropical seas.

Myliobatis is represented in the Mediterranean by two species, and one of them, the almost cosmopolitan M. aquila (Fig. 266), has been taken at various points on the eastern and southern coasts of England. Aëtobatis is also widely distributed in tropical seas, but is unknown in European waters. Rhinoptera has one species in the Mediterranean, while others have been recorded from Brazil, the Atlantic and Pacific coasts of North America, and the East Indies. The two tropical genera Dicerobatis and Ceratoptera have the cephalic fins prolonged anteriorly into a pair of horn-like appendages, which are said to be used in conveying food to the mouth. The teeth are small, flat or tubercular, and are arranged in numerous rows. In Ceratoptera they are wanting in the upper jaw. The Eagle-Rays feed principally on Molluscs, the shells of which they crush with their large grinding-teeth. Some of them attain {466}an enormous size, and are among the largest of Fishes. Ceratoptera vampyrus of the West Indies, for example, grows to a width of 20 feet, and an embryo extracted from the oviduct of a gravid female 15 feet wide, and from 3 to 4 feet in thickness, measured 5 feet across the disc and weighed twenty pounds. This Fish is much dreaded by the divers engaged in the pearl fisheries near Panama, whom it is said to devour after enveloping them with its vast wings.

The family is exclusively Tertiary, and with the exception of an extinct genus, Promyliobatis, from the Eocene of Monte Bolca, all the fossil species belong to the existing genera Myliobatis, Rhinoptera, and Aëtobatis.

ORDER V. HOLOCEPHALI

The propriety of including the Holocephali in the sub-class Elasmobranchii is scarcely open to doubt. Like the Acanthodei {467}they seem to represent a divergent and specialised offshoot from some primitive Elasmobranch type, and while retaining most of the essentially distinctive features of their ancestors, they have acquired, perhaps independently, certain characters distinctive of the Teleostomi, combined with others peculiar to themselves. In the few surviving genera agreement with the Elasmobranchs is to be seen in the wholly cartilaginous condition of the endoskeleton and the complete absence of cartilage- and membrane-bones. The vertebral column is acentrous and ribless, and the notochord is persistent; the dorsal arcualia include supradorsals and regularly alternating basi- and inter-dorsals. The limbs and limb-girdles are essentially Elasmobranch. Dermal denticles are present, either locally, or, as in some of the fossil types, in the form of a general investment. The brain and the reproductive organs agree more closely with the corresponding structures in the Elasmobranchs than with those of any other Fishes, and the agreement extends to the large size of the eggs and their enclosure in horny egg-cases. In both groups the nostrils are connected with the mouth by oro-nasal grooves; the hyoidean hemibranch is a true gill, and there is no air-bladder. The Holocephali also agree with the Elasmobranchs in retaining such primitive features as an intestinal spiral valve and a conus arteriosus. On the other hand, indications of specialisation in the Teleostome direction are to be noticed in the tendency to the concentration of the branchial arches towards and beneath the skull; the reduction of the interbranchial septa to the extent that they are no longer continuous with the skin, and the gill-filaments project beyond their outer margins; the presence of an operculum; the suppression of the spiracles; and the absence of a cloaca, the rectum opening externally by an anus in front of the urino-genital apertures. Among the more notable features evolved within the limits of the group mention may be made of the autostylic condition of the skull, probably an adaptive modification induced by the large size of the crushing dental plates which have taken the place of ordinary teeth; and the singular development of anterior and frontal "claspers."

The group is one of great antiquity. Apart from the isolated spines or "ichthyodorulites" common in Devonian and Carboniferous strata, some of which are probably the frontal or the {468}fin-spines of ancient Holocephali, dental plates, closely resembling those of modern Chimaeroids and referred to the Ptychodontidae, are probably the earliest indications of the existence of the group. The Holocephali become more abundant in the Mesozoic period, but of the four families usually recognised, only one, the Chimaeridae, has survived.

FAM. 1. PTYCTODONTIDAE.—This Palaeozoic family is known only by the dental plates, of which there is a single pair in each jaw, meeting at the symphysis. Ptyctodus and Rhynchodus occur in the Devonian of either Russia or Germany, and in North America, and Palaeomylus only in the Devonian of North America.

FAM. 2. SQUALORAIIDAE.—General shape of the body similar to the existing Harriotta. There is a long, depressed, preoral rostrum, and in the male the head carries a long slender frontal spine. Conical denticles are sparsely present on the head and body. No dorsal fin-spine. Dental plates similar to those of the living Chimaeroids, but thinner, the tritoral areas being less well defined. The only genus is Squaloraia from the English Lias, of which nearly complete skeletons are known.

FAM. 3. MYRIACANTHIDAE.—Body elongate, but less depressed. A dorsal fin-spine is present, and in the males a frontal spine. The dentition consists of a median incisor-like tooth at the symphysis of the lower jaw, in addition to dental plates similar to those of Squaloraia. There is a symmetrical series of tuberculated dermal plates on the lateral surfaces of the head, which probably represent groups of fused denticles. One species (Myriacanthus granulatus) has its rostrum terminating in a cutaneous flap, as in Callorhynchus. Myriacanthus, from the Lower Lias of Lyme Regis, and Chimaeropsis, from the Lithographic Stone of Bavaria, are the only two genera.

FAM. 4. CHIMAERIDAE.—Body elongate and shark-like in form, but the head is compressed and the mouth is small. Pectoral and pelvic fins large, especially the former, which are somewhat ventrally placed. Two dorsal fins, the anterior over the pectorals, with a stout spine in front; and a small anal fin. Dermal denticles restricted to the claspers, and to localised areas {469}on the dorsal surface in young forms. Dental plates large and thick, including a single pair in the lower jaw and two pairs, vomerine and palatine teeth, above, which combine trenchant edges with well-marked grinding areas. Three genera are known.

In Chimaera (Fig. 267) the mouth and nostrils are ventral, posterior to a bluntly conical snout. Head surmounted in the males by a club-shaped appendage armed with a pad of recurved denticles, the frontal clasper; there is also an anterior clasper armed with similar denticles and retractile into a shallow glandular pouch in front of each pelvic fin, in addition to the ordinary clasper behind the fin. The caudal fin consists of nearly equal-sized dorsal and ventral lobes, between which the slightly up-tilted caudal axis is prolonged as a long tapering filament: hence the tail appears to be nearly diphycercal. C. monstrosa occurs off the coasts of Europe from Norway to Portugal, including the Mediterranean, and also in the neighbourhood of the Azores, as far south as the Cape of Good Hope, and eastwards off the coast of Japan. It is the largest of the living species, reaching a length of 3 feet. C. affinis was first taken off the coast of Portugal, and subsequently on the North American side of the Atlantic, at depths ranging from 200 to 1200 fathoms. C. (Hydrolagus) colliei is restricted to the North Pacific, and is especially plentiful off South-eastern Alaska, and about the wharves at Esquimalt. Unlike most other Chimaeroids this species swims at the surface, and there is no evidence that it is a deep-sea form. In its breeding habits, and in the mode in which its eggs are fertilised, Chimaera probably resembles the oviparous Sharks and Dog-Fishes.

{470}[Illustration: FIG. 268.—Egg-case of a species of Chimaera. a, Transverse section across the case at x, showing the lateral valvular slits; b, similar section across x′, showing the vertical ridges. (From Günther.)]

The eggs appear to be deposited on the sea-bottom in deep water, but they are very rarely obtained. An egg-case dredged up off the south-west coast of Ireland, at a depth of 315 fathoms, and about 6½ inches in length, is shown in Fig. 268. It consisted of a broad, somewhat oval, flattened portion which contained the egg, and terminated at one end in a truncated margin, while at the other it was produced into a long tapering styliform process, traversed by dorsal, ventral, and lateral ridges. The cavity of the egg-case was open in front, and also along each side, where linear, slit-like valvular apertures freely admitted sea-water into the central cavity. A similar egg-case from Japan, measuring 9 inches in length, had its surface traversed by longitudinal and {471}transverse ridges, and no doubt belonged to a Japanese Chimaera. In neither egg-case was there any trace of tendrils. The eggs probably lie on the sea-bottom, or, when the cases have styliform prolongations, it is possible that they are implanted in the ooze.

Callorhynchus (Fig. 269) is distinguished by a singular prolongation of the rostrum, which terminates in a downwardly-directed cutaneous flap, evidently from its abundant nerve-supply an important tactile organ. A frontal clasper is present in the male. The prolonged caudal axis is up-tilted, and the tail is more distinctly heterocercal than in Chimaera. The only species, C. antarcticus, is confined to the Antarctic basin and the South Pacific. The egg-cases of Callorhynchus differ considerably from those of Chimaera, and so large are they that one may measure 25 cm. in length, or nearly as long as the abdominal cavity of the Fish. Each case is ovoid in shape, surrounded by a wide flat margin which is covered on one side with yellow hair-like fibres, thus giving to the case a protective resemblance to a mass of seaweed (Fig. 270). In the central part of the case there is a pear-shaped cavity in which the egg or the embryo is contained. From one end of this cavity a passage, guarded by a valve, leads to the exterior, and provides for the escape of the young. While in the egg-case the nearly ripe embryo has long external gills, and its body is nearly sessile on a large and singularly lobed yolk-sac.

{472}[Illustration: FIG. 271.—Harriotta raleighana. A, lateral view; B, ventral view of a male. (From Goode and Bean.)]

The third genus, Harriotta (Fig. 271), is remarkable for its elongated, tapering, and depressed rostrum, and for the large size and wing-like appearance of the pectoral fins. There is no frontal clasper, and the ordinary claspers in the young male examined {473}were very small and simple. The caudal filament, which is longer in older specimens than in the younger, and is not developed at all in the youngest examples at present known (Fig. 272, A), is not uptilted, although the lower lobe of the caudal fin is much larger than the upper. Young forms have a double row of stout spine-like denticles in front of the second dorsal fin, and also in the interval between the latter and the upper caudal lobe. Similar denticles are also present on the upper surface of the head between the orbits (Fig. 272). H. raleighana is found in the North Atlantic. Individuals varying in length from 4 to 25 inches have been taken at depths ranging from 707 to 1081 fathoms. A species of Harriotta has also been recorded as occurring in Japanese waters.

With the probable exception of Chimaera colliei the surviving Holocephali are denizens of deep water; hence their comparative rarity and our almost complete ignorance of their habits. Young forms of C. monstrosa, 1½ to 5 inches in length, have been dredged in the Färoe Channel at depths from 505 to 555 fathoms; and the youngest specimen of Harriotta was obtained from 991 fathoms. Egg-cases are rarely obtained, and then only from considerable depths. It is therefore reasonable to {474}infer that these Fishes breed in deep water. As might be expected, little is known of the embryology of any of the Holocephali, but that little adds further proof of the Elasmobranch relationship of the group. The segmentation of the egg of Chimaera and the overgrowth of the yolk by a circular blastoderm are essentially as in Elasmobranchs. The early embryos are said to be shark-like, and to possess both spiracles and "external gills," and the primary upper jaw is less completely confluent with the skull than in the adult. It is also said that the palatine dental plates are represented at an early stage by series of small, more or less conical elements, which, outwardly at least, resemble the rudiments of the grinding teeth of the Cestraciont Sharks.

The Chimaeridae first appear in the Lower Oolites, and attain their maximum development in the Cretaceous and the Eocene. Ganodus is an Oolitic genus. Ischyodus ranges from the Lower Oolites to the Lower Cretaceous. Edaphodon is Cretaceous and Eocene, extending, however, into the Miocene, and Elasmodus ranges from the Upper Cretaceous into the Eocene. Teeth of the existing genus Callorhynchus occur in the Cretaceous of New Zealand, and of Chimaera in the Upper Tertiary of Europe and Java. The fossil Holocephali afford little evidence of the origin of the group from more typical or more primitive Elasmobranchs. So far as their structure is known, they all possess the essentially distinctive features of their modern representatives, and offer little evidence of transitional forms. The surviving Chimaeroids seem to have acquired a more specialised dentition, but in other respects they are either more primitive, or possibly somewhat degenerate.

{475}CHAPTER XVIII

TELEOSTOMI: GENERAL CHARACTERS—CROSSOPTERYGII—CHONDROSTEI—HOLOSTEI

SUB-CLASS II. TELEOSTOMI.

In this group of Fishes the primary upper and lower jaws (palato-quadrate and Meckelian cartilages) are supplemented by the addition of certain tooth-bearing membrane bones which form secondary jaws corresponding to the functional jaws of the higher Craniates. The chondrocranium and the primary jaws are usually more or less completely ossified by cartilage bones, and there is always a secondary cranium of dermal bones, of which paired parietals and frontals above, and a median vomer and a parasphenoid below, are amongst the most constant. The skull is hyostylic. An operculum covering the gill-clefts and supported by a special opercular skeleton is a constant feature. The vertebral column is often acentrous, and when centra are present they are invariably arch-centra. There is a well-developed secondary pectoral girdle, connected dorsally with the hinder part of the skull. As a rule the pelvic girdle is absent altogether, and when present it is rarely more than a rudiment or a vestige. The endoskeletal supports of the paired fins are uniserial. The dermal fin-rays of the paired and median fins are probably modified scales or lepidotrichia. In the median fins the fin-rays are at first more numerous than their supporting radials, but in the more specialised Teleostomes they ultimately equal them in number. The body is usually invested by an exoskeleton of articulated rhombic or imbricated cycloid scales. Claspers are unknown. In the surviving members of the group there is usually an {476}air-bladder. The gill-filaments project freely beyond the outer edges of the greatly reduced interbranchial septa. The external opening of each nasal sac is usually divided into two distinct apertures, and there is no oro-nasal groove leading from the sac to the mouth. The brain has no proper cerebral hemispheres, but retains an undivided prosencephalon with a non-nervous roof. A cloaca is not developed, the rectum opening externally by an anus in front of, and distinct from, the separate or united urino-genital apertures. The ova are small and numerous, and the segmentation is either holoblastic and unequal, or meroblastic. Besides a large number of fossil forms the group includes the vast majority of living Fishes.

The Teleostomi include four "Orders," the CROSSOPTERYGII, the CHONDROSTEI, the HOLOSTEI, and the TELEOSTEI. Of these the Crossopterygii occupy a remarkably central position. Remotely connected with the Elasmobranchs on the one hand, and more intimately related to the Holostei and Teleostei on the other, they also probably represent the ancestral stock from which the Stegocephalan Amphibia and the Dipneusti have had their origin. Of the three remaining groups, often collectively spoken of as "Actinopterygii," the Chondrostei are the oldest and most primitive. Like the Crossopterygii, they are not without evidence of a remote kinship with the Elasmobranchs, but in a broad general sense they also represent the initial stages in a sequence of structural modifications, of which the Teleostei, the dominant Fishes of the present day, are the final outcome.

ORDER I. CROSSOPTERYGII.

Pectoral fins obtusely lobate and probably uniserial, or acutely lobate and probably biserial. Pelvic fins abdominal in position, uniserial, non-lobate, or obtusely lobate. Scales rhombic or cycloid, and, like the dermal cranial bones, they are generally invested by a layer of enamel-like ganoin. Tail heterocercal, or apparently diphycercal or gephyrocercal. Vertebral column acentrous, or with ring-like centra, or even with complete bony amphicoelous centra. Lower jaw with dentigerous splenials. As a rule, the opercular series includes an operculum and a suboperculum. Branchiostegal rays absent, their place being taken by a remarkable armature of jugular plates (Fig. 274). Secondary pectoral girdle {477}complete, including a pair of infra-clavicles. With rare exceptions the fin-rays of the median fins retain their numerical preponderance over the supporting radials. The group is divisible into two "sub-orders," the OSTEOLEPIDA and the CLADISTIA.

SUB-ORDER 1. OSTEOLEPIDA.

The obtusely or acutely lobate pectoral fins articulate with the pectoral girdle by a single basal endoskeletal element. Nostrils on the ventral surface of the snout. Two dorsal fins and an anal fin. Dermal bones of the ethmoid region often fused with one another and with the premaxillae in front and the frontals behind to form a continuous rostral shield. Infra-dentary bones may be present. A series of lateral jugular plates often present in addition to the pair of principal plates. The Osteolepida first make their appearance in the Old Red Sandstone and Devonian formations, where they become abundant. They are also well represented in the Carboniferous, but only one family survived to the Mesozoic period, finally becoming extinct in the Upper Cretaceous. The following are the more important families:—

FAM. 1. OSTEOLEPIDAE.—Scales rhombic and thickly enamelled. Pectoral and pelvic fins obtusely lobate. Tail heterocercal. Teeth simple, not complicated by surface infoldings except quite at the base. Genera:—Osteolepis (Fig. 273), Thursius, Diplopterus (Middle Old Red Sandstone, Scotland), Glyptopomus (Upper Old Red Sandstone, Scotland), {478}Megalichthys (Carboniferous and Lower Permian of Europe and North America).

FAM. 2. RHIZODONTIDAE.—Scales cycloid and overlapping. Paired fins obtusely lobate. Tail heterocercal, sometimes apparently gephyrocercal. Teeth with the external enamelled layer of dentine infolded towards the axis in the form of radially arranged folds. In some genera ring-like vertebral centra have been recognised and also a preoperculum. Genera:—Rhizodus, Lower Carboniferous of Scotland and Northumberland; Tristichopterus (Fig. 275), Old Red Sandstone of Scotland; Eusthenopteron (Fig. 276), Upper Devonian of Scaumenac {479}Bay, Canada; Gyroptychius, Old Red Sandstone, Scotland; Rhizodopsis (Fig. 274), Carboniferous of England, Scotland, Silesia, and North America; Strepsodus, Carboniferous of Great Britain, Ireland, and North America.

FAM. 3. HOLOPTYCHIDAE (Dendrodontidae).—Scales cycloid. Pectoral fins acutely lobate; pelvic fins short and somewhat obtusely lobate. Tail heterocercal. Teeth similar to those of the Rhizodontidae but more specialised, the enamelled dentine infoldings being much more complicated, presenting a radiating {480}arborescent appearance in transverse sections. Vertebral column acentrous. Genera:—Holoptychius (Fig. 277), Old Red Sandstone of Scotland; Devonian of Belgium, Russia, North America, and East Greenland. Glyptolepis has a similar range.

FAM. 4. COELACANTHIDAE.—Scales cycloid. Paired fins obtusely lobate. Tail symmetrical but apparently gephyrocercal, usually with a protruding axial vestige of the disappearing terminal part of the tail and of the proper caudal fin. Radialia of the functional caudal lobes agree in number with the contiguous neural and haemal arches and dermal fin-rays, the diagnostic feature of Smith Woodward's Actinistia. Proximal radials of the dorsal and anal fins fused into a single, internally-forked basipterygium in each fin. Teeth simple. Vertebral column acentrous. The skull presents several interesting features. The hyomandibular and the palato-quadrate bar, for example, are fused on each side into a continuous triangular bone, articulating with the cranium above and with the lower jaw below. The opercular skeleton is reduced to an operculum and two jugular plates. A very singular feature in these Fishes is the ossification of the walls of the air-bladder (Fig. 278), a structural modification which has no parallel in Fishes, except in certain Teleosts (Siluridae and Cyprinidae) {481}in which the organ becomes encapsuled by bone owing to the partial ossification of its walls.

From their first appearance in the Lower Carboniferous the Coelacanthidae range, practically unchanged, through the intervening formations to the Upper Cretaceous. Coelacanthus itself occurs in the Carboniferous and Permian of England, Scotland, and Germany, and in the Carboniferous of North America. Undina (Fig. 278) is a Jurassic genus. Diplurus is found in the Trias of North America, and Macropoma is a well-known form from the Middle and Upper Cretaceous beds of England, and other parts of Europe.

SUB-ORDER 2. CLADISTIA.

Pectoral fins uniserial and abbreviate, with three basal endoskeletal elements. Nostrils on the upper surface of the snout. Entire skeleton well ossified. Notochord replaced by bony, amphicoelous vertebral centra. Bones of the ethmoid region not fused to form a rostral shield. Infra-dentary bones absent. Jugular plates reduced to a single pair of large plates. As this group includes the only Crossopterygii which have survived to the present day, it is noteworthy that they retain certain primitive features indicative of their remote origin. The spiracles are persistent; the intestine has a spiral valve; and the conus arteriosus is furnished with several rows of valves. Amongst other characters of contrary significance, the air-bladder is double; its oesophageal aperture is ventral; and its afferent arteries are pulmonary arteries derived from a posterior aortic arch.

FAM. 5. POLYPTERIDAE.—Pectoral fins obtusely lobate. Pelvic fins non-lobate. Scales rhombic and thickly enamelled. Dorsal fin in the form of a series of isolated finlets, each consisting of a stout spine-like fulcral scale supporting a single soft ray, or a fringe of several rays, along its hinder margin. Tail symmetrical, apparently gephyrocercal. Teeth simple. Nostrils tubular.

The only representatives of the sub-order and the sole {482}surviving family of Crossopterygii, the Polypteridae, are restricted to the Nile and to the river basins of tropical Africa which drain into the Atlantic (Fig. 280). Only two genera are known, Polypterus and Calamichthys, neither of which has yet been discovered in any geological deposits, ancient or recent.

In Polypterus each of the spines of the dorsal fin supports several soft rays. Pelvic fins and a suboperculum are present. Ten species are known, of which six pertain to the Congo and its tributaries. P. bichir is said to attain a length of four feet.

Until recently little was known of the habits of Polypterus, but the observations of Budgett on the widely distributed P. senegalus and those of Harrington on P. bichir, have brought to light many interesting facts about these most interesting Fishes.

P. bichir haunts the deeper holes and depressions of the muddy bed of the Nile, although it is "not essentially a bottom-liver or a mud-fish." It is most active at night when in search of food, and then it may readily be taken by trawl lines. The lobate pectoral fins are used for progression, but their primary function is to act as balancers, and they exhibit the characteristic trembling movements so often seen in the balancing fins of Teleosts. Polypterus does not readily live out of water, rarely longer than three to four hours, and then only when covered with damp grass or weeds. P. bichir is said to feed on small Teleosts, which it swallows whole, and to these there may be added in other species, Batrachians and Crustaceans. The observations of Budgett show that in captivity Polypterus often remains motionless for a long time at the bottom of the water, the anterior part of the body resting upon the tips of the {483}pectoral fins. According to the same observer, the air-bladder is an accessory respiratory organ, supplementary to the gills, rather than a hydrostatic organ.

In P. bichir the eggs ripen from June to September, inclusive, and, as in most other Nile Fishes, the breeding season is during or just after the period of inundation. P. senegalus and P. lapradei spawn during the rainy season in the months of July, August, and September, but nothing is certainly known as to the place or mode of deposition of the eggs. During the breeding season Polypterus is unusually active and excitable, and at this period the anal fin of the male becomes greatly thickened and enlarged, and has its surface thrown into deep folds between the successive fin-rays. The use of the modified fin is not known. During his stay at McCarthy Island, about 160 miles up the River Gambia, Budgett was fortunate in securing a larva of P. senegalus, 1 to 1¼ inches in length, or only about one-third the length of any larval Polypterus previously known (Fig. 281). The larva is described as a most beautiful object, "marked with black stripes on a golden ground, with a conspicuous golden stripe on each side above the eye, across the spiracle, and along the dorsal surface of the external gill." The pinnate external or cutaneous gills were relatively of much greater size than in the considerably more advanced stage figured elsewhere, and reached half-way to the tail. The dorsal fin is not divided into finlets, and behind it is continuous with the caudal, while the anal fin is scarcely distinct from the {484}lower lobe of the caudal. The fin-rays which support the ventral portion of the caudal fin are more numerous and longer than those in relation with the dorsal lobe, and hence at this stage the tail is really heterocercal.

In the genus Calamichthys the body is greatly elongate and Eel-like in shape. Pelvic fins are absent, and normally there is no suboperculum. The dorsal finlets are more isolated than in Polypterus, and each spine supports but a single soft ray. Only a single species is known, C. calabaricus (Fig. 282).

Calamichthys has a more restricted distribution than Polypterus, and is confined to certain rivers of West Africa. First obtained at Creek Town on the Old Calabar river, it is now known to occur in the delta of the Niger, on the coast of Cameroon, and as far south as the river Chiloango, frequenting the smaller muddy rivers opening into the estuaries. It is a {485}very agile Fish, swimming like a snake, and subsisting on insects and crustaceans. The anal fin is enlarged in the male, and the young are provided with cutaneous gills. Calamichthys may attain a length of nearly 40 cm.

* * * * *

In the remaining Teleostomi (ACTINOPTERYGII) the paired fins are invariably non-lobate, with abbreviate, multibasal endoskeletal supports. Fin-rays are the main support of both the median and paired fins. Jugular plates are usually replaced by branchiostegal rays, but both may co-exist. The Actinopterygii are the successors of the Crossopterygii in palaeontological sequence, and when the latter began to decline in Carboniferous and Permian times, the former, mainly represented by the earlier Chondrostei, had already become the dominant Fishes of the period.

ORDER II. CHONDROSTEI (ACIPENSEROIDEI).

In these Fishes, the oldest and the most primitive of the Actinopterygii, the fin-rays of the median fins still continue to retain their primitive numerical superiority over the radials, and the tail is heterocercal. There is a single dorsal and an anal fin, which, like the upper lobe of the caudal fin, are generally provided with fulcra. Pelvic fins abdominal. Squamation typically rhombic and ganoid. Vertebral column acentrous. So far as is known the chondrocranium is but little ossified, and the cranial bones are mainly dermal. The secondary pectoral girdle still includes a pair of infra-clavicles.

The Chondrostei are first represented in the Lower Devonian by the solitary Palaeoniscid genus Cheirolepis, a contemporary of the earliest Crossopterygii. They occur throughout the Mesozoic period, except in the Cretaceous, and also in the Eocene, and while steadily diminishing in number and variety they gradually approximate to their degenerate and in some respects highly specialised descendants, the Sturgeons and Paddle-Fishes of the existing Fish fauna. Of the seven families included in the group the Palaeoniscidae are the oldest and the most generalised. The Platysomidae are a specialised offshoot from the Palaeoniscidae, and, if they are rightly to be considered as Chondrostei, perhaps the same may be said of the problematic Belonorhynchidae. On {486}the other hand, there are certain features in the Catopteridae which indicate an approach to Fishes of an altogether more modern type. Finally, the Chondrosteidae represent a stage in a career of degeneration, the climax of which is reached by the modern Polyodontidae and Acipenseridae.

FAM. 1. PALAEONISCIDAE.—Fishes with fusiform bodies, short dorsal and anal fins, and usually with a complete investment of articulating rhombic, rarely cycloid, ganoid scales (Fig. 283). Fulcra generally present at the bases of the median fins, and especially along the dorsal border of the upper caudal lobe. Ribs are not known to be present. Skull invested by a very complete series of paired dermal bones, which in number and disposition conform to the normal Teleostome type (Fig. 284). The secondary upper jaw includes both premaxillae and large maxillae; and, as a rule, both the dentary and splenial bones {487}of the lower jaw are dentigerous. Except for the absence of an interoperculum, the opercular series of bones is complete, including numerous branchiostegal rays. There is a single small median jugular plate.

The Palaeoniscidae are remarkable both for their individual and specific abundance and for their extensive range in time. Represented only by Cheirolepis in the Middle Old Red Sandstone and Devonian, the family attained its maximum development in the later Palaeozoic rocks (Carboniferous and Lower Permian), became rare in the Mesozoic, finally dwindling away at the close of the Jurassic period. Their geographical distribution in the past is hardly less remarkable. In various geological formations they have been found in Great Britain and Ireland, in widely remote parts of continental Europe, and in North America, South Africa, and Australia. Cheirolepis, Amblypterus, Canobius, Phanerosteon, Elonichthys, Cryphiolepis, Palaeoniscus, and Trissolepis are Palaeozoic genera. Gyrolepis, Urolepis, Coccolepis, Oxygnathus, and Centrolepis are characteristic Mesozoic forms.

FAM. 2. PLATYSOMIDAE.—More or less deep-bodied Fishes, with elongated dorsal and anal fins, a high head, short jaws, usually armed with bluntly conical tritoral teeth, and a complete investment of high, narrow, rhombic scales. They agree with the Palaeoniscidae in their osteology and in most other essential {488}features, and they flourished in large numbers during the Carboniferous and Permian periods. Platysomus ranges from the Lower Carboniferous to the Upper Permian in Great Britain and continental Europe, and also occurs in the Carboniferous of North America. Eurynotus (Fig. 285), and the singularly deep-bodied Cheirodus (Fig. 286), in which pelvic fins are unknown, are British Carboniferous genera.

FAM. 3. BELONORHYNCHIDAE.—The systematic position of these Triassic forms is very doubtful, and it is by no means clear that they are Chondrostei at all.

FAM. 4. CATOPTERIDAE.—It is very probable that this widely-distributed Triassic family is an offshoot from the Palaeoniscidae. It agrees with the latter in the general character of the head and pectoral girdle and in the rhombic squamation, but differs from its progenitors and approaches the more modern Holostei in the semi-heterocercal condition of the tail, and in the approximate numerical agreement between the fin-rays and radialia of the dorsal and anal fins.

{489}FAM. 5. CHONDROSTEIDAE.—This family affords an interesting annectant link between the Palaeoniscidae and their degenerate living representatives the Polyodontidae and Acipenseridae. They agree with the latter in the general shape of the body, the growth of a preoral rostrum, and in the relatively small size of their ventrally-placed and probably protrusible mouth (Fig. 287). The skin is entirely scaleless, except on the upper lobe of the caudal fin, where, as in Polyodon and Acipenser, the primitive rhombic squamation and a series of fulcra are retained.

On the other hand, their relationship to the Palaeoniscidae is indicated by the general disposition of the dermal bones of the cranial roof, and the presence of a transverse row of supra-temporals and of an extensive series of branchiostegal rays (Fig. 288). The family is represented by Chondrosteus from the Lower Lias of Dorset and Leicestershire, and Gyrosteus from the Upper Lias of Yorkshire. From an evolutionary point of view it is significant that the Chondrosteidae do not make their appearance until the Palaeoniscidae are approaching extinction.

The two remaining families, the Polyodontidae and the Acipenseridae, agree in presenting a remarkable leaven of characters otherwise distinctive of the typical Elasmobranch, associated with certain primitive features which they have doubtless inherited from some remote ancestral stock common both to existing Elasmobranchs and to the other primary groups of Fishes, and also with others obviously due to degeneration.

{490}[Illustration: FIG. 288.—Lateral view of a restored skull and pectoral girdle of Chondrosteus acipenseroides. a, Angular; br, branchiostegal rays; c.h, cerato-hyal; h.m, hyomandibular; j, jugal; p.f, post-frontal; s.op, suboperculum; s.t, supra-temporal; other reference letters as in Fig. 284. (After Traquair.)]

The most interesting illustration of the first point is to be found in the condition of the primitive upper jaw which, especially in the Polyodontidae, is typically Elasmobranch in the median union of the palato-quadrate bars beneath the basis cranii, but Teleostome in the presence of a secondary upper jaw formed by two maxillae. Both families also agree in possessing an acentrous vertebral column which, if it does so far resemble that of Teleostomes in being potentially arco-centrous, nevertheless has a better developed series of distinct inter-dorsal and inter-ventral cartilages, regularly alternating with only partially bony basi-dorsals and basi-ventrals, than is to be met with in any other adult Fishes except Elasmobranchs. Primitive features are apparent in the presence of spiracles, sometimes associated with pseudobranchs; the presence in one family (Acipenseridae) of a hyoidean hemibranch supplied with blood directly from the ventral aorta, and the existence of a multi-valvular conus arteriosus and an intestinal spiral valve. Finally, the massive growth of the chondrocranium wholly devoid of cartilage bones, except in so far as they may be represented by splint-like membrane bones, the fragmentation of the investing dermal bones, the degeneration of the opercular skeleton and the loss of branchiostegal rays, and the almost complete disappearance of the primitive rhombic squamation, are probably to be regarded {491}as the outcome of a long-continued career of degeneration from some remote Palaeoniscid ancestor.

FAM. 6. POLYODONTIDAE.—The Polyodontidae are more generalised, and in some features decidedly more Selachioid than the Acipenseridae. Body fusiform and apparently scaleless, but the primitive squamation is still represented by isolated vestigial scales imbedded in the otherwise soft skin, and by a continuous series of rhombic scales on the upper caudal lobe, which also has a dorsal fringe of large fulcra. Rostrum exceptionally long, spatulate or somewhat conical, with a rigid axis and thinner and more flexible margins. Barbels absent. Mouth wide, not spout-like. Pectoral fins devoid of spines. Two pairs of membrane-closed vacuities separate the paired dermal bones of the cranial roof (possibly parietals and frontals) from the more laterally-placed post-temporals and squamosals, and there are no median plates posterior to the orbits, nor any representatives of supra-temporals. A feeble suboperculum is retained in addition to a small rayed operculum. Hyoidean hemibranch completely suppressed. Two genera only are known, each with a single species.

The Paddle-Fish or Spoon-Bill, Polyodon folium (Fig. 289) inhabits the rivers of the Southern States of North America, the Mississippi, Ohio, and Missouri, and their numerous tributary rivers and streams. A Fish of sluggish habits, Polyodon feeds chiefly on mud and the minute organisms it contains, the exceptionally long gill-rakers probably forming an efficient filter to prevent the food particles escaping through the gill-clefts with the expiratory water current. The singular rostrum is apparently used for stirring up the mud when feeding, but in view of the muddy waters the Fish frequents, and the very small size of {492}the eyes, its value as a tactile organ must not be overlooked. Polyodon may attain a length of 5 to 6 feet. The time of spawning varies, according to locality, from March to June. Nothing is known of the development of Polyodon. Young less than 6 to 8 inches in length are unknown, and specimens of this size are very rarely seen. The jaws are furnished with minute teeth until the Fish is about half-grown, when they become edentalous. Caviare is made from the eggs, and the centres at which this industry is carried on are chiefly situated along the course of the Mississippi. The second species, Psephurus gladius, inhabits the Yang-tse-Kiang and Hoangho rivers of China, and differs from Polyodon in the conical shape of its rostrum and the smaller number and larger size of its fulcra. Psephurus is stated to reach a length of 20 feet. The family is represented in the Eocene of Wyoming by the genus Crossopholis, which is note-worthy for the retention of trunk scales in the form of small, somewhat quadrate denticulated discs, arranged in oblique rows.

FAM. 7. ACIPENSERIDAE.—In the Sturgeon family the body is elongate, cylindrical, and somewhat bulky. Rostrum well developed and often massive, with a transverse row of simple or branched preoral barbels on its ventral surface. Mouth small and remarkably protrusible. Jaws devoid of teeth except in the larvae. As in the preceding family, the primitive rhombic squamation is confined to the upper lobe of the tail, which, like the dorsal and anal fins, is furnished with fulcra. Elsewhere the scales are represented by five longitudinal rows of large bony scutes and by intervening small scattered ossifications. The anterior dermal ray of the pectoral fin is stout and spine-like. The dermal bones of the cranial roof suturally articulate with one another to form a continuous shield, uninterrupted by lateral vacuities. A median dermal bone in the occipital region transmits the occipital sensory canal. The opercular series is represented only by an opercular bone.

The family includes but two genera, Acipenser (Fig. 290) and Scaphirhynchus, and about twenty species, confined to the seas, estuaries, and rivers of the temperate and north temperate regions of the northern hemisphere. Acipenser includes the more typical Sturgeons, and is distinguished by the presence of spiracles, and by the fact that the longitudinal rows of scutes remain distinct to the base of the caudal fin.

{493}[Illustration: FIG. 290.—The Sterlet (Acipenser ruthenus). o, Barbels; c.f, caudal fin; d.f, dorsal fin; pct.f, pectoral fin; pv.f, pelvic fin; sc, scutes; v.f, ventral or anal fin. (From Parker and Haswell, after Cuvier.)]

There are probably about fifteen species, but the exact number is uncertain. Sturgeons are abundant in the Black Sea, the Sea of Azov, the Caspian, and their tributary rivers, notably the Danube, Don, Dnieper, Ural, and Volga. They are also present in the rivers and on the coasts of Northern Europe and of China. Five species occur in North America, on the Atlantic and Pacific coasts, and in the rivers of these regions as well as in the Great Lakes. One or two species are almost exclusively fresh-water, but most Sturgeons are migratory Fishes, living in the sea, but ascending rivers for spawning. Their food consists of worms, molluscs, the smaller Fishes and aquatic plants; and in feeding the mouth is protruded downwards in the form of a cylindrical, spout-like structure and thrust into the mud. The only species certainly known to frequent the British coasts is the common Sturgeon (A. sturio), which is also found in the Black Sea and the Mediterranean, and is abundant on the Atlantic coast of North America from Maine to South Carolina. The species occurs all round our coasts, more plentifully, perhaps, on the northern and eastern shores. In the spring and summer the Fish ascends the rivers, often to a considerable distance. Its presence has been recorded in the Severn, near Shrewsbury; in the Trent at Nottingham, and also, but not in recent years, in the Thames above London Bridge. In this country the species is a "Royal Fish," and by an unrepealed Act of Edward II. it is enacted that "the King shall have the wreck of the sea throughout the realm, Whales and Great Sturgeons, except in certain places privileged by the King." If not so large as some of its Russian relatives, A. sturio often attains a great size. Even on {494}our own coasts the capture of individuals 8 to 10 feet in length has been recorded. The great Russian Sturgeon (A. huso), which is common in the Black Sea, the Sea of Azov and the Caspian, and in the rivers flowing into them, is the largest of all the Sturgeons, individuals weighing 2760 and 3200 pounds having been captured. The Sterlet (A. ruthenus), similarly distributed and often ascending the Danube to Vienna, is much smaller, rarely exceeding a length of three feet.

In Europe A. sturio spawns about July, but in North America (Delaware river) during May. Small in size, the eggs are produced in enormous numbers, a single female, it is said, producing about 3,000,000 in one season. They are invested by a gelatinous sheath, so that they readily stick to one another or to other objects, and, when deposited, they adhere in streaks or sheet-like masses to the bed of the river. The young are hatched very early, about the third or fourth day in A. sturio, and in the Sterlet between the ninth and twelfth, the length of the larva then varying from 7 to 10 mm. When they are a few days old the larvae closely resemble those of existing Holostei except that the small opercular folds leave the gills freely exposed (Fig. 291). A shallow pigmented groove in front of the mouth apparently represents the sucker of the young Amia and Lepidosteus. Although toothless in the adult, both the Sturgeon and Sterlet possess vestigial rudimentary, uncalcified, larval teeth, which in shape resemble the teeth of a Dog-Fish, consisting of a broad base and a sharp spine.

The Sturgeon is a Fish of considerable economic importance. The flesh is an article of food, and from the ovaries of certain Russian and American species thousands of hundredweights of {495}caviare are prepared annually. Large quantities of isinglass are obtained from the air-bladders, in the United States and in Russia. The organ is split open and washed; the inner lining is then stripped off and the bladder dried as rough isinglass.

The second genus, Scaphirhynchus, which includes the Shovel-nosed Sturgeons, differs from Acipenser in the long, flattened, and almost spatulate shape of the rostrum, the suppression of the spiracles, and the union of the longitudinal rows of scutes beneath the dorsal fin to form a scaly armature completely investing the tail. The distribution of the genus affords an interesting parallel to that of the Polyodontidae. Of the four species, one (S. platyrhynchus) is common in the Mississippi valley and in the rivers of the Western and Southern States of North America, while the remaining species, also exclusively fresh-water, frequent the rivers of Tartary.

The Acipenseridæ are not known to occur earlier than the Tertiary. Scutes, pectoral spines and fragmentary bones, indistinguishable from the corresponding parts of existing species, have been recorded from the London Clay of the Isle of Sheppey (Lower Eocene), and from later Eocene deposits in the Isle of Wight and Hampshire; and also from the Pliocene of England (Red Crag of Suffolk) and Virginia.

ORDER III. HOLOSTEI (LEPIDOSTEOIDEI).

The Holostei include a large and somewhat heterogeneous assemblage of Fishes, most of which are now extinct. As a group they are by no means easy to define or delimit. Widely separated from the Chondrostei, there is little evidence of the existence of connecting links between the two groups, although in some respects the Catopteridae may be regarded as transitional. On the other side, however, the Holostei shade off almost imperceptibly into the Malacopterygian Teleostei. In different fossil and recent Holostei there may be traced the gradual acquisition of the more special Teleostean characters and the elimination of the more archaic features of their remote Teleostome ancestors; and in a general sense this may be taken as the key to the more salient attributes of the group. It is not suggested that all the families of Holostei are on the direct lines of Teleostean descent. Some families, like the Eugnathidae and {496}Amiidae, may possibly occupy this position, but others, such as the Pycnodonts, for example, seem to be highly specialised and terminal offshoots which have left no descendants. Of the more generalised features which different Holostei retain, mention may be made of the prevalence of rhombic scales which, like the dermal cranial bones, are generally invested by a variously ornamented coat of ganoin; the presence of fulcra, cheek-plates, post- or sub-orbital ossicles, and of a complex lower jaw, which includes dentigerous splenials; and the abdominal position of the pelvic fins. On the other hand, indication of advancing specialisation in the Teleostean direction are to be noted in the numerical agreement between the dermal fin-rays of the median fins and their supporting radialia, and in the character of the vertebral column. Some Holostei, especially the earlier forms, are acentrous, but between this primitive condition and the possession of well-ossified centra, associated with equally bony arcualia, almost every gradation is to be found. The chondrocranium is more or less completely replaced by cartilage bones corresponding to those generally present in Teleosts, while the palato-pterygoid cartilages, likewise modified by the growth of cartilage bones, separately articulate with the lateral ethmoid regions instead of meeting in a ventral symphysis beneath the basis cranii. With rare exceptions (e.g. certain Pycnodonts) the opercular skeleton is complete, and includes branchiostegal rays; and although a single gular plate is often present, it may be absent in entire families. Like so many other structures, the tail is in a transitional state: really heterocercal, but incipiently homocercal, it may be described as semi-heterocercal. Infra-clavicular plates no longer form part of the secondary pectoral girdle, their place being taken by cleithra which, as in most Teleosts, meet in a ventral symphysis.

Indications of transition are not wanting in the squamation in certain families, and may be seen in the partial or complete replacement of the rhombic type by thin, imbricated, cycloid scales. Lastly, the soft parts of the two surviving genera are not without features of similar significance. A multivalvular conus arteriosus, it is true, is still retained, but the spiral valve is vestigial, the spiracles are closed, and in the female of one genus (Lepidosteus) the gonoducts are peritoneal tubes, continuous, as in most Teleosts, with the investments of the ovaries.

{497}The Fishes here included in the Holostei constitute the Protospondyli and Aetheospondyli of Smith Woodward. In the former group vertebral centra are either entirely absent, or, if present, their components in the form of alternating hypo- and pleuro-centra invariably remain distinct in the tail. The latter group has been instituted for the provisional reception of two highly specialised families of uncertain relationships, which differ from the Protospondyli in their higher grade of vertebral structure, the centra always being complete without any indication of distinct hypo- and pleuro-centra.

The Holostei first appear in the Permian, where they are represented by a single genus (Acentrophorus). During the Mesozoic period they were abundant in the Trias, reaching their maximum development and becoming the dominant Fishes of the period in the Jurassic. In the Cretaceous they began to decline, and in the Tertiaries became reduced to the two families which at the present day are the sole survivors of the group.

Of the six families of Protospondyli the Semionotidae are the oldest and most generalised, and the Macrosemiidae a closely allied group. The Pycnodontidae are a highly specialised and terminal offshoot. The Eugnathidae obviously lead to the Amiidae, and from the same stock it is probable that the Pachycormidae have been derived. The relations of the Aspidorhynchidae and Lepidosteidae (Aetheospondyli) are extremely doubtful. That the two families are allied seems probable, but beyond the possibility of a remote connection with the Protospondyli there is no clue to their ancestry.

FAM. 1. SEMIONOTIDAE.—Small-mouthed, fusiform or {498}deep-bodied Holosteans with rhombic scales, rarely, as in Aetheolepis, cycloid in the caudal region. All the fins possess fulcra. Teeth more or less conical, with a tendency to become tritoral in certain genera. Jugular plate present or absent. Acentrophorus (Upper Permian); Semionotus (Trias of England, Germany, S. Africa, and N. America); Lepidotus (Fig. 292) (Trias of Germany, Jurassic of Europe and India, Cretaceous of Brazil); the deep-bodied Dapedius (Lias of Dorset, Fig. 293), and Aetheolepis (Jurassic of New South Wales) are characteristic genera.

FAM. 2. MACROSEMIIDAE.—Closely related to the Semionotidae, but with a more extended dorsal fin. Macrosemius (Upper Jurassic of England, Germany, France); Notagogus (Upper Jurassic of Naples, Bavaria, France); Petalopteryx (Upper Cretaceous of Syria).

FAM. 3. PYCNODONTIDAE.—Highly specialised deep-bodied Fishes, with a small mouth and characteristic grinding or tritoral teeth. Scales rhombic. Fulcra absent. Dorsal and anal fins long. There is no jugular plate. The family ranges from the Lower Lias to the Lower Eocene, inclusive. Mesodon, Mesturus, Gyrodus, and Microdon are Jurassic genera. Coccodus and Xenopholis occur in the Upper Cretaceous of Syria (Mount Lebanon), and Pycnodus in various European Eocene formations.

FAM. 4. EUGNATHIDAE.—Large-mouthed, elongate fusiform {499}predaceous Fishes, with pointed teeth, rhombic scales, short dorsal and anal fins, a single jugular plate and prominent fulcra. The vertebral centra are represented by distinct hypo- and pleuro-centra, which may form complete alternating rings in the tail.

The family first appears in the Trias and ranges throughout the Jurassic period. Eugnathus (Jurassic) and Eurycormus (Upper Jurassic). Caturus (Fig. 294) has a more extensive range, occurring in the Upper Trias of the Tyrol and in the Upper Jurassic of England and Bavaria. Caturus and Eurycormus, with their relatively thin, imbricated, cycloid scales, which have lost the peg-and-socket articulation, form connecting links between the more typical Eugnathus and the Amiidae.

FAM. 5. AMIIDAE.—Body fusiform and somewhat compressed. Scales uniformly thin, cycloid, and imbricated. Single dorsal fin long and low. Anal fin short. Tail nearly homocercal, with a rounded hinder margin. Fulcra absent from all the fins. Moderately large conical teeth are present on the premaxillae, maxillae, palatines and dentaries, and smaller teeth on the vomers, pterygoids, splenials and parasphenoid. Pre- and post-centra fused in the trunk, forming complete bony amphicoelous centra, but distinct in the tail. A single large jugular plate is present. In the solitary living species the air-bladder is cellular, and its afferent arteries are derived from a posterior aortic arch. Pyloric caeca absent. Two peculiar comb-like structures are present on the throat.

The Bow-Fin (Amia calva), the sole existing representative of the family, is abundant in the rivers and lakes of Central and Southern North America, including the great lakes Huron and {500}Erie. It is a voracious, carnivorous Fish, preying upon other Fish as well as upon fresh-water Crustaceans and Insects, very tenacious of life, and of no economic value. The male is smaller than the female, about 18 inches in length, and is distinguished by the presence of a round black spot, encircled by a margin of orange, at the base of the caudal fin (Fig. 295). The female may exceed 24 inches.

Amia frequently rises to the surface, especially when the water is foul, and takes in large mouthfuls of air, and it is probable that the air is subsequently passed into the spacious cellular air-bladder which acts as a lung. The breeding season, during which the coloration of the Fish is more brilliant than at other times, lasts from the beginning of May to June, but it may begin and end somewhat earlier if the temperature be favourable. The Fish makes its way from the deeper water, where it has remained sluggish during the winter, to the spawning ground. This is usually at the swampy end of a lake where there is an abundance of aquatic herbage intersected by channels of clear water. There the Fish is said to circle round until the soft weeds and rootlets are bent and crushed aside, so as to leave an area having the appearance of a crude form of nest, in which the eggs are deposited. They may be found in enormous numbers adhering to the leaves and rootlets of the weedy home. After oviposition the male remains on guard until the young are hatched out, when they appear to leave the nest in a body, still under the protection of their watchful parent. At all events a little later the male has been observed to be accompanied by a swarm of young fry, which he keeps together by circling round them. The development of the eggs is remarkably rapid. From the first cleavage of the egg to the hatching of the embryo the whole {501}process may be completed within from 4 to 8 days. When hatched the larvae are about 5 to 6 mm. long. They possess a large yolk sac and a preoral sucker for attachment. The pectoral fins are conspicuous structures before there is any trace of the pelvic fins.

The Amiidae, represented by Megalurus, first appear in the Upper Jurassic of Dorset, France, and Bavaria. In the Cretaceous period the family is represented by Amiopsis. Species of Amia occur in the Eocene of Europe and North America. In the former area the genus became extinct at the close of the Lower Miocene period, but in the latter Amia calva still survives.

FAM. 6. PACHYCORMIDAE.—Large-mouthed, predaceous Amioid Fishes with a more or less prominent snout and a short dorsal fin. Scales rhombic but thin, rounded behind, and overlapping, sometimes absent. A single large jugular plate.

In the earlier forms (e.g. Pachycormus, Lias) the snout is but slightly produced, but in Hypsocormus (Upper Jurassic), and {502}especially in Protosphyraena (Cambridge Upper Greensand and the Cretaceous of Europe and North America), it becomes greatly elongated and associated with an exceptionally strong dentition.

FAM. 7. ASPIDORHYNCHIDAE.—Long-bodied Fishes, with a pointed preoral rostrum, sharp teeth, and deep rhombic scales. Fins small, the dorsal and anal being remote from the pelvic fins. Fulcra vestigial or absent. Jugular plates not known.

Two genera only are known. Aspidorhynchus is a Jurassic form. Belonostomus is Upper Jurassic and Cretaceous. Species of the latter genus have a very wide distribution (Europe, North and South America, and Australia).

FAM. 8. LEPIDOSTEIDAE.—Body elongate, with a relatively short caudal region. Tail semi-heterocercal. Scales rhombic, thick, ganoin-coated and articulated, not vertically elongated on the sides of the body. Dorsal and anal fins short and remote from the pelvic fins. Median fins with fulcra. Both the upper and lower jaws more or less elongated, forming a broad and depressed or a long tapering beak, near the anterior end of which the nostrils are placed. Eyes small. Vertebral centra well {503}ossified, opisthocoelous and fused with the neural arches. The metapterygoid bones have a secondary articulation with the skull. Maxillae segmented into numerous pieces. Jugular plates absent. Branchiostegal rays reduced to three on each side. Teeth numerous, slender, and of unequal size. In the larger teeth the dentine is intricately folded. Pyloric caeca branched and compacted together into a gland-like mass. Air-bladder cellular, but its blood is not derived from a posterior aortic arch.

The only known genus is Lepidosteus, the existing species of which frequent the fresh waters of North America. The common or Long-nosed Gar-Pike (L. osseus), remarkable for its long and slender beak, is generally abundant in the rivers and lakes of the United States from Vermont to the Rio Grande, and it may reach a length of five feet. The "Short-nosed Gar" (L. platystomus, Günther) has a much shorter and broader beak, and a similar distribution (Fig. 299). The "Great" or "Alligator Gar" (L. viridis, Günther) has a more southerly habitat, frequenting the rivers of the Southern States, Northern Mexico, and Cuba. It is by far the largest species, sometimes reaching a length of 8 to 10 feet.

Lepidosteus is a voracious Fish, preying upon smaller Fishes, and, except in the breeding season, it frequents the deeper parts of the rivers or lakes. The Fish is constantly in the habit of rising to the surface and emitting bubbles of gas, either through the mouth or by the branchial clefts, and it is probable that this gas is air which has been previously swallowed at the surface and passed into the air-bladder. About May Lepidosteus resorts in large numbers to shallower water, where the temperature is {504}higher, for the purpose of spawning, each female being attended by from one to four males. During brief recurring periods of excitement, accompanied by convulsive lashing movements, the eggs and sperm are emitted. The eggs are extremely sticky, and adhere tenaciously to the rocks and stones on which they are deposited. In a few days the embryos hatch out, and at this stage the larva has a huge mouth surmounted by a terminal preoral disc, fringed with a row of marginal wart-like suckers (Fig. 300). The yolk sac is so large as greatly to hamper the movements of the larva; hence, by means of its suckers, the young Lepidosteus attaches itself to surrounding objects, and remains almost entirely motionless for some little time after hatching. Later, about a fortnight after escaping from the egg, the yolk becomes completely absorbed, the suckers degenerate and eventually disappear, and the larva, freed from its load of nutritive reserve, assumes a more active life. After the absorption of the yolk the larvae of Mosquitos appears to form the exclusive diet of the young Lepidosteus for some time, but very soon young Fishes are readily devoured.

Lepidosteus seems to have been abundant in Europe during the Eocene and Miocene periods, but became extinct before the Pliocene. In North America, also, the genus dates from the Eocene, and still survives.

ORDER IV. TELEOSTEI.

{505}CHAPTER XIX

DIPNEUSTI

SUB-CLASS III. DIPNEUSTI (DIPNOI).

These singularly interesting Fishes are distinguished by their more or less acutely lobate paired fins and their overlapping cycloid scales, and by the fact that the bony dermal fin-rays of the median fins are much more numerous than their supporting radialia. Tail heterocercal or apparently diphycercal. Nostrils inferior. Vertebral column acentrous. The radialia of the median fins articulate with the contiguous neural or haemal spines and agree with them numerically. Skull autostylic. Premaxillae and maxillae absent, but a secondary lower jaw is represented by certain dermal bones of which tooth-bearing splenials are the most important, the dentary bones being absent altogether, or, if present, toothless and small. The cranial dermal bones include median as well as paired lateral plates, but their relations to those of other Fishes are very obscure. Two opercular bones are always present, but branchiostegal rays are unknown. One of the most important diagnostic features is the dentition. All Dipneusti agree in possessing large tritoral dental plates supported by the palato-pterygoid and splenial bones. The secondary pectoral girdle includes only cleithra and infraclavicles (clavicles). There is a pelvic girdle. Claspers absent. Of the four families of Dipneusti, two, the Ctenodontidae and the Uronemidae, are exclusively Palaeozoic. The third, the Ceratodontidae, is Mesozoic, and still survives. The fourth, the Lepidosirenidae, is known only by two existing genera.

{506}[Illustration: FIG. 301.—Restoration of Dipterus valenciennesi. × ⅕. (From Traquair.)]

FAM. 1. CTENODONTIDAE.—Body fusiform. Tail heterocercal or apparently diphycercal. Excluding the anal fin, which is always distinct, the remaining median fins are either distinct or continuous. Dental plates traversed by radiating transverse ridges terminating in rows of conical denticles (ctenodont dentition, Fig. 303). Vomerine teeth not known. Cranial bones numerous and small, and, like the squamation, with or without an investment of ganoin. Jugular plates present or absent. The oldest genus is Dipterus from the Old Red Sandstone of Scotland, where it is contemporaneous with the earliest Crossopterygii and also with the oldest known Actinopterygii (Cheirolepis). Phaneropleuron (Old Red Sandstone of Scotland, Upper Devonian of Canada, Fig. 302), Scaumenacia (Upper Devonian of Canada), Ctenodus (Carboniferous of Great Britain and North {507}America), and Sagenodus (Carboniferous of Great Britain and Lower Permian of Bohemia) belong to the same family.

FAM. 2. URONEMIDAE.—Body fusiform. Dentition non-ctenodont, consisting of patches of distinct rounded denticles with a row of basally-confluent conical denticles along the outer margin of each. Scales thin. All the median fins are continuous. Tail apparently diphycercal. Cranial dermal bones as in Dipterus. Uronemus (Lower Carboniferous of Scotland), and perhaps Conchopoma (Lower Permian of Prussia), are the only known genera.

The two remaining families possess certain features which cannot be affirmed to have existed in their extinct allies. Thus, both agree in exhibiting those striking and, so far as Fishes are concerned, unique modifications of the air-bladder and vascular system, and the olfactory organs, which are more or less closely associated with air-breathing habits and indicate a marked convergence towards the Amphibia. Side by side with such indications of advancing specialisation in certain directions, ample evidence of a remote ancestry is to be seen in such primitive features as the presence of a spiral valve and a multi-valvular conus arteriosus, and in the short and simple alimentary canal. Of other points of agreement mention may be made of the absence of jugular plates, the presence of vomerine teeth, the continuity of all the median fins, and the apparently diphycercal but probably gephyrocercal character of the tail.

FAM. 3. CERATODONTIDAE.—Body elongated and compressed. Scales large, thin, non-ganoid, and partially enclosed in dermal pouches. Paired fins biserial. Chondrocranium complete. Dermal bones wholly devoid of ganoin, reduced in number but increased in size. Circumorbital bones present. Dental plates oval, crescentic or triangular, traversed by several radiating enamelled ridges, terminating in smooth or feebly denticulated biting margins. Lower jaw with a small toothless dentary on each side. The hyoid arch includes a small hyomandibular and a hypo-hyal in addition to a cerato-hyal. Branchial arches five in number and bisegmented. The gills exhibit little evidence of {508}degeneration. Hyo-branchial cleft open, and associated with a pseudobranch. The first four branchial arches carry holobranchs. Air-bladder single. Young not provided with cutaneous gills. Two genera only are known, the Mesozoic Ceratodus and the still living Neoceratodus. The former genus includes numerous species, for the most part known only by their dental plates, and has a remarkably wide distribution in different geological formations. Species occur in the Trias of England, Germany, India, South Africa (Upper Karoo strata), and also, but more rarely, in certain Jurassic deposits in England and in Colorado. Neoceratodus is represented by a solitary species, N. forsteri (Fig. 304, A), which is now restricted to the Burnett and Mary rivers in Queensland. A somewhat wider distribution of the species in recent times is indicated by the presence of teeth in the later Tertiary (alluvial) deposits of Darling Downs, near the borders of New South Wales.

The Neoceratodus of the Burnett frequents the comparatively stagnant pools or water-holes which alternate with shallow runs and are usually full of water all the year round. In these pools, filled with a rich growth of aquatic vegetation, and often the favourite haunt of the Platypus (Ornithorhynchus), the Fish is fairly abundant. Inactive and sluggish in its habits, usually lying motionless on the bottom, the Fish is easily captured by the natives with hand-nets or baited hooks. Neoceratodus lives on fresh-water Crustaceans, worms, and molluscs, and to obtain them it crops the luxuriant vegetation of the water-holes much in the same way that a Polychaet or a Holothurian swallows sand for the sake of the included nutrient particles. Apparently the air-bladder is a functional lung at all times, acting in conjunction with the gills. At irregular intervals the Fish rises to the surface and protrudes its snout in order to empty its lung and take in fresh air. While doing so the animal makes a peculiar grunting noise, "spouting" as the local fishermen call it, which may be heard at night for some distance, and is probably caused by the forcible expulsion of air through the mouth. Useful as the lung is as a breathing organ under normal conditions, there can be little doubt that its value as such is much greater whenever gill-breathing becomes difficult or impossible.

{509}[Illustration: FIG. 304.—A, Neoceratodus forsteri, Queensland; B, Protopterus annectens, Gambia. C, Lepidosiren paradoxa, Paraguay. The lozenge-shaped markings on the surface of B do not represent scales but areas of the skin outlined by pigment cells. In a fresh specimen the scales are as completely invisible as in C. (A, from Günther; B and C, from Lankester.)]

{510}[Illustration: FIG. 305.—A young Neoceratodus four weeks after hatching. c, Cloacal aperture; l.l, lateral line; m, mouth; op, operculum; p.f, pectoral fin. (From Semon.)]

This seems to be the case during the hot season, when the water becomes foul from the presence of decomposing animal or vegetable matter. Semon records a striking illustration of this in the case of a partially dried-up water-hole, in which the water had become so foul that it was full of dead fishes of various kinds. Fatal as these conditions were to ordinary Fishes, Neoceratodus not only survived but seemed to be quite healthy and fresh. Such observations are of exceptional interest. Not only do they afford a clue to the conditions of life which, in the course of time, probably led to lung-breathing in Neoceratodus, but they also suggest the possibility that a similar environment has been conducive to the evolution of air-breathing Vertebrates from gill-breathing and Fish-like progenitors. In spite of its pulmonary respiration, Neoceratodus more closely resembles the typical Fishes in its habits than any other Dipneusti. It lives all the year round in the water. There is no evidence that it ever becomes dried up in the mud, or passes into a summer sleep in a cocoon, and the well-developed condition of its gills suggest that these organs play a more important rôle in breathing than in either Protopterus or Lepidosiren. The Fish is not known to leave the water, and the paired fins, useful no doubt as paddles, are quite incapable of supporting the bulky body on terra firma. In fact, when Neoceratodus is taken out of its natural element it seems to be more helpless than most other Fishes, and, in spite of its capacity for lung-breathing, soon dies unless kept moist by artificial means. Spawning takes place from April to November, principally in September and October. The eggs, invested by a jelly-like coat, secreted by the oviducal walls, are deposited {511}not in a nest, but singly amongst aquatic vegetation, and, as they are not adherent, it is probable that they finally rest on the mud. The early developmental stages exhibit a general resemblance to those of Amphibia. There is no larval metamorphosis, and at no period does the young Neoceratodus (Fig. 305) possess cutaneous gills or a cement organ. The tail is apparently diphycercal from the first, and the pelvic limbs do not appear until about six weeks after the pectoral members. It is interesting to note that the dental plates are first represented by lines or patches of separate denticles (non-ctenodont), which subsequently fuse basally (ctenodont) before the adult condition is reached. Neoceratodus is stated to grow to a length of 5 to 6 feet.

FAM. 4. LEPIDOSIRENIDAE.—Body elongate, cylindrical and more or less Eel-like, with small cycloid scales completely enclosed in the skin. Paired fins so acutely lobate as to present the appearance of tapering cylindrical filaments, equally devoid of scales and fin-rays. In a general way the cranial dermal bones correspond with those of Neoceratodus, but the place of the posterior median bone is taken by a large, gable-like fronto-parietal bone, situated internal to the head muscles, and in direct relation with the chondrocranium, which is largely aborted in the interorbital region. Circumorbital bones absent. Opercular bones much reduced. Lower jaw without dentary plates. Palatine and splenial dental plates with three non-denticulate, trenchant ridges. Hyoid arch consists of cerato-hyals only. Hyoidean cleft closed. Certain of the anterior branchial arches devoid of branchial filaments; when present the latter are leaf-like and free. Air-bladder a double lung. There is a larval metamorphosis, and the young possess cutaneous gills. The family includes two genera, Protopterus and Lepidosiren. In the former genus the paired fins are either uniserial or they consist of axial mesomeres only; there are six branchial arches and five clefts; and the larval gills are usually retained as vestiges throughout life. In Lepidosiren the paired fins are reduced to the segmented axis, without pre- or post-axial radials. There are five branchial arches and four clefts, and the cutaneous gills disappear soon after the larval metamorphosis.

{512}[Illustration: FIG. 306.—Map showing the distribution of the surviving Dipneusti.]

Protopterus has a wide distribution over the middle portion of the great African continent, ranging from the river Senegal and the White Nile on the north to the Congo basin, Lake Tanganyika, and the Zambesi on the south. Three species are known, P. annectens (Fig. 304), P. aethiopicus, and P. dolloi. Protopterus is usually found in marshes in the vicinity of rivers. Voracious in its habits the Fish is mainly carnivorous, subsisting principally on Frogs, worms, insects, and crustaceans. It is by no means averse to preying upon its own kind, and if several of these Fishes are confined in the same aquarium they are apt to give free vent to their cannibal instincts by biting off the tails or limbs of their fellows. The missing parts are soon regenerated, but the new members are usually somewhat abnormal, the tail, for instance, never regaining its original length, while a new pectoral limb may be bifid or even trifid. The tail is the principal organ of locomotion, and by its means the Fish is capable of remarkably quick, agile movements. When slowly moving over the bottom of an aquarium the paired limbs are observed to move to and fro on opposite sides alternately in a somewhat bipedal fashion. The limbs are useless for swimming, although it is possible that they may be helpful in creeping over the bottom, or in balancing, or as tactile organs. Protopterus is said to breathe by its lungs as well as by its gills, and to rise to the surface at short intervals to take in fresh air.

{513}[Illustration: FIG. 307.—Diagram of a torpid Protopterus, in situ. c, Cocoon; e, earth; f, funnel leading to the mouth of the Fish; l, lid; m, mouth; m.b, mouth of the burrow; t, tail. (From Newton Parker.)]

In the dry seasons the marshes in which Protopterus lives become dried up, and to meet this adverse change in its surroundings the Fish hibernates, or passes into a summer sleep, until the next rainy season brings about conditions more favourable to active life. Preparatory to this summer sleep, and before the ground becomes too hard, the Fish makes its way into the mud to a depth of about 18 inches, and there coils itself up in a flask-like enlargement (Fig. 307) at the bottom of the burrow, which is lined by a capsule of hardened mucus secreted by the glands of the skin. The mouth of the flask is closed by the capsular wall or lid, which is perforated by a small aperture. The margins of this aperture are pushed inwards, so as to form a tubular funnel for insertion between the lips of the Fish. While encapsuled in its cocoon the Fish is surrounded by a soft slimy mucus, no doubt for the purpose of keeping the skin moist, and its lungs are the sole breathing organs, the air passing from the open mouth of the burrow through the hole in the lid directly to the mouth of the animal. The nutrition of the dormant Fish is effected by the absorption of the fat stored up about the kidneys and gonads, somewhat after a fashion not unknown in the fat-bodies of Insects and the hibernating glands of Rodents. Even portions of the caudal muscles undergo fatty degeneration, and thus, in a way which recalls the mode of nutrition of the Salmon during the breeding season, and of the Tadpole during its metamorphosis, a further store of nutritive material becomes available for the sustenance of the Fish during {514}its long summer nap. It is highly probable that the exceptionally numerous leucocytes act as carriers in the work of transporting the fatty particles to the different organs and tissues of the body. The length of the summer sleep naturally varies with the duration of the dry season, and probably it lasts on an average nearly half the year (August to December). The cocoons, imbedded in an outward casing of hardened mud, have often been brought to Europe, and when placed in water of suitable temperature the long torpid Protopterus escapes from its prison in a perfectly healthy condition, and resumes its partly branchial and partly pulmonary mode of breathing. The negroes of the West Coast of Africa are very partial to these Fishes, which they dig out of the dried marshes and preserve in their clumps of mud for food. With the advent of the rainy season, when the marshes become flooded, the Protopterus emerges from its cocoon, and returning to its former active life, soon enters upon the task of reproducing its kind. The important observations of Budgett have thrown much light on the curious breeding habits and development of these Fishes. The Fish makes a nest near the edge of a swamp. The nest is simply a hole of irregular shape, about a foot in depth, filled with water and surrounded by long grass (Fig. 308). There is no lining to the nest, and the eggs are deposited on the bare mud. Until the eggs are hatched, which occurs about the eighth day, and while the larvae are in the nest, the male remains on guard, and is apt to bite severely an incautious intruder. Probably with the view of aerating the eggs the water is continually lashed about by the tail of the guardian parent. The male has no trace of the peculiar vascular filaments which adorn the pelvic limbs of the male Lepidosiren during the breeding season. The early developmental stages are similar in their main outlines to those of Neoceratodus, but the young are very different. When the young Protopterus (Fig. 309) is hatched it is provided with a crescentic glandular sucker or cement-organ, situated on the under side of the head behind the mouth, by means of which the larva attaches itself to the sides of the nest, or of the vessel in which it is confined, much in the same way as the young Lepidosteus, and probably for the same reason. It may be remarked that the sucker agrees in structure, position, and function with that found in Amphibian tadpoles, but it differs both in position and structure from its preoral analogue in the young of Acipenser, Amia, and Lepidosteus.

{515}[Illustration: FIG. 308.—Nest of the Protopterus of the Gambia. (From Budgett.)]

A month-old larva has much the aspect of a larval Newt. It has four pairs of vascular plumose cutaneous gills (Fig. 309), which are retained as vestiges for a long time or even throughout life, and two pairs of synchronously-developed limbs. As an interesting instance of a nocturnal and protective change of colour, it may be mentioned that the dark chromatophores of the skin of the larva expand in the day-time and the young Fish becomes darker in colour, and therefore less conspicuous when seen against a background of black mud or soil. At night the contraction of the colour-sacs renders the larva more transparent and probably less easily {516}visible than if opaque. The commencement of pulmonary respiration is coincident with the degeneration of the cutaneous gills, which takes place about seven weeks after the deposition of the eggs, and about a month after the larvae leave the nest. Protopterus is said to attain a length of six feet.

Lepidosiren paradoxa, probably the only species of the genus, is confined to South America. It occurs along the course of the main Amazon river, entering some of its larger affluents, such as the Ucayale, the Madeira, the Rio Negro, and the Tapajóz, and also in the Chaco Boreal to the west of the Upper Paraguay river. The home of the Lepidosiren (or "Lolach," as the natives call the Fish) of the Chaco country is to be found in the wide-spreading marshes and swamps, which for a great part of the year are almost choked by a luxuriant growth of their own peculiar vegetation and covered by a floating carpet of surface weeds, with here and there deeper and clearer water and slow-flowing streams. In the dry season the water gradually shrinks and the swamps eventually become dried up. Of sluggish habits, the Fish wriggles slowly about at the bottom of the swamp like an Eel, using its hind limbs in an irregular bipedal fashion as it wends its way through the dense network of subaqueous plants. Lepidosiren is not exclusively carnivorous. The large fresh-water snail, Ampullaria, which lives in the swamps in enormous numbers, seems to be its favourite food; but masses of confervoid Algae are also eaten, and in its earlier stages it is probable that the Fish is more herbivorous than carnivorous. The Jacare (Caiman sclerops) feeds on Lepidosiren, and this fact, and probably also the cannibal habits of the Fish itself, may explain the capture of specimens with mutilated tails and regenerated, branched, pectoral limbs. Like other living Dipneusti, Lepidosiren rises to the surface to breathe. The intervals are, however, very variable, and no doubt depend on the relative purity or impurity of the water. Both expiration and inspiration are said to take place through the mouth. The snout is protruded on the surface, and the creature expires. After being withdrawn for a moment the head is again projected, and inspiration takes place through the partially open lips.

{517}[Illustration: FIG. 310.—Pelvic limb of the male Lepidosiren during the breeding season. (From Graham Kerr.)]

When the Fish finally sinks a few bubbles of surplus air escape through the gill-clefts. A nocturnal and protective change of colour, similar to that described in Protopterus, has been observed, and although most strikingly manifest in the larvae, it also occurs in individuals of older growth. The flesh is much esteemed as food by the Indians, who wade into the swamps and transfix the Fishes with spears. During the rainy season the Lepidosiren eats voraciously, and a reserve of fat is stored up in the tissues. Like its African relative, the Fish ceases to feed on the approach of the dry season, and eventually hibernates at the dilated extremity of a deep tubular burrow, the entrance to which is plugged by a small lump of clay perforated by several round holes. On the rising of the water at the next rainy season the Lepidosiren pushes out the plug and soon emerges from its burrow. The breeding season begins soon after the escape of the Fish. The eggs are deposited in nests in the form of underground burrows excavated in the black peaty soil at the bottom of the swamp, with an entrance about 4-5 inches wide. At a depth of about a foot the burrow takes a horizontal course, its total length varying from 2-5 feet. After the eggs are laid the male remains to guard them. During the breeding season the pelvic limbs of the male enlarge and become covered by a rich growth of highly vascular, blood-red filaments 2-3 inches in length (Fig. 310). The use of these curious structures is uncertain, but it is not improbable that they act as accessory gills to enable the male to guard the eggs in the nest without being forced to resort to the surface to breathe air. The development is essentially similar to that of Protopterus. The larva (Fig. 311) has four pairs of cutaneous gills in relation with the first, second, third, and fourth branchial arches, inclusive, the first three pairs being the homologues of the cutaneous gills of the tailed Amphibia; and also a cement-organ {518}which disappears shortly before the larval metamorphosis. At that period the circulation in the cutaneous gills becomes sluggish, and very soon these organs completely atrophy. About the same time the hyo-branchial cleft closes up, as in Protopterus. The young Lepidosiren soon begins to breathe air and to become more active and lively in its habits. The adult may attain a length of four feet.

The relations of the different genera of Dipneusti to one another has been discussed by Dollo in a remarkably suggestive paper. Until the publication of this treatise it was generally believed that the modern Dipneusti, Neoceratodus, Protopterus, and Lepidosiren, especially the first mentioned, were the most primitive and the more nearly related to the ancestral stock, while the older types, such as Dipterus, were regarded in the light of highly specialised offshoots. The continuity of the median fins, the apparently diphycercal character of the tail, and the wholly cartilaginous condition of the chondrocranium in the modern Dipneusti, were contrasted with the divided median fins, the heterocercal tail, and the more extensively ossified chondrocranium of the Palaeozoic forms, and the belief seemed inevitable. Dollo has shown, however, that there is good reason for the view that the evolution of the group has taken place in exactly the opposite direction; that, in fact, the older Dipneusti are the more archaic, and that their modern representatives have been derived from them by a sequence of retrogressive changes; or, in other words, the latter have much the same relation to the former as the degenerate Sturgeons and Paddle-Fishes to their Palaeozoic ancestors, the Palaeoniscidae. Taking Dipterus, the {519}most ancient of all the known Dipneusti, as a starting-point, it is possible to select a series of genera which illustrate the evolution of the group both in structure and in palaeontological sequence. The series is as follows:—Dipterus, Scaumenacia, Phaneropleuron, Uronemus, Ceratodus (Neoceratodus), Protopterus and Lepidosiren. Briefly, the more important structural modifications observable in the transition from the older to the recent genera are (a) the gradual union of isolated median fins to form a continuous fin; (b) the substitution of a gephyrocercal tail for a heterocercal; (c) the degeneration of the squamation, the thick ganoid scales of the earlier types being replaced by thin, non-ganoid scales; (d) a reduction in the number of cranial dermal bones and the loss of their original ganoid investment; (e) the suppression of the jugular plates; and (f) a reduction in the size of the opercular bones. In the last two genera of the series, in which specialisation in some respects and degeneration in others have reached their maximum, the body no longer retains the fusiform and more typically Fish-like shape of the older genera, but, in accordance with Eel-like habits and mode of progression, has become more or less Eel-like in form. The paired fins are almost vestigial, while the scales, so deeply insunken in the skin as to be externally invisible, suggest that the modern Dipneusti are approximating to a final scaleless as well as to an ultimately limbless condition. As to the origin of the Dipneusti as a group, it seems reasonable to look for their ancestors in the early Devonian Crossopterygii with acutely lobate fins, or, with greater probability, to some still more primitive Crossopterygian with simple, non-rhizodont teeth, capable by fusion of giving rise to massive tritoral plates, and involving as a consequence the substitution of an autostylic for an originally hyostylic skull, and the suppression of the secondary upper jaw. In fact, when our knowledge of the development of the surviving Dipneusti and Crossopterygii is more complete, it is not improbable that the inclusion of the two series of Fishes in subordinate divisions of the Teleostomi will prove to be amply justified. The relations of the Dipneusti to the Amphibia are {520}somewhat deceptive, and it seems improbable that the former group stands in the direct line of Amphibian descent. In most of their structural features not directly or remotely associated with air-breathing the Dipneusti are true Fishes, and the striking resemblances which they present to the Amphibians in the vascular system and lungs seem to be rather the outcome of physiological convergence, associated with adaptive and parallel modifications in structure, and due to the influence of a similar environment, than indicative of direct ancestral relations. With more reason it may be inferred that both the Dipneusti and the Amphibia have been derived from some primitive Crossopterygian ancestor with Elasmobranch tendencies, and subsequently became modified in certain respects on parallel lines.

{521}CHAPTER XX

APPENDIX TO THE FISHES: PALAEOSPONDYLIDAE—OSTRACODERMI—HETEROSTRACI— OSTEOSTRACI—ANASPIDA—ANTIARCHI—ARTHRODIRA.

In this chapter it is proposed to treat of certain fossil "Fishes" which, from our ignorance of much that is essential to a proper estimate of their true relationships, cannot at present be referred to any of the recognised primary groups of Fishes.

I. PALAEOSPONDYLIDAE.

The interesting little fossil, Palaeospondylus gunni, discovered in the Lower Old Red Sandstone of Caithness, and first described by Traquair, represents the calcified endoskeleton of an elongated fish-like organism about an inch, or not exceeding two inches, in length. The vertebral column consists of a series of broad, calcified ring-like centra, destitute of ribs, but possessing neural arches and spines, and in the caudal region haemal arches and spines in addition. The skull, of which only the ventral surface is known, has a complete basis cranii, laterally expanded behind by periotic capsules, and in front by what seem to be bulging olfactory capsules. Anteriorly, the skull terminates in a ring of calcified cirri. Behind the skull there are two singular post-occipital plates, one on each side of the anterior section of the vertebral column. The tail was apparently furnished with a fringing caudal fin, supported dorsally by the long forked, neural spines, and below by the much shorter haemal spines. There is no trace {522}of limbs, limb-girdles, jaws, or branchial arches, nor any evidence of the existence of scales, denticles, or other exoskeletal structures.

Palaeospondylus has been regarded as a Cyclostome, a view which derives its principal support from the resemblance of the cirri-encircled orifice at the anterior end of the skull to an unpaired nasal or naso-pituitary aperture, and perhaps some measure of credence from such purely negative evidence as the apparent want of limbs, and of any structures comparable to jaws. But even if it be admitted that there is some reason for this view, it is certain that Palaeospondylus obtained a far higher grade of specialisation in certain respects than any of the existing Cyclostomata; the presence of calcified vertebral centra and neural arches is conclusive on this point. Palaeospondylus has also been compared with a larval Arthrodiran and with a larval Dipnoid.

II. OSTRACODERMI.

The Palaeozoic fish-like forms, which, more as a matter of convenience than as an expression of real kinship, it has been customary to include in this group, are amongst the earliest Craniates of which we have any precise knowledge. {523}Of the three subordinate groups or "Orders" into which they have usually been divided hitherto, two, the Heterostraci and the Osteostraci, may, with some show of reason, be considered as related forms, and although they are characterised by much specialisation on independent lines, there is yet some evidence of connecting links between the two. The organisms comprising the third group, the Antiarchi, stand upon a very different footing, and at present it cannot be said that they are in any way related to either the Heterostraci or the Osteostraci, or indeed to any other Craniates whatsoever. The association of the Ostracodermi with the Cyclostomata, a view which has received more influential support than it deserves, is based on the presumed absence of jaws and paired fins. The absence of jaws, which, if present, were almost certainly cartilaginous, has yet to be proved, and even in the latter group it is by no means certain that they do not possess structures which, morphologically if not functionally, are veritable jaws. Nor is it quite certain that the lateral lobes of some Ostracodermi are neither pectoral flaps nor lateral fin-folds, to say nothing of the lateral appendages of the Antiarchi. And to these objections there is the further difficulty that there is absolutely no evidence that the Ostracodermi are monorhinal in the sense in which this term is applied to the Cyclostomata. On these grounds it would seem more in accordance with our present knowledge to regard the Ostracodermi as an independent group whose exact position in the system has yet to be determined, including, however, besides the generally accepted orders Heterostraci and Osteostraci, the recently founded provisional order Anaspida, but excluding the Antiarchi as a separate and distinct section; rather than to crystallise in a definite system of classification views which are either purely conjectural or wholly unjustifiable. Even with this limitation the Ostracodermi are by no means easy to define, especially if we include those remarkable shark-like forms from the Upper Silurian rocks of the south of Scotland which have been so admirably described in the recent classical memoirs of Dr. Traquair. As a rule, the head and the anterior part of the body are laterally expanded, and more or less sharply defined from the rest of the body by prominent postero-lateral angles. The exoskeleton, which exhibits an extraordinary variety of {524}structure in the different families, ranges from a uniform covering of dermal denticles to a condition in which the denticles fuse to form anteriorly a highly characteristic tessellated or continuous dorsal shield, while posteriorly they become replaced by a nearly typical rhombic squamation. The tail is heterocercal. Paired fins of the ordinary piscine type are absent. In some Ostracodermi it seems probable that the gill-clefts opened into a common branchial chamber on each side, with a single external aperture, but in others they may have been ventral. The endoskeleton, jaws, dentition, and the nostrils are unknown.

ORDER I. HETEROSTRACI.

The exoskeletal structures consist of dentine, or of a tissue resembling it, never of true bone. The orbits are marginal or lateral in position. With the exception of a caudal fin there are no median fins.

FAM. 1. COELOLEPIDAE.—Head and anterior portion of the body flattened and expanded, with prominent lappet-like postero-lateral lobes, which may represent continuous lateral fin-folds or a very primitive type of pectoral fin. Nothing is known of the mouth, but it must have been ventral, nor of the position of the orbits. Branchial apertures unknown, but transverse markings on each side of the anterior part of Thelodus pagei may be indications of a branchial apparatus. The exoskeleton consists of a uniform covering of hollow pointed spines, devoid of a basal plate and open below (Lanarkia); or of minute shagreen-like tubercles (Thelodus). The tubercles or spines consist of dentine coated by ganoin. Of the only two known genera, Thelodus is a characteristic Upper Silurian genus {525}from the Ludlow and Downtonian Beds of Lanarkshire. Detached scales are also known in the Upper Silurian of England. One species (Th. pagei) occurs in the Lower Old Red Sandstone of Forfarshire, and another (Th. tulensis) in the Upper Devonian of Russia. Lanarkia has only been found in the Downtonian Beds. None of the Coelolepidae exceed fourteen to fifteen inches in length.

FAM. 2. DREPANASPIDAE.—This family affords an interesting transition to the more highly specialised and carapaced Pteraspidae. The head and anterior part of the trunk now form a broad oblong shield, rounded in front and abruptly marked off from the tail by conspicuous rounded angles. The exoskeleton is no longer uniform. In the caudal region the scattered spines or shagreen tubercles of the Coelolepids have become transformed into tuberculated quadrangular scales, which are further differentiated along the dorsal and ventral margins into ridge scales or fulcra; and from a similar source by a process of basal fusion a series of larger or smaller dermal plates are formed as components of large dorsal or ventral shields. The dorsal shield (Fig. 314) is formed by a large central plate; the postero-lateral portions by two narrow falciform plates; and the anterior margin by a series of smaller rostral plates. Between the larger plates the shield is completed by numerous {526}small polygonal plates. All the plates are superficially ornamented by small stellate tubercles. The ventral armature (Fig. 315) is similar to the dorsal. A large mental plate forms the hinder margin of the transverse slit-like mouth, the anterior limit of which is defined by the rostral plates already mentioned. Laterally may be seen a pair of small plates (x), each perforated by a small aperture, and probably indicating the position of some kind of sense-organ. Posteriorly there is a large median ventral plate, in relation with a pair of anterior and a pair of posterior ventral plates. The areas between the larger plates are filled in by numerous small polygonal plates. It is possible that there is a single external branchial aperture on each side, near the postero-lateral angle of the shield and behind the posterior ventro-lateral plate. The sole representative of the family is Drepanaspis gemündenensis, from the Lower Devonian of Gemünden in Rhenish Prussia. Large examples of this fossil must have exceeded two feet in length.

FAM. 3. PSAMMOSTEIDAE.—To this family are referred certain dermal plates occurring, in a more or less fragmentary condition, in the Old Red Sandstone and Devonian formations of Great Britain and Russia. In their size and shape, and in their stellate tubercles, these have been compared to the dorsal, postero-lateral, and ventral plates of {527}Drepanaspis. That Psammosteus is closely allied to Drepanaspis seems certain, but for the present the two genera may be retained in separate families.

FAM. 4. PTERASPIDAE.—Until the recent inclusion of the three preceding families, the Pteraspidae were the only representatives of the Heterostraci. In the best known genus, Pteraspis, there is a marked reduction in the number of the component plates of the carapace, and only seven can now be distinguished (Fig. 316): (a) a large posterior dorsal plate, supporting behind a stout spine; (b) a conical rostral plate, covering the preorbital part of the head; (c) a pair of small marginal orbital plates, each with a small aperture, probably for the eye; (d) a pair of posterior lateral or cornual plates, each of which is perforated by a large oblique foramen, conjecturally an external branchial aperture; and (e) a large ventral plate. There is probably, also, a small median "parietal," or "pineal," plate, with a pit on its inner surface, situated between the rostral and posterior dorsal plates. Externally the plates are sculptured into fine ridges, which in their minute structure and their crenated free margins are suggestive of linear series of fused denticles. The tail appears to have been invested by imbricated rhombic scales. Pteraspis (Lower Old Red Sandstone of Scotland and England, and the Lower Devonian of Galicia); Cyathaspis (Upper Silurian and Lower Old Red Sandstone), known only by its dorsal and ventral shields; and Holaspis (Lower Old Red Sandstone of Monmouthshire, and the Upper Silurian of Pennsylvania), are the only genera.

ORDER II. OSTEOSTRACI.

While agreeing with the more specialised Heterostraci in the division of the body into an anterior carapaced portion and a free {528}hinder part invested by a rhombic squamation, the Osteostraci are distinguished by the presence of bone as a histological component of the dermal hard parts; by the position of the orbits, which, instead of being marginal in position, are close together on the dorsal aspect of the carapace; and by the possession of a median dorsal fin.

FAM. 1. ATELEASPIDAE.—The general shape of the body is much the same as in the Coelolepidae, but the exoskeleton consists of numerous polygonal tuberculated plates in front of the postero-lateral lobes, and of sculptured rhombic scales behind. A pair of crescentic markings, placed close together about the middle of the dorsal surface of the head, probably indicate the outer margins of orbital recesses (Fig. 317). The only species at present known (Ateleaspis tessellata) occurs in the Downtonian beds.

FAM. 2. CEPHALASPIDAE.—In this family the dorsal shield is rounded in front, strongly arched above, with its postero-lateral angles produced into highly characteristic cornua (Fig. 318). The shield consists of a single piece, but as the outer surface is ornamented by small tubercles arranged in polygonal areas, it is probable that it has been formed by the basal fusion of {529}numerous primitively distinct polygonal plates (Fig. 319, A). Between the orbits there is a separately calcified but fixed plate, which bears a hollow prominence, probably for the reception of a parietal organ. In some genera certain of the anterior dorsal and ventral scales of the trunk fuse into a continuous plate. Internally to the postero-lateral cornua the middle layer of the shield is prolonged backwards into a pair of singular flap-like lobes, which have been variously interpreted as corresponding to the lateral lobes of the Coelolepidae, to pectoral fins, or to opercula. The scales of the trunk and tail are rhombic and imbricated; on the sides of the body they are remarkably high and narrow.

The best known genus is Cephalaspis. The earliest remains are found in the Ludlow Tilestones. The genus is also represented in the Ledbury Passage Beds, the Lower Old Red Sandstone of Scotland, and the Upper and Lower Devonian of Canada. Most of the species are of small size, but C. magnifica, from the Caithness Flagstones, the largest of all the Cephalaspids, has a shield 8½ inches long, and 12 inches across the widest part. Auchenaspis occurs in the Ludlow Tilestones and the Ledbury Passage Beds, and also in the Upper Silurian of the Isle of Oesel in the {530}Baltic. Another genus, Didymaspis, has been found in the Lower Old Red Sandstone of Ledbury.

FAM. 3. TREMATASPIDAE.—The interorbital plate is free, and hence it is often lost in the fossils. Several species of Tremataspis occur in the Upper Silurian of the Isle of Oesel.

As regards the origin and mutual relationships of the different families comprising the Heterostraci, it has been urged with great force by Dr. Traquair that they constitute a natural sequence of forms, beginning with organisms whose Elasmobranch ancestry is extremely probable, and leading to highly-specialised types, which, considered by themselves, possess little to justify any conclusions whatever as to their origin or kinship. The Coelolepidae form the starting-point, and in the light of their exoskeleton of dermal denticles, their derivation from some primitive Elasmobranch prototype seems a reasonable inference. From the Coelolepids the path of specialisation through the Drepanaspidae and Psammosteidae to the Pteraspidae is marked (i.) by the basal concrescence of isolated denticles to form, first, numerous small polygonal plates, and then larger and less numerous plates, as the constituent elements of a characteristic dorsal shield, leaving, however, the denticles of the rest of the body to become converted into a rhombic squamation; (ii.) by modifications in the "lateral fin-lobes," which may become enclosed in the developing dermal armour (e.g. Drepanaspis), or cease to be recognisable (e.g. Pteraspis). The affinities of the Osteostraci are very obscure, and their inclusion with the Heterostraci in the same group (Ostracodermi) has hitherto rested mainly on such negative evidence as the supposed absence of paired limbs, jaws, and teeth; in fact, it has been affirmed that "there is absolutely no reason for regarding Cephalaspis as allied to Pteraspis beyond that the two genera occur in the same rocks." It is possible, however, that in Ateleaspis we have an annectent form, which in some measure combines the structural peculiarities of the two groups. That this singular genus belongs to the Osteostraci is proved by the presence of bone lacunae in its dermal hard parts, a conclusion which is strengthened by the apparently dorsal position of the orbits and the presence of a dorsal fin. On the {531}other hand, its close resemblance to the Coelolepids in the general contour of its laterally-lobed body, and the probability that its mosaic and tuberculated head-shield has been formed by the concrescence of Coelolepid denticles, is at least significant of a relationship to the more primitive Heterostraci. Little can be conjectured as to the habits of these ancient "Fishes." The form and regional proportions of the body, which in some respects often remind one of organisms so diverse as a King Crab, or a Loricaroid Teleost (such as Liposarcus), are strongly suggestive of a grovelling, bottom-feeding, sluggish habit of life, in sharp contrast to the more active and predaceous Fishes whose appearance is coincident with the extinction of the Ostracodermi at the close of the Devonian period. Habits such as these may well be associated with much structural degeneration, even, it may be, with the loss of paired fins, and hence it is not altogether improbable that the Ostracodermi are outcasts from the Elasmobranchs, a degenerate race which has sought safety in a sequestered life and a coat of mail.

ORDER III. ANASPIDA.

This group has been instituted by Traquair for the provisional reception of two remarkable genera, which, owing to the absence of precise knowledge of the histology of their exoskeletal structures, cannot at present be referred either to the Heterostraci or the Osteostraci, and for which, as their discoverer remarks, no place can be found in the system unless they are admitted to the Ostracodermi.

FAM. 1. BIRKENIIDAE.—Body fusiform and fish-like. Head bluntly rounded, without a cranial shield. Caudal fin bilobate and heterocercal A median row of scales with recurved spines arranged along the ventral surface. Orbits, jaws, teeth, paired fins, and endoskeleton unknown.

In Birkenia (Fig. 320) the body is invested by longitudinal rows of narrow scales arranged in oblique transverse rows, which are replaced on the head by much smaller, peculiarly disposed, spindle-shaped scutes. On the side of the hinder part of the head there is an oblique row of small apertures, possibly branchial. A small remote dorsal fin, invested by the trunk scales, is present. {532}Birkenia elegans, the only species known, does not exceed 3½ inches in length. Less is known about the second genus, Lasanius, of which there are two species. Except for the mid-ventral series of spiny scutes, and a row of slender, parallel, rod-like structures, the body appears to have been naked (Fig. 321). The two genera belong to the remarkable series of fossil Fishes from the Silurian rocks of Lanarkshire. Rare in the Ludlow series, Birkenia is by far the most common of the Fishes of the over-lying Downtonian Beds. Lasanius is confined to the latter horizon. Euphanerops, from the Upper Devonian of Canada, is probably related to this family, but lateral branchial apertures are not known.

III. ANTIARCHI.

The organisms comprising this group resemble the Ostracodermi in possessing a well-developed carapace of bony plates and a heterocercal tail, as well as in many of the purely negative features which are characteristic of the latter group.

{533}[Illustration: FIG. 322.—Restored outline of Pterichthys milleri. The upper figure represents a dorsal view, and the lower a lateral view. The dotted lines indicate the course of the lateral line system. a.d.l, Antero-dorso-lateral; ag, angular; a.m.d, anterior median dorsal; a.v.l, anterior ventro-lateral; e.l, extra-lateral or operculum; l, lateral; l.occ, lateral occipital; m, median or interorbital plate; m.occ, median occipital; o, orbit; p.d.l, posterior dorso-lateral; p.m, pre-median; p.m.d, posterior median dorsal; pt.m, post-median; p.v.l, posterior ventro-lateral. ——, Plates investing the limbs: c, central; d.a, dorsal anconeal; d.ar, dorsal articular; e.m, external marginal; i.m, internal marginal; m.m, marginals; t, terminal. (From Traquair.)]

The remarkable dorsal shield is divided into a small cephalic portion and a much larger hinder part investing the greater part of the trunk, both of which are strongly arched above and flattened ventrally, with a movable articulation between the two. The cephalic shield is formed by numerous symmetrically-disposed tuberculated plates, suturally connected with one another, and, like the other exoskeletal structures, containing bone lacunae (Fig. 322). The orbits are close together, near the middle of the dorsal surface, and between them there is a small median interorbital plate, with a deep pit on its inner surface, possibly for a parietal organ. A small lateral plate (e.l.), evidently free behind, suggests the presence of an operculum. Nothing is certainly known about the jaws or the nostrils. The mouth is situated just behind the anterior margin of the cephalic shield on the ventral surface, and in front of it there are two plates, {534}which in Bothriolepis canadensis have their oral margins fringed by small "denticles"; it is possible that these plates represent the components of a secondary upper jaw. The dorsal armature of the trunk is shown in Fig. 322. Ventrally it is completed by a pair of anterior ventro-lateral plates and a pair of posterior ventro-lateral plates with a small median plate between the two pairs. Articulating with the anterior ventro-lateral plates by means of a complex hinge joint there is a pair of pectoral appendages of a kind entirely without parallel in any other vertebrated animals. Each appendage is completely encased by numerous suturally connected plates, and about the middle of its length there is a second movable joint. The appendages are hollow, and their cavities probably contained the muscles by which the limbs were moved, and the blood-vessels and nerves for their nutrition and innervation. A lateral line system of the normal type is present in Pterichthys, consisting of a lateral groove along the side of the trunk, and of supra-orbital and infra-orbital grooves, and post-temporal and infra-orbital commissures, on the head. The free portion of the body and the tail are invested by imbricated and finely tuberculated scales, which form fulcra in front of and behind the small dorsal fin. There are no pelvic fins. The caudal fin is heterocercal.

FAM. 1. ASTEROLEPIDAE.—The best known genera are Pterichthys from the Lower Old Red Sandstone of Scotland and the Devonian of Eifel, and Bothriolepis, a more widely distributed genus which occurs in the Upper Old Red of Scotland and Shropshire, and in the Upper Devonian of Russia and Canada. Two other genera, Asterolepis and Microbrachius, are also found in the Old Red Sandstone of Scotland.

Beyond an uncertain and shadowy relationship to the Ostracodermi, and perhaps some points of resemblance to the Arthrodira, the Antiarchi stand alone among Craniates. Nothing is known of their origin; no intermediate forms link them to any other groups, and the high specialisation they have attained is sufficient to negative any idea that they can "be credited with any share in the evolution of the Fishes of more recent periods."

{535}IV. ARTHRODIRA

This group has been instituted for the reception of a number of remarkable armoured Fishes of uncertain relationships which flourished in Europe during the Devonian and Old Red Sandstone periods, and in North America from the Devonian to the Lower Carboniferous. The head (e.g. in Coccosteus) is invested dorsally by a series of median and lateral symmetrically-disposed tuberculated plates (Fig. 323). Two of the lateral plates are notched for the orbits, and between them there is an interorbital plate which either has a pit on its inner surface or is perforated by an open funnel, as in Dinichthys, possibly for a parietal or a pineal organ. Some of the bones present some analogy, to say the least, to certain of the dermal bones of a typical Teleostome, apparently representing such elements as paired parietals and frontals, a dermal mesethmoid, and toothless premaxillae and maxillae (Fig. 324, A). As in the Antiarchi, the anterior portion of the trunk is also armoured, above by a dorsal shield, formed by median and lateral plates, and below by a similarly constructed ventral shield (Fig. 324, B). A huge joint connects the head and trunk shields: hence the term Arthrodira or "joint-neck." The rest of the body is naked.

Pectoral fins are unknown, but pelvic fins, each supported by a stout basal plate or basipterygium, and with traces of radials, are present. There is a small dorsal fin. Little is known of the primary cranium, but in the trunk and tail it is evident {536}that there are well-developed and partially calcified neural and haemal arches associated with a persistent notochord. It is possible that the skull is autostylic. Gill-arches are not known. A pair of plates (Fig. 324, A, j) at the postero-lateral angles of the cephalic shield may perhaps be opercula. The teeth are conical. Those in the upper jaw are supported by two pairs of plates, probably vomers and palatines. In the lower jaw there are two series of teeth, one in front near the symphysis, and the other behind, supported by a single bone in each ramus. There is a well-developed lateral line system, indicated by surface markings on the head and trunk shields.

FAM. 1. COCCOSTEIDAE.—Coccosteus occurs in the Devonian of Europe and North America, and includes species of relatively small size, not exceeding half a metre in length. C. decipiens, the best known species, is a characteristic fossil in the Old Red Sandstone of Scotland. Phlyctaenaspis is found in the Lower Devonian of Canada, England, and Poland. A larger Arthrodiran, with slender toothless jaws, Homosteus, is met with in the Lower Old Red Sandstone of the North of Scotland, and in the Devonian of Germany and Russia. The Old World Arthrodira must yield, however, to those of the New World for variety in size and shape, and in the character of their dentition. {537}Some of the North American genera (e.g. Dinichthys) probably attained a length of ten feet, or even, as in Titanichthys, a much greater size. Some are fusiform in shape, but Mylostoma is flattened and Ray-like, and, judging from the dentition, their food and habits must have been equally varied. Mylostoma has tritoral plates not unlike those of Neoceratodus or Chimaera. In others the teeth are single, and conical or pointed; in Titanichthys the front teeth in both jaws are beak-shaped structures. It is highly probable that Titanichthys, Mylostoma, and perhaps other genera, are types of distinct families.

The Arthrodira have been regarded as armoured Dipneusti, a view which is mainly based on their supposed autostylism and the nature of the dentition. But this autostylism has yet to be verified, and, if proved, the possibility that it may be a secondary feature, associated with the evolution of a peculiar dentition, must not be forgotten. Much more may be said for their claim to be regarded as a highly specialised race of primitive Teleostomi. Besides a well-developed lower jaw, bones comparable to the elements of a secondary upper jaw are known, and in a general way the disposition of the cranial roofing bones, and the arrangement of the endoskeletal elements of the pelvic fins, tend to conform to the normal Teleostome type. In fact, Dr. Traquair has expressed the opinion that the Arthrodira are Teleostomi and Actinopterygii.

TELEOSTEI (SYSTEMATIC PART)

G. A. BOULENGER, F.R.S., V.P.Z.S.

Of the British Museum (Natural History)

{541}CHAPTER XXI

TELEOSTEI: GENERAL CHARACTERS—MALACOPTERYGII—OSTARIOPHYSI

ORDER IV. TELEOSTEI.

As stated above (p. 495), the Holostean Ganoids pass very gradually into the Teleosteans, the lower groups of which appear to have been directly derived from them. The precise definition of the Order Teleostei, as compared with the Ganoid Order Holostei, is a matter of some difficulty. The most important character appears to be the presence of an ossified supraoccipital bone. Remnants of primitive characters, such as ganoid scales, fulcra, rudiments of a splenial bone, spiral valve to the intestine, multivalvular conus arteriosus, are still found in some lower Teleosteans, but no longer in that combination which serves to define the preceding order. Although Albula is exceptional among all Teleosteans in having two transverse series of valves to the bulbus arteriosus instead of one, no Ganoid has fewer than three.

The first remains of Teleosteans appear scantily in the Upper Trias, and it is not before we reach the Upper Cretaceous that they assume preponderance over other Teleostomes; whilst in the Upper Eocene they have already attained a development and variety of types comparable to their present condition. Out of some 12,000 well-established species of Fishes known to exist at {542}the present day, about 11,500 belong to this order. The classification of such an array of forms is, of course, a matter of great difficulty, and gives scope for much difference of opinion among those who have attempted to grapple with the subject. It is now recognised that the study of the skeleton affords the safest guide to a natural arrangement of the families and higher divisions. Much has been done in this line by Cope, Gill, Sagemehl, A. S. Woodward, and Jordan and his pupils; but the osteology of many important types still remains unknown. For some years a large number of skeletons have been prepared in the British Museum with the object of settling open questions, and this material has enabled me to draw up a scheme of classification which, whatever its defects, and however provisional, I feel sure is on the whole an improvement on those hitherto proposed, and especially on that generally in use in this country. The latter was, to a great extent, based on physiological principles; the present aims at being phylogenetic. In its preparation I have derived great benefit from the labours of the authors quoted above, but have endeavoured in every instance to verify their statements on a larger osteological material than appears to have been available to them. I have also had the advantage of the criticism, on many points, of my young colleague, Mr. C. Tate Regan, who has himself endeavoured to settle some important questions of classification.

The Order Teleostei is divided into thirteen sub-orders, the probable relations of which are expressed in the following diagram:—

11. Opisthomi. 13. Plectognathi. 12. Pediculati. –––+ | | | | +––––––––––––––––––+––––––––––––––––––––+ | | | 9. Anacanthini. 10. Acanthopterygii. 8. Percesoces. | | | | | +––––––––––––––––––+––––––––––––––––––––+ |––Teleostei. | | 7. Catosteomi. 5. Haplomi. 6. Heteromi. | | | | | +––––––––––––––––––+––––––––––––––––––––+ | | | | | 4. Apodes. | | 3. Symbranchii. | | | 1. Malacopterygii. 2. Ostariophysi. | | | –––+ +––––––––––––––––––––––+ | Ganoidei Holostei.

{543}In the classification of Günther, which has been generally in use in this country for the last thirty years, the Teleosts were divided into six principal groups, of ordinal rank: I. Acanthopterygii; II. Acanthopterygii Pharyngognathi; III. Anacanthini; IV. Physostomi; V. Lophobranchii; VI. Plectognathi. Group I. corresponds to Sub-Order 6 (part), 7 (part), 8 (part), 10 (part), 11 and 12 of the present work; Group II. to Sub-Order 10 (part); Group III. to Sub-Order 9 and 10 (part); Group IV. to Sub-Order 1, 2, 3, 4, 5, 6 (part), and 8 (part); Group V. to Sub-Order 7 (part); and Group VI. to Sub-Order 13.

SUB-ORDER 1. MALACOPTERYGII.

Air-bladder, if present, communicating with the digestive tract by a duct. Opercle well developed. Pectoral arch suspended from the skull; mesocoracoid arch present. Fins without spines, the ventrals abdominal, if present. Anterior vertebrae distinct, without Weberian ossicles.

This sub-order, which corresponds to the Isospondyli and Scyphophori of Cope and to a part of the Isospondyli of A. S. Woodward, embraces the most generalised of the Teleosts, and is intimately connected with the Ganoids by the fossil forms which are placed at the base of the series of families. The physostomous condition of the air-bladder, the connexion of the pectoral arch with the skull, the presence of the mesocoracoid arch, the backward position of the many-rayed ventral fins, the normal condition of the anterior vertebrae, the absence of true spines to the fins, and the separation of the supraoccipital bone from the frontals by the parietals, are primitive characters which among the Teleosts occur combined in some families of this suborder only. The mesocoracoid arch is retained by the Ostariophysi, which differ in the remarkably modified condition of the anterior vertebrae, but it disappears in all other Teleosts, which gradually acquire a more forward position of the ventral fins and a reduction in the number of their rays, develop spines in the vertical fins, and lose the communication of the air-bladder with the outside.

The Malacopterygii may be divided into twenty-one families, the characters of which are contrasted in the following synopsis:—

I. Fins fringed with fulcra, or scales coated with ganoin; {544} notochord usually continuous through the vertebrae.

Vertebral centra not more than rings; fins with fulcra; scales rhombic, united by peg-and-socket joints 1. Pholidophoridae.†

Vertebral centra not more than rings; fins with fulcra; scales cycloid 2. Archaeomaenidae.†

Vertebral centra complete or with minute perforation; fins with fulcra; scales cycloid 3. Oligopleuridae.†

Vertebral centra nearly complete, but with perforation; no fulcra; scales cycloid 4. Leptolepididae.†

II. Fins without fulcra; scales without ganoin; vertebral centra complete.

A. Supraoccipital separated from the frontals by the parietals or upper surface of skull; no adipose fin.

1. Ventral fins with 10 to 16 rays.

An intergular bone; parasphenoid narrow 5. Elopidae.

No intergular bone; parasphenoid very broad 6. Albulidae.

2. Ventrals with not more than 7 rays.

a. Supratemporal very large, plate-like.

Praemaxillary single, its posterior extremity free from the maxillary; symplectic absent; basis cranii simple 7. Mormyridae.

Praemaxillary paired, its posterior extremity firmly attached to the maxillary; symplectic present; basis cranii double 8. Hyodontidae.

b. Supratemporal small; maxillary firmly attached to posterior extremity of praemaxillary.

Praemaxillary paired; a large hole on each side of the skull, between the post-frontal and the squamosal; basis cranii double; suboperculum absent; ribs sessile 9. Notopteridae.

Praemaxillary paired; basis cranii simple; suboperculum reduced; ribs inserted on parapophyses 10. Osteoglossidae.

Praemaxillary single; basis cranii simple; suboperculum and interoperculum absent; ribs inserted on parapophyses 11. Pantodontidae.

c. Supratemporal small; maxillary movable; ribs sessile; ventral fins below the pectorals 12. Ctenothrissidae.†

B. Supraoccipital separating parietals, or adipose fin present.

1. Interoperculum enormous; symplectic absent; basis cranii simple 13. Phractolaemidae.

2. Interoperculum normal; symplectic present; basis cranii double.

a. Teeth in sockets; maxillary firmly attached to praemaxillary. Symplectic exposed 14. Saurodontidae.†

b. Teeth not in sockets.

Symplectic concealed between the quadrate and the hyomandibular 15. Chirocentridae.

Postclavicle on outer side of clavicle; no adipose fin 16. Clupeidae.

Postclavicle on inner side of clavicle; an adipose dorsal fin 17. Salmonidae.

Postclavicle absent; no adipose fin 18. Alepocephalidae.

3. Interoperculum normal; basis cranii simple. {545}

Maxillary large, toothed; praecaudal vertebrae without well-marked parapophyses; scales cycloid or absent; adipose dorsal fin present or absent 19. Stomiatidae.

Mouth small, toothless; vertebrae with strong parapophyses; head and body covered with spiny scales 20. Gonorhynchidae.

Mouth small, toothless; no symplectic; head and body naked 21. Cromeriidae.

† This sign indicates that the group is represented by fossil forms only.

FAM. 1. PHOLIDOPHORIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter large and loosely attached; teeth small and conical. Parietal bones separating the supraoccipital from the frontals; opercular bones well developed. Vertebral centra never advanced beyond the annular stage; ribs delicate; no fused or expanded haemal arches at the base of the tail; no epipleurals or epineurals. Fin-fulcra present, but usually small; dorsal and anal fins small, the former above or behind the ventral fins, which are small or may be absent. Scales ganoid, rhombic, those on the flanks united by peg-and-socket joints.

This family, which appears to me to be related to the Dapediidae, is provisionally placed here by A. S. Woodward on account of its resemblance to the Leptolepididae, but it is not yet quite clear that the mandible was destitute of splenial and coronoid elements, while the bones at the base of the pectoral fin have not hitherto been observed. The principal genera are Pholidophorus, ranging from the Upper Trias to the Purbeck; Thoracopterus, from the Upper Trias; and Pleuropholis, from the Upper Jurassic. The species of Pholidophorus are very numerous in the Jurassic period, and Woodward has observed that the scales of the later species are more elaborately ornamented than those of earlier date.

FAM. 2. ARCHAEOMAENIDAE.—Distinguished from the preceding by the thin, cycloid scales. Conspicuous obtuse ridge-scales are present along the dorsal and ventral lines. Archaeomenes, from the Jurassic (?) of New South Wales.

FAM. 3. OLIGOPLEURIDAE.—Characters as in Pholidophoridae, but scales cycloid and vertebrae completely or nearly completely ossified.

Oligopleurus, from the Jurassic of England and France; Oenoscopus, from the Jurassic and Cretaceous of France, Germany, and Italy; and Spathiurus, from the Cretaceous of Mount Lebanon and Dalmatia.

{546}FAM. 4. LEPTOLEPIDIDAE.—Praemaxillaries very small; maxillaries large, loosely attached; teeth small and conical. Parietal bones separating the supraoccipital from the frontals; opercular bones well developed. Vertebral centra well ossified, but always pierced by the notochord; ribs delicate; epipleurals present; no fused or expanded haemal arches at the base of the caudal fin. Dorsal and anal fins small, the former above or behind the ventrals. Ventrals with 5 to 10 rays. Scales thin, cycloid and deeply imbricate, usually coated with ganoin in their exposed portion.

Leptolepis, with numerous species, from the Jurassic and Cretaceous of Europe and New South Wales; Vidalia, Jurassic of France; Aethalion, Jurassic of Bavaria; Thrissops, Jurassic and Cretaceous of Europe; and Lycoptera, Jurassic of Asia.

FAM. 5. ELOPIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter the more developed, and movably articulated above the former to the ethmoid. Parietal bones in contact behind the frontals; opercular bones well developed. Basis cranii double. A bony intergular or sublingual plate. Jaws, palatines, pterygoids, vomer, parasphenoid, glossohyal, and pharyngeals toothed. Ribs mostly sessile, inserted very low down, behind parapophyses; epineurals similar to the ribs, but directed upwards. Pectorals low down, folding like the ventrals. Post-temporal forked, the upper branch attached to the epiotic, the lower to the opisthotic; post-clavicle small; scapular foramen in the scapula; pterygials well developed, three in contact with coracoid. Ventrals with 10 to 16 rays. Branchiostegal rays very numerous (over 20). Air-bladder large.

{547}This family is abundantly represented in Cretaceous times by the genera Osmeroides and Elopopsis, and from the Lower Eocene to the present day by Elops and Megalops. Elops saurus is a handsome elongate silvery Fish, found in all the warm and tropical seas; the young are ribbon-shaped like those of Albula. A second species, E. lacerta, is from the West Coast of Africa, entering rivers. Megalops, distinguished by larger scales, the absence of pseudobranchiae, and the curious prolongation of the last ray of the dorsal fin, includes the well-known Tarpon M. atlanticus, and the Indian M. cyprinoides. The Tarpon occurs from the south-eastern coasts of North America and the West Indies to Brazil, and reaches a length of 6 feet and a weight of 110 lbs. It often leaps out of the water, after the manner of Grey Mullets, and its chase when hooked affords good sport, the landing of so active a giant being attended with great difficulties. Its remarkably large scales, over two inches in diameter, are much prized for fancy work in the Florida curiosity shops.

FAM. 6. ALBULIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter movably articulated above the former to the ethmoid. Parietal bones separating the supraoccipital from the frontals; suboperculum large; interoperculum small and entirely or nearly entirely hidden below the praeoperculum. Basis cranii double. Praemaxillaries, mandible, vomer, palatines, pterygoid, parasphenoid, glossohyal, and pharyngeals toothed. Ribs sessile, inserted behind and somewhat below small parapophyses, which are absent or merely indicated on the anterior vertebrae, and gradually increase in size towards the caudal region; these parapophyses, as well as the neural and haemal arches, are autogenous bones; epineurals, no epipleurals. {548}Pectorals low down, folding like the ventrals. Post-temporal trifid, the upper branch attached to the epiotic, the median to the squamosal, the lower to the opisthotic; post-clavicle large (formed of three pieces); scapular foramen between scapula and clavicle; pterygials well developed, two in contact with coracoid. Ventrals with 10 to 14 rays. Branchiostegal rays 6 to 14. Air-bladder large, not communicating with the ear.

Elongate fusiform Fishes, covered with large silvery scales forming regular longitudinal series; head naked; mouth rather small, with thick lips; gill-openings wide. Dorsal fin originating in front of ventrals; anal short; caudal well developed, forked.

The type of this family, Albula or Butirinus, is remarkable among Teleosts in possessing a rudiment of a conus arteriosus to the heart, provided with two rows of valvules. Its single species inhabits all the warm and tropical seas. Prof. Gilbert has ascertained that the young pass through a metamorphosis, analogous to that of the Eels. The "Leptocephalid" described as Esunculus by Kaup is probably a larval Albula.

The deep-sea Japanese Pterothrissus (Bathythrissa) must be placed in this family; its skeleton is very similar to that of Albula. It differs in the elongate dorsal fin, in the presence of small teeth on the maxillary bone, and in the small number of branchiostegal rays (6 instead of 12 to 14).

{549}Albula is represented in the Eocene (London Clay and Bruxellian); and the Cretaceous Istieus and Anogmius are believed to be possibly related to Pterothrissus. Four Cretaceous types (Plethodus, Thryptodus, Syntegmodus, and Ancylostylus) are referred with doubt to the Albulidae.

FAM. 7. MORMYRIDAE.—Margin of the upper jaw formed by the single praemaxillary and the maxillaries, the latter articulated above the former to the ethmoid. Parietal bones separating the supraoccipital from the frontals; a large hole on each side of the skull, between the squamosal, the epiotic, and the opisthotic, covered by a large, thin, bony plate (the supratemporal), which may extend over a part of the parietal; symplectic absent; suboperculum small and hidden under the operculum, or absent; interoperculum well developed. Basis cranii simple. No pharyngeal teeth. Opercular bones hidden under the skin; gill-clefts narrow. Anterior ribs sessile; epineurals, no epipleurals. Pectorals directed upwards. Ventrals with 6 or 7 rays. Air-bladder communicating with the ear.

Curious-looking Fishes, very variable in the form of the head and body and in the extent of the fins. Mouth often very small; teeth in jaws usually few; teeth usually present on the parasphenoid, working against a similar patch on the glossohyal; eye covered over by skin, sometimes very indistinct; scales small, cycloid; branchiostegal rays 4 to 8. The dorsal and anal fins may be nearly equally developed (Genyomyrus, Gnathonemus); or the former (Mormyrus) or the latter (Hyperopisus) are several times the longer. Gymnarchus, Eel-shaped, apodal, and deprived of the caudal fin, lacks the anal fin, the dorsal extending over the whole extent of the body. Some species of Mormyrops show how a form {550}like Gymnarchus may have been evolved out of a more typically-formed Fish. Nothing is more striking than the variation in shape of the snout within one and the same genus, and the names given to some of the species (ovis, caballus, elephas, tamandua, numenius, ibis) are suggestive of resemblances with the heads of various animals.

The Mormyrids are highly remarkable for the enormous development of the brain, the weight of which equals 1/52 to 1/82 of the total, a thing unparalleled among lower Vertebrates; and for the problematic organ which surmounts it; also as being among the few Fishes in which an electric organ has been discovered. The organ, situated on each side of the caudal region, is derived from the muscular system and is of feeble power, as ascertained by Babuchin and by Fritsch; it was long considered as "pseudo-electric." The natural affinities of this family appear to be with the Albulidae, and there is nothing to justify the term "Nilhechte" (Nile-pike) which has been bestowed on them by German {551}authors. Ninety-three species are known from the fresh waters of Africa north of the Tropic of Capricorn, and may be referred to two sub-families and ten genera:—

(i.) MORMYRINAE, with teeth on the parasphenoid and tongue, with ventral, anal, and caudal fins, and a simple air-bladder; vertebrae 37 to 64; peculiar (Gemmingerian) linear bones, without known homologues, along each side of the tail, above and beneath the electric organ; scapular foramen in the scapula, or between the scapula and the coracoid. Mormyrops, Petrocephalus, Isichthys, Marcusenius, Stomatorhinus, Myomyrus, Gnathonemus, Genyomyrus, Mormyrus.

(ii.) GYMNARCHINAE, without teeth on the parasphenoid and tongue, without ventral, anal, or caudal fins, and with a cellular air-bladder; vertebrae about 120; Gemmingerian bones absent; scapular foramen in the coracoid. Gymnarchus.

Fossil Mormyrids are unknown.

Venerated by the ancient Egyptians, the Mormyrs of the Nile are frequently represented on hieroglyphics and mural paintings as well as in bronze models. Very little is known of the habits of these Fishes. Prof. G. Fritsch, of Berlin, during his stay in Egypt for the purpose of experimenting on electric Fishes, observed that they perish very rapidly when removed from the river, and he had the greatest difficulty in keeping some alive in an aquarium for two or three days. The species with comparatively large mouths (Mormyrops, Gymnarchus) feed principally on fishes and crustaceans, the others on tiny animals and vegetable and more or less decomposed matter. Delhez, on the Congo, found that many are attracted to the borders of the river in the neighbourhood of human dwellings, where they feed on the refuse thrown into the water. It is probable that the species with a rostrum use it to procure small prey hidden between stones or buried in the mud, and that the fleshy mental appendage with which many are provided is a tactile organ compensating the imperfection of the vision in the search for food. A small Mormyrid from the Congo (Stomatorhinus microps) has the eyes so much reduced and the skin so feebly pigmented as to convey the impression of a cave Fish. Until quite recently, absolutely nothing was known {552}of the breeding habits and development in this important family. To the late J. S. Budgett we owe some very interesting observations made in the Gambia on Gymnarchus niloticus. The Fish makes a floating nest, emerging on three sides, over which the male keeps a fierce watch; the recently-hatched larvae are remarkable for the enormous size of the yolk-sac, which hangs down, acting as a sort of anchor, and for the presence of long external branchial filaments, as in Selachian embryos. The Fish propels itself through the water entirely by the action of its dorsal fin, forwards and backwards with equal facility; when swimming rapidly backwards, it may be seen to use the end of its tail as a feeler to guide the way. Budgett has also identified, with some doubt, the eggs of Hyperopisus bebe, out of which emerged embryos not unlike those of some tailless Batrachians, which hung suspended to rootlets of grass in swamps by means of threads of viscid mucus secreted from glands on the top of the head.

FAM. 8. HYODONTIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter the more developed and firmly united to the end of the former. Parietal bones separating the supraoccipital from the frontals; a large hole on each side of the skull, between the parietal, the squamosal, and the epiotic (paroccipital), closed by a large, thin, bony plate (the supratemporal), which extends over the greater part of the parietal; suboperculum and interoperculum small, the latter partly hidden below the praeoperculum. Basis cranii double. Jaws, palatines, pterygoids, vomer, parasphenoid, and glossohyal toothed; no pharyngeal teeth. Ribs sessile, inserted above and behind well-developed parapophyses; epineurals, no epipleurals. {553}Pectorals low down, folding like the ventrals. Post-temporal forked; the upper branch attached to the epiotic, the lower to the squamosal; no post-clavicle; coracoids forming together a ventral keel; scapular foramen between scapula and clavicle; pterygials well developed, three in contact with coracoid. Ventrals with 7 rays. Branchiostegal rays in moderate number (8 to 10). Air-bladder communicating with the ear. No oviducts, the eggs falling into the abdominal cavity before exclusion.

Elongate, compressed, silvery Fishes, covered with moderate-sized cycloid scales; head naked; mouth large, with strong dentition; gill-openings wide; dorsal fin short, posterior to the ventrals; anal rather elongate; caudal well developed, forked.

A single genus (Hyodon) with three species (Moon-Eyes of the Americans), all reaching the length of about a foot, inhabiting the fresh waters of North America east of the Rocky Mountains.

{554}FAM. 9. NOTOPTERIDAE. The Fishes which form this family may be regarded as an eccentric modification of a type very similar to the preceding, with which they agree in most osteological features as well as in the dentition, in the connexion between the air-bladder and the ear, and in the absence of oviducts. They differ in the absence of the lid-like supratemporal, the hole which it covers in Hyodon being here bordered by the post-frontal and the squamosal (fused with the opisthotic), sometimes also by the epiotic, in the absence of the suboperculum, in the absence or incomplete ossification of the upper branch of the post-temporal (the lower articulating with the opisthotic), and in the presence of accessory bones (named adpleurals) attached to or fused with the distal extremity of the anterior ribs, which they prolong to the mid-ventral line, where they are embraced by dermal ossifications forming a doubly serrated crest.

The bones of the head are cavernous, the mouth is large; the anterior nostril is produced into a tentacle. The body is very strongly compressed, with very short precaudal region, attenuate behind; the ventral fins are much reduced or absent; the dorsal is short or absent, {555}and the anal is much elongate and confluent with the caudal, which may be regarded as aborted. The scapular foramen is entirely in the scapula. The air-bladder is very large, with several divisions, forked in front and behind, and prolonged along each side of the caudal region; its extraordinary condition has been described by Bridge.

These Fishes live in marshes and lakes, fresh-water or brackish, and feed on worms and insects. Nothing is known of their breeding habits and development.

Two genera: Notopterus, with a dorsal fin and 6 to 9 branchiostegal rays—three species from India, Burma, and the Malay region, and one from West Africa; Xenomystus, without dorsal fin and with only 3 branchiostegal rays, the unique species inhabiting the White Nile and West Africa. Remains of Notopterus have been found in the marl slates (Tertiary) of Padang, Sumatra. The largest species, the Indian N. chitala, attains 4 feet in length; its flesh is said to be uncommonly rich and well flavoured, but a strong prejudice exists against it, owing to the Fish being supposed to live on human carcases.

FAM. 10. OSTEOGLOSSIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter the more developed and firmly attached to the end of the former. Parietal bones separating the supraoccipital from the frontals; suboperculum much reduced, and entirely or partially concealed under the praeoperculum. Basis cranii simple. Teeth in jaws, and on pterygoid and hyoid bones; no pharyngeal teeth. Head scaleless, the thin skin confluent with the bones; body covered with large bony scales, composed of pieces like mosaic. Ribs inserted on the strong parapophyses; epineurals, no epipleurals. Pectoral fins low down. Post-temporal forked, the upper branch attached to the epiotic, the lower to the squamosal; post-clavicle present; scapular foramen in scapula; pterygials well developed, only one in contact with coracoid. Dorsal and anal fins originating behind the ventrals; latter with 5 or 6 rays. No oviducts, the eggs falling into the abdominal cavity before exclusion (at least in Heterotis, as observed by Budgett).

This family is represented at the present day by five species, referred to four genera; thus characterised:—

{556}[Illustration: FIG. 335.—Principal forms of Osteoglossids. A, Dapedoglossus testis (restoration); B, Scleropages leichardti; C, Osteoglossum bicirrhosum; D, Arapaima gigas; E, Heterotis niloticus. All much reduced.]

{557}Scleropages.—Mouth large; vomer, palatines, pterygoids, and glossohyal toothed; mandibular barbels; branchiostegal rays 15 to 17; body compressed, with trenchant abdomen; coracoids forming a ventral keel; dorsal fin short; ventral fins nearly equally distant from end of snout and caudal fin; vertebrae 29 to 31 + 30; air-bladder not cellular. One species from the northern parts of Australia, and one from Sumatra, Banka, and Borneo.

Osteoglossum.—Mouth large; vomer, palatines, pterygoids, and glossohyal toothed; mandibular barbels; branchiostegal rays 10; body compressed, with trenchant abdomen; coracoids forming a ventral keel; dorsal fin long; ventral fins nearly twice as far from the caudal as from the end of the snout; vertebrae 28 + 59; air-bladder not cellular.—South America (Guianas, Brazil).

Arapaima.—Mouth rather large; vomer, palatines, pterygoids, and glossohyal toothed; branchiostegal rays 16; belly rounded; dorsal fin rather long; ventral fins equidistant from head and caudal fin; vertebrae 36 to 38 + 41 to 42; air-bladder cellular.—South America (Guianas, Brazil).

Heterotis.—Mouth moderate; branchiostegal rays 7; belly rounded; dorsal fin rather long; ventral fins nearer end of snout than caudal fin; vertebrae 27 + 42 to 43; air-bladder cellular; fourth branchial arch with an accessory breathing-organ. Africa (Nile, Senegal, Gambia, Niger).

Dapedoglossus, from the Eocene of Wyoming, appears to be nearest to Scleropages, and Brychaetus, from the Eocene (London Clay) of Sheppey, Kent, to Arapaima, so far as the state of preservation of these fossils enables us to form an opinion.

Dr. Günther has directed attention to the remarkable {558}coincidence of the geographical distribution of this family and the Dipneusti, although, however, the latter are not known to be represented in the Malay Archipelago. "Not only," he adds, "are the corresponding species found within the same region, but also in the same river systems; and although such a connexion may and must be partly due to a similarity of habit, yet the identity of this singular distribution is so striking that it can only be accounted for by assuming that the Osteoglossidae are one of the earliest Teleosteous types which have been contemporaries of and have accompanied the present Dipnoi since or even before the beginning of the Tertiary epoch."

The Queensland species of Scleropages (S. leichardti) is known to the settlers by the name of Barramunda, which has also been applied to Neoceratodus. Arapaima gigas is one of the largest fresh-water Fishes known, exceeding a length of 15 feet and a weight of 400 pounds. Its flesh is much valued. Sir R. Schomburgh has observed that the mother protects the young, who, for some time after their birth, always swim in front of her. A similar observation has been made in the Gambia on Heterotis niloticus by the late J. S. Budgett, who states that the Fish builds enormous nests in swamps, in about two feet of water; the walls of the nest are made of the stems of the grasses removed by the Fish from the centre; the floor is the swamp-bottom, and is made perfectly smooth and bare. The nest appears to be used for at most four or five days; the newly-hatched larvae are provided with long external gill-filaments of a blood-red colour.

FAM. 11. PANTODONTIDAE.—The little West African Fish described by Peters as Pantodon buchholzi is the unique representative of a family closely related to the Osteoglossidae, but distinguished by the very small, single praemaxillary and the absence of suboperculum and interoperculum. The pectoral fins are very large and are remarkable for the fleshy process to which the inner ray is adnate; the ventrals, formed of 7 rays, some of which are simple and prolonged into filaments, are placed more forward than in any other type of this sub-order, the Ctenothrissidae excepted, viz. immediately behind the pectorals. Teeth in the jaws and on the vomer, palatines, pterygoids, parasphenoid, {559}glossohyal, and pharyngeal bones. Mesocoracoid arch slender, strongly curved, and meeting its fellow on the median line; coracoids forming a ventral keel. Vertebrae few (16 + 14).

Observed by M. de Brazza to be a freshwater Flying-Fish.

FAM. 12. CTENOTHRISSIDAE.—A curious type characterised by small praemaxillaries, large maxillaries, with feeble dentition, {560}the parietals in contact on the median line, vertebral centra without transverse processes, a moderately large dorsal with simple anterior rays, and large ventrals advanced far forwards and formed of 8 rays. Its affinities are still obscure, but the condition of the jaws decides its allocation to the suborder Malacopterygii, whilst in the position of the ventrals it is most nearly approached by the Pantodontidae. Small Fishes known only by two genera, of the Cretaceous period (England and Mount Lebanon), one with ctenoid scales (Ctenothrissa), the other with cycloid scales (Aulolepis).

FAM. 13. PHRACTOLAEMIDAE.—Mouth edentulous, projectile, bordered by the very slender praemaxillaries and maxillaries. Supraoccipital in contact with the frontals, widely separating the small parietals; operculum and suboperculum well developed; praeoperculum small; interoperculum enormous, covering the gular region and overlapping its fellow; symplectic absent. Basis cranii single. No pharyngeal teeth. Only 3 slender branchiostegal rays. Ribs stout, sessile, nearly completely encircling the body; slender epineurals; no epipleurals; caudal region very short. Supratemporal small, simple, fixed to the parietal and squamosal; no postclavicle; scapular foramen in the scapula. Pectoral fin inserted low down, folding like the ventrals; latter with 6 rays.

The remarkable little Fish, Phractolaemus ansorgii, discovered by Dr. W. J. Ansorge in the Niger Delta in 1900, and which has since also been found in the Congo, stands quite apart among the Malacopterygians, its nearest allies being apparently the Osteoglossidae. The body is elongate and subcylindrical, covered with large striated scales; the head is small, the skull strongly ossified, covered with thin skin; the mouth small, proboscidiform, {561}capable of being thrust forwards, when at rest folded over and received into a depression on the upper surface of the head; the narial orifice is single, and preceded by a barbel; the gill-openings are narrow, restricted to the sides. The ventral fins are inserted far back, the dorsal and anal are short. The air-bladder is very large, and the intestine extremely long and much convoluted. Vertebrae 26 + 8.

FAM. 14. SAURODONTIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter the more developed and firmly united to the former; these bones, as well as the mandible, with teeth implanted in deep sockets; palate toothless. Supraoccipital separating the small parietals; opercular bones well developed; symplectic present, exposed. Basis cranii double. Ribs sessile, very low down on the centra; no parapophyses; neural arches not fused with the centra. Pectorals inserted very low down; postclavicle apparently absent. Caudal fin deeply forked, without fused hypurals.

This family, comprising several Cretaceous genera, may be regarded as ancestral to the Chirocentridae, with or near which it has been placed by Cope and various later authors. The normal position of the symplectic, however, entitles its members to rank as a separate family, and the autogenous neural arch, as well as the distinctness of the bones supporting the caudal fin, are also indicative of a greater generalisation. The restoration of Ichthyodectes as given by Loomis, shows a general form similar to an ordinary Herring, but it does not appear to be reliable.

The members of the Saurodontidae have been referred to two groups: (a) with praedentary (praesymphysial) bone, Saurocephalus, Saurodon; (b) without praedentary, Chirocentrites, Portheus, Ichthyodectes, Spathodactylus, Cladocyclus. These Fishes are from the Chalk of Europe and North America, and some among them attain a very large size, perhaps not less than two metres in length.

FAM. 15. CHIROCENTRIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter the more developed and firmly united to the former; these bones, as well as the mandible, with large teeth not implanted in true sockets; minute teeth on the palatines, pterygoids, and hyoid bones, Supraoccipital in contact with the frontals, separating the small parietals; opercular bones well developed; symplectic hidden {562}between the inner surface of the quadrate and a descending process of the hyomandibular. Basis cranii double. Ribs very slender, sessile, very low down on the centra; no parapophyses; epipleurals and epineurals. Pectorals inserted very low down. Post-temporal forked; postclavicle absent; a thin bony lamina, similar to the postclavicle, above the pectoral fin, attached to the scapula; scapular foramen in scapula; coracoids in contact with each other, forming a keel. Ventrals very small, with 7 rays. Brachiostegal rays 8. Air-bladder large, not communicating with the ear, incompletely divided into cells. Mucous membrane of the intestine forming a spiral fold.

The body is very elongate and strongly compressed, covered with thin, deciduous scales; the vertebrae number 75. The dorsal fin is short and opposite to the anal, which is long.

Chirocentrus dorab, the only representative of this family, inhabits the Indian Ocean and the seas of China and Japan.

FAM. 16. CLUPEIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries. Supraoccipital separating the small parietals; opercular bones well developed. Basis cranii double. Ribs mostly sessile, inserted behind parapophyses; intermuscular bones (epineurals, epipleurals, adpleurals) usually numerous. Post-temporal forked, the upper branch attached to the epiotic, the lower to the opisthotic; post-clavicle applied to outer side of clavicle. Ventrals with 6 to 11 rays. Air-bladder large, communicating with the ear.

Four sub-families:—

(i.) THRISSOPATRINAE.—Mouth large; praemaxillaries very small; maxillaries large, with rather narrow supplemental bone, firmly attached to praemaxillaries; branchiostegals about 30; abdomen compressed to an edge, without serration; no lateral line. Thrissopater, from the Gault of Folkestone.

{563}(ii.) ENGRAULINAE.—Mouth moderate or large; praemaxillaries very small; maxillaries large, with narrow supplemental bones, more or less firmly attached to praemaxillaries; branchiostegals 6 to 19; abdomen rounded or more or less compressed, with or without serration; no lateral line. Recent genera: Dussumieria, Etrumeus, Engraulis, Cetengraulis, Heterothrissa, Coilia. Fossil: Spaniodon, Upper Cretaceous.

(iii.) CLUPEINAE.—Mouth small or moderate; maxillaries freely movable behind the praemaxillaries, usually with large supplemental bones; branchiostegals 5 to 10; abdomen usually serrated; lateral line usually absent. Recent genera: Clupea, Hyperlophus (Diplomystus), Opisthonema, Brevoortia, Pellonula, Clupeichthys, Odaxothrissa, Pellona, Chirocentrodon, Pristigaster, Raconda, Chatoessus. Fossil: Pseudoberyx, Histiothrissa, Scombroclupea, Leptichthys, Upper Cretaceous.

(iv.) CHANINAE.—Mouth small, toothless; maxillaries firmly attached to praemaxillaries; branchiostegals 4, very broad; abdomen rounded or flattened; lateral line distinct. Chanos, recent; Chanoides, Upper Eocene; Prochanos, Cretaceous.

{564}Heralded by the genus Thrissopater, which may be regarded as a connecting type between the Elopidae and the Clupeidae, this family is largely represented in Cretaceous times, more abundantly still in the Eocene and Miocene, where Clupea and Engraulis occur in numerous species; Hyperlophus, distinguished from Clupea by the presence of a dorsal serrated ridge similar to the ventral, occurs in the Upper Cretaceous of Syria, Southern Europe, and South America, in the Eocene of North America and Europe, and is represented at the present day on the West Coast of South America and on the coast and in the rivers of New South Wales. About 200 Clupeids are known to live at the present day, mostly marine species, but a few are confined to fresh-waters; none may be termed deep-sea forms; some, like the Allis Shad (Clupea alosa) and Twait Shad (C. finta), are anadromous, ascending rivers to spawn. The range of the family is almost cosmopolitan. Several species are remarkable for the extreme abundance of individuals, as for example the Herring (Clupea harengus), the Pilchard or Sardine (C. pilchardus), and the Anchovy (Engraulis encrasicholus). The Herring inhabits the northern parts of the Atlantic and the seas north of Asia. As Dr. Günther first showed, the so-called "Whitebait" consists chiefly of the fry of Herrings, which, like those of the Sprat (C. sprattus), have a predilection for brackish water. The Anchovy and the Pilchard, on the other hand, seldom if ever enter estuaries. The eggs of the Herring, contrary to those of most British marine food-fishes, are heavy and adhesive, sticking firmly to stones or fixed objects on the sea bottom, whilst those of the Sprat and Pilchard float on the surface. The larvae are long, slender, and transparent. The Sardine, which affords so valuable a fishery on the West Coast of France, is the immature state of the Pilchard, which grows to a length of 10 to 14 inches. Its movements are not yet well understood, and its scarcity during certain years in the waters where it usually swarms has caused periodical crises in an important industry. Ripe Pilchards are mostly found at a considerable distance from the coasts. The Anchovy is especially abundant in the {565}Mediterranean, but it is also regularly fished in Holland, especially in the Zuydersee, where it breeds, as well as in the Mediterranean; it makes only temporary appearances, and has not been observed to spawn, in the English Channel, although eggs have recently been obtained off the coast of North Lancashire.

The imperfectly known Cretaceous Crossognathidae (Crossognathus and Scyllaemus), referred by some authors to the Percesoces, should probably be placed with or near the Clupeidae.

FAM. 17. SALMONIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries. Supraoccipital in contact with the frontals, but frequently overlapped by the parietals, which may meet in a sagittal suture; opercular bones all well developed. Basis cranii double. Ribs sessile, parapophyses very short or absent; epineurals, sometimes also epipleurals, present. Post-temporal forked, the upper branch attached to the epiotic, the lower to the opisthotic; postclavicle, as usual, applied to inner side of clavicle. A small adipose dorsal fin. Air-bladder usually present, large. Oviducts rudimentary or absent, the ova falling into the cavity of the abdomen before exclusion.

Marine and fresh-water Fishes, mostly from the temperate and Arctic zones of the northern hemisphere: one genus (Retropinna) on the coasts and in the rivers of New Zealand; a few deep-sea forms (Argentina, Microstoma, Nansenia, Bathylagus) occur in the Arctic Ocean, the North Atlantic Ocean, the Mediterranean, and the Antarctic Ocean, down to 2000 fathoms. Apparently of comparatively recent age, no remains older than Miocene (Osmerus, Thaumaturus, Prothymallus) being certainly referable to this family. The recent genera may be grouped as follows:—

A. Air-bladder present.

a. Branchiostegal rays 8 to 20; ventral rays 9 to 13; stomach siphonal; pyloric appendages more or less numerous (17 to 200). Breed in fresh water. Salmo, Brachymystax, Stenodus, Coregonus, Phylogephyra, Thymallus.

b. Branchiostegal rays 6; ventral rays 11 to 14; stomach caecal; pyloric appendages in moderate numbers (12 to 20). Argentina.

{566}c. Branchiostegal rays 6 to 10; ventral rays 6 to 8; stomach caecal; pyloric appendages few (2 to 11) or rather numerous. Osmerus, Thaleichthys, Mallotus, Plecoglossus, Hypomesus.

d. Branchiostegal rays 3 or 4; ventral rays 8 to 10; stomach caecal; pyloric appendages absent. Microstoma, Nansenia, Bathylagus.

B. Air-bladder absent; branchiostegal rays 3 to 6; ventral rays 6 or 7; stomach siphonal; pyloric appendages absent. Retropinna, Salanx.

Only about 80 species can, at present, be regarded as valid.

The beauty, gameness, and great economical value of the Salmonids have caused more attention to be bestowed on them than probably upon any other group of fishes. As Professor Smitt tells us, a Swedish proverb says "A dear child has many names," and this applies well to our Salmon and Trout, the species of which have been unduly multiplied by some writers. The genus Salmo, characterised by a large mouth and powerful dentition, is divided into three sections: Oncorhynchus, Quinnat Salmon, of the North Pacific, ascending rivers in North America and Asia, with 12 to 17 developed rays in the anal; Salmo, Salmon and Trout, with 8 to 12 rays in the anal, and teeth not only on the head of the vomer but also along its shaft, at least in the young, {567}represented in the seas and freshwaters of Europe, Asia, and North America, extending southwards to North-West Africa, Asia Minor, Northern Persia, the Hindu Kush, the head of the Gulf of California, and the Rio Grande; Salvelinus, Charr, with 8 to 10 rays in the anal, and teeth on the raised head of the vomer only, of the lakes of Northern and Central Europe and the rivers of the northern parts of Asia and North America as far north as 82° 34´, sometimes descending to the sea.

The changes in form and colour which these fishes undergo when passing from fresh water into the sea or when artificially transported from one place to another are very great, and this plasticity, together with the connecting links which render the naming of not a few specimens impossible, have caused most recent students of the genus Salmo, in Europe at least, to reduce many of the so-called species to the rank of local varieties, and even our common Brown Trout or Brook Trout (S. fario) is now generally regarded as not specifically separable from the anadromous Sea Trout (S. trutta). The anadromous true Salmon (S. salar) may be distinguished by its somewhat larger scales, there being only 11 or 12 in a transverse series running from the posterior border of the adipose fin forwards to the lateral line, Trout having 13 to 16. The Charr of the lakes of Wales, the North of England, Scotland, and Ireland are also regarded as mere varieties of the common Northern migratory Charr (S. alpinus), of which the "Omble Chevalier" of the Swiss lakes and the "Saeblings" of the Alpine lakes of Germany and Austria are likewise varieties. An allied species (S. fontinalis) has been introduced into England from North America, as well as a true Trout (S. irideus). The large size of the eggs, their lack of {568}adhesiveness, and the fact that the ova fall into the abdominal cavity, out of which they may easily be squeezed, renders artificial impregnation particularly easy, and the species of Salmo have always occupied the first place in the annals of fish-culture. Fertilised eggs are transported in ice, the development being simply suspended for several weeks, and several forms of British and American Salmonidae have thus been introduced into New Zealand and Tasmania, where some have thoroughly established themselves.

The White-Fish, Coregonus, are more numerous in species than Salmo, and as a rule more readily defined. They are easily recognised by their large silvery scales and their smaller mouth without or with minute teeth. Some, like the Houting (C. oxyrhynchus) of Northern Europe, occur in the sea, entering rivers to spawn, whilst others, like the Sik, Weiss, Felchen, or Lavaret (C. lavaretus), are confined to lakes. British species are the Gwyniad (C. clupeoides), of Loch Lomond, Haweswater, Ullswater, and Bala, the Vendace (C. vandesius), of Loch Maben, and the Pollan (C. pollan) of Lough Neagh in Ireland.

The Grayling (Thymallus vulgaris or vexillifer), with its high dorsal fin formed of about 20 rays, one of the handsomest British fishes, inhabits the rivers and lakes of Northern and Central Europe, and is represented by a few allied species in Asia and North America. It derives its name from having the odour of thyme.

The Smelt (Osmerus eperlanus) breeds in salt water, and although it often enters rivers, it does not ascend beyond tidal influence. The Capelin (Mallotus villosus), of the coasts of Arctic America and North-eastern Asia, deposits its eggs in the sand along the shores in incredible numbers, the beach becoming a {569}quivering mass of eggs and sand. Plecoglossus, from Japan and Formosa, is highly remarkable for its lamellar, comb-like, lateral teeth. The Siel-Smelts (Argentina) are deep-sea Salmonids of which examples have occasionally been taken off the coasts of Scotland and Ireland; large numbers have been brought from Norway to English markets. Bathylagus is still better adapted for life at great depths (down to 1700 fathoms), the eyes being of enormous size. As Dr. Günther has observed, "these fishes must be entirely dependent for vision on the phosphorescent light which is produced by other abyssal creatures. Not being fish of prey themselves, or only to a slight degree, they would be attracted by the light issuing from the Pediculates and Stomiatids of the deep, and thus form an easy prey to these fishes."

Secondary sexual characters are very strongly developed in many Salmonids. In adult males of Salmon, Trout, and Quinnat the snout becomes greatly distorted, both jaws being hooked and the base of the teeth more or less enlarged; in the latter species a fleshy hump is developed before the dorsal fin, and the scales of the back become embedded in the flesh. Pearl-like excrescences appear on the scales of many of the White-Fish during the breeding season, being more prominent in males than in females, and Mallotus villosus is so called from the villous bands formed by the scales of mature males, the scales above the lateral line and along each side of the belly becoming elongate-lanceolate, densely imbricated and produced into free, projecting points.

The Pachyrhizodontidae, with the Cretaceous genus Pachyrhizodus, are placed by some authors with the Salmonidae, but the remains at present known are too fragmentary to afford a correct idea of their exact systematic position. There seems to be less justification for placing them among the Elopidae.

FAM. 18. ALEPOCEPHALIDAE.—Deep-sea Fishes similar in general structure to the Clupeidae and Salmonidae, but destitute of a postclavicle and of an adipose dorsal fin, the rayed fin being situated far back on the body, in the caudal region, and opposed {570}or slightly anterior to the anal. The skeleton of Alepocephalus is remarkable for its feeble ossification. Epipleurals and epineurals are present, and the bilateral division of the neural arch remains perfectly distinct throughout the praecaudal region, both halves being very loosely apposed. The air-bladder is absent. Ventrals are absent in Platytroctes, and the snout is much produced in Aulostomatomorpha.

Eleven genera are distinguished:—A, with scales: Alepocephalus, Conocara, Bathytroctes, Leptochilichthys, Narcetes, Platytroctes, Aulostomatomorpha. B, without scales:—Xenodermichthys, Aleposomus, Leptoderma, Anomalopterus.

Represented by about 35 species in nearly all the seas; as usual with deep-sea forms, individuals of the same species have been obtained from stations very remote from one another.

FAM. 19. STOMIATIDAE.—I would unite under this name the Stomiatidae and Sternoptychidae of Günther, an assemblage of aberrant deep-sea Fishes which agree in having the maxillary bone more developed than the praemaxillary, and beset with teeth, a character which differentiates them at once from all other deep-sea forms of this sub-order, as well as from the Scopelidae among the Haplomi. The ventral fins are usually inserted very far back, and the number of their rays varies from 5 to 8. Contrary to what occurs in other groups of fishes, the pectoral fins have a tendency to reduction, and actually disappear in some genera, whilst the ventrals remain well developed; whenever the pectoral fins are fully developed, as in Maurolicus, Chauliodus, Astronesthes, and Photichthys, the mesocoracoid arch is present. The form of the body varies exceedingly, even within the smaller groups into which this family has been divided; it may be excessively short and compressed, or excessively elongate, {571}but the mouth and eyes are always large, these fish being essentially predatory; the dentition is often very powerful, and may extend to the palate or be confined to the jaws. The body is naked or scaly; luminous spots (photophores) are more or less developed. The development and position of the vertical fin is highly variable within this group, and the several families which have been founded upon this character have no more taxonomic importance than in the better-understood groups Characinidae and Siluridae. All authors, besides, have been compelled to admit that the presence or absence of an adipose dorsal fin has no high significance in this case, a view which is further strengthened by Dr. Gilchrist's discovery, off the Cape of Good Hope, of a deep-sea Fish agreeing in every respect with Astronesthes, but for the presence of a small adipose fin, absolutely similar to the dorsal, but situated on the ventral side, immediately in front of the anus. Two species with similar ventral adipose fins have just been discovered by Dr. Brauer and referred to Astronesthes. I am therefore unable to adopt the elaborate arrangement in favour with the modern American school.

The genera may be arranged in five sub-families:—

I. Anal not exactly opposed to the rayed dorsal, or much longer than the latter; no hyoid barbel.

A. Rayed dorsal far forward, between pectorals and ventrals; pectorals well developed (CHAULIODONTINAE). Chauliodus.

B. Rayed dorsal above or behind the ventrals; pectorals well developed.

1. Body more or less elongate; ventrals well developed (GONOSTOMATINAE).

a. A hyoid barbel. Astronesthes.

b. No barbel. Bathylychnus, Gonostoma, Cyclothone, Triplophos, Photichthys, Bathylaco, Diplophos, Maurolicus, Ichthyococcus.

2. Body short and deep; ventrals rudimentary or absent (STENOPTYCHINAE). Argyropelecus, Sternoptyx, Polyipnus.

II. Dorsal and anal opposed to each other and very far back on the caudal region; pectorals often reduced or absent; hyoid barbel often present. {572}(STOMIATINAE). Stomias, Macrostomias, Echiostoma, Opostomias, Pachystomias, Photonectes, Malacosteus, Thaumatostomias, Photostomias.

This family, comprising about 55 species, has a world-wide distribution, but most of the known forms have been obtained from the Atlantic; some of the species occur both in the Atlantic and the Indo-Pacific. Chauliodus, Astronesthes, and Stomias are among the fishes with the most formidable dentition.

FAM. 20. GONORHYNCHIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter articulated above the former to the ethmoid. Supraoccipital in contact with the frontals, widely separating the small parietals; opercular bones well developed; symplectic present. Basis cranii simple. Mouth small and toothless, inferior, surrounded by thick, fringed lips. Four branchiostegal rays. Head and body entirely covered with small spiny scales. Praecaudal vertebrae with strong parapophyses, to the extremity of which slender ribs and epipleurals are attached. No postclavicle. Pectoral fins inserted low down, folding like the ventrals; latter with 10 rays.

The single existing species, Gonorhynchus greyi, is characterised by an elongate, cylindrical body, a pointed projecting snout bearing a single barbel, short dorsal and anal fins, the former opposed to the ventrals, and the gill-membranes broadly attached to the isthmus. Teeth are present on the pterygoid and hyoid bones. No suborbital arch. Vertebrae, 45 + 20. Air-bladder absent. Its distribution is a very wide one, the species being on record from the coasts of the Cape of Good Hope, Australia, New Zealand, and Japan.

The genus Notogoneus, from the freshwater Eocene beds of France and North America, has been referred to this family by Cope, and has been shown by A. S. Woodward to be closely related to Gonorhynchus, differing only in the absence of teeth on the palate and tongue, and in the more forward position of the dorsal fin. The genus Charitosomus, with several species {573}from the Upper Cretaceous of Westphalia and Mount Lebanon, has also been included in this family, but the precise shape and character of the scales have not yet been ascertained.

FAM. 21. CROMERIIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries. Supraoccipital large and widely separating the very small parietals; opercular bones well developed; symplectic absent. Basis cranii simple. Mouth small and toothless, inferior; gill-opening narrow. Three branchiostegal rays. Body naked. Praecaudal vertebrae with parapophyses; ribs and epipleurals slender. No postclavicle. Pectoral fin inserted low down, folding like the ventrals.

A single genus, Cromeria, recently discovered in the White Nile. In its elongate, naked body and the posterior position of the dorsal fin, it resembles the Galaxiidae, to which it was at first referred. But this allocation has proved to be incorrect, now that the osteological structure of the minute Fish (only about 30 mm. long) has been worked out by Swinnerton. The vertebrae number 42 to 45 (28-30 + 14-15). A long, slender air-bladder is present.

SUB-ORDER 2. OSTARIOPHYSI.

Air-bladder, if well developed, communicating with the digestive tract by a duct. Pectoral arch suspended from the skull; mesocoracoid arch present. Fins without spines, or dorsal and pectoral with a single spine formed by the co-ossification of the segments of an articulated ray. The anterior four vertebrae strongly modified, often co-ossified and bearing a chain of small bones (so-called Weberian ossicles) connecting the air-bladder with the ear.

This is one of the most natural groups of the Class Pisces, although its members are so diversified in outward appearance as to have been widely separated in the systems of older authors. It is to Sagemehl that is due the credit of having first grouped, under the above name, the Characines, the Carps, the Cat-Fishes, and the Gymnotids, the relations of which had been realised, to a certain extent, by Cope. But it was not until the homology throughout the group of the ossicula auditus, first described by E. H. Weber in 1820, had been demonstrated by Sagemehl that the justification for the course here followed appeared in its full {574}strength, as such an agreement in the structure of so complicated and specialised an apparatus can only be the result of a community of descent of the families which are possessed of it. It is invariably the anterior four vertebrae that take part in the support of the Weberian apparatus. The first vertebra is much reduced; its upper arch is absent and replaced by the ossicles termed claustrum and scaphium (the former being perhaps nothing but the modified neural arch), which fill in the space between the exoccipital and the neural arch of the second vertebra; the principal piece of the apparatus, the tripus, variable in form, is related to the third vertebra, of which it is regarded as a modified rib; a fibrous ligament extends from the anterior extremity of the tripus to the scaphium, and in this ligament is inserted the fourth piece, the intercalarium. The various forms of this sub-order also show a complete agreement in the spinal nerves which pass through these ossicles. The parietal bones either separate the frontals from the supraoccipital or are fused with the latter.

This sub-order is divided into six families. The Characinids are the most generalised, and the others are probably derived from them in the manner expressed by the following diagram:—

Loricariidae Aspredinidae | | +––––––––––––+––––––––––+ | Cyprinidae Siluridae Gymnotidae | | | +–––––––––––+–––––––––––+ | | | +–––––––––––––––––––+–––––––––––––––––+ | Characinidae.

SYNOPSIS OF THE FAMILIES

I. Parietal bones distinct from the supraoccipital; symplectic present; ribs mostly sessile, all or the greater number of the praecaudal vertebrae without parapophyses.

Mouth not protractile, usually toothed; pharyngeal bones normal; body scaly; an adipose dorsal fin often present 1. Characinidae.

Mouth not protractile, usually toothed; pharyngeal bones normal; body Eel-shaped, naked or scaly; vent under the head or on the throat 2. Gymnotidae.

Mouth usually more or less protractile, toothless; lower {575} pharyngeal bones large, falciform; body naked or scaly; no adipose dorsal fin 3. Cyprinidae.

II. Parietal bones usually fused with the supraoccipital; symplectic absent; body naked or with bony scutes; mouth usually toothed, with barbels; adipose fin often present.

Ribs attached to strong parapophyses; operculum well developed 4. Siluridae.

Ribs sessile; parapophyses absent; operculum more or less developed; mouth inferior 5. Loricariidae.

Ribs sessile; strong parapophyses to the vertebrae; operculum absent 6. Aspredinidae.

FAM. 1. CHARACINIDAE.—Mouth non-protractile, usually bordered by the praemaxillaries and the maxillaries, rarely by the praemaxillaries only; jaws usually toothed. Parietal bones united in a sagittal suture or separated by a fontanelle; opercular bones well developed; symplectic present. Pharyngeal bones normal, with small teeth. Ribs mostly sessile; no parapophyses in the thoracic region; epipleurals and epineurals, mostly free floating. Pectoral fins inserted very low down, folding like the ventrals. Body covered with scales. An adipose dorsal fin often present.

This is a very generalised type, although perhaps not directly derived from the bony Ganoids, as believed by Sagemehl. The species number about 500, and are confined to the freshwaters of Africa and Central and South America. The classification of the family is still in an unsatisfactory state, but the division into the following groups (hardly deserving the rank of sub-families), although quite provisional, appears preferable to the highly artificial arrangement hitherto adopted:—

I. No adipose fin.

A. ERYTHRININAE.—Carnivorous; teeth strong; maxillary large; gill-openings wide; scales cycloid. American: Macrodon, Erythrinus, Lebiasina, Pyrrhulina, Corynopoma.

II. Adipose fin usually present.

B. HYDROCYONINAE.—Entirely or partially carnivorous; teeth strong; maxillary well developed; scales cycloid; lateral line usually nearer ventral than dorsal outline (sometimes only on the tail). African: Sarcodaces, Hydrocyon, Bryconaethiops, Alestes, Micralestes, Petersius. American: Acestrorhynchus, Boulengerella, Acestrorhamphus, Crenuchus, Chalceus, Brycon, Bryconops, Bryconodon, Creagrutus, Chalcinus, Brachychalcinus, Pseudocorynopoma, Stichonodon, Gastropelecus, Tetragonopterus, Scissor, Chirodon, Piabucina, Iguanodectes, Aphiocharax, Salminus, Oligosarcus, Agoniates, Paragoniates, Leptagoniates, Anacyrtus.

{576}C. SERRASALMONINAE.—Carnivorous; teeth strong; belly serrated; scales cycloid. American: Serrasalmo, Myletes, Myleus, Metynnis, Catoprion.

D. ICHTHYOBORINAE.—Carnivorous; teeth strong; maxillary very small; upper jaw movable; scales ciliated. African: Eugnathichthys, Paraphago, Mesoborus, Phago, Ichthyoborus, Neoborus.

E. XIPHOSTOMINAE.—Carnivorous; teeth very small; maxillary rather small; scales ciliated. American: Xiphostoma.

F. ANOSTOMINAE.—Herbivorous, entirely or partially; teeth well developed in both jaws; maxillary very small; gill-openings narrow; scales cycloid. American: Anostomus, Leporinus, Characidium, Chorimycterus, Nanostomus, Nanognathus.

G. HEMIODONTINAE.—Partially herbivorous; dentition imperfect; maxillary well developed; scales cycloid. American: Hemiodus, Caenotropis, Saccodon, Parodon.

H. DISTICHODONTINAE.—Entirely or partially herbivorous; teeth small but well developed; maxillary well developed; scales ciliated. African: Nannaethiops, Neolebias, Distichodus, Nannocharax, Xenocharax.

I. CITHARININAE.—Herbivorous; teeth minute or absent; maxillary small; scales cycloid or ciliated. African: Citharinus, Citharidium. American: Prochilodus, Curimatus.

The genera in the above sub-families are mostly founded on the dentition and the extent of the praemaxillary and maxillary bones, which are astonishingly varied, as may be seen from the annexed figures showing the open mouths of a few of the most remarkable types. As I have already pointed out, the character often given as diagnostic of this family, viz. the maxillary forming part of the oral border, is not absolutely constant; this bone is often much reduced, and it is entirely excluded from the mouth in Ichthyoborus and Neoborus. The branchiostegal rays {577}number 3 to 5 only. The fins never bear pungent spines, and the ventrals have 6 to 13 rays. Barbels are absent. In most of the herbivorous forms the brain-case is produced forward to the nasal capsule, whilst in most of the carnivorous forms they are separated by an interorbital septum; but there are exceptions to this correlation, and as otherwise closely related genera may differ in this respect, I have not been able to make use of the character in defining sub-families.

{578}The air-bladder is divided into an anterior and a posterior part by a constriction; the posterior part is the longer, and its anterior portion is cellular in Erythrinus and Lebiasina. Pyloric appendages to the stomach, which are constantly absent in the Cyprinids, are more or less numerous. An accessory respiratory organ in a diverticulum above the fourth branchial arch has been observed in Xenocharax and Citharinus.

The appearance and habits of the genera which compose this family vary greatly. Some resemble the Cyprinids and are mainly vegetarians, whilst others recall Salmonids and Pike. Among the most formidable are Hydrocyon, the Dogs of the Water, or Kelb-el-Bahr of the Arabs, with their powerful jaws with shark-like teeth, visible when the mouth is closed, and which grow to the size of the Salmon. The five known species inhabit the Nile and the rivers and lakes of tropical Africa. No less ferocious are the "Piranha" or "Cariba" (Serrasalmo) of South America, whose bite has been compared to the cut of a razor. They abound in some rivers and are much dreaded by people having to enter the water, as they fiercely bite off big pieces of flesh as with a pair of scissors, and the smell of blood is said to attract them by thousands; they show a great tenacity of life and can remain for hours out of the water. Serrasalmo niger has been observed by Schomburgk to produce a grunting noise in the water. Salminus orbignianus, of the Plate River, "Dorado" of the Spaniards, which reaches a length of 3 feet, has the predacious habits of the Pike, and follows other fishes moving in shoals; its flesh is much valued, although very full of bones, like that of all Characinids.

{579}As an example of phytophagous types may be mentioned the Moon-Fish of the Nile (Citharinus geoffroyi), with its feeble dentition, deep compressed body, and falciform dorsal fin; it is often represented on the monuments of the ancient Egyptians.

FAM. 2. GYMNOTIDAE.—Mouth non-protractile, bordered by the praemaxillaries and the maxillaries, the latter sometimes much reduced; jaws usually toothed. Parietal bones united in a sagittal suture, or separated by a fontanelle; opercular bones well developed; symplectic present. Pharyngeal bones normal, with small teeth. Anterior ribs sessile, the posterior inserted on transverse processes; epipleurals and epineurals. Body much elongate, Eel-like, naked or scaly; dorsal fin absent or reduced to an adipose strip; anal very long; caudal rudimentary or absent; ventrals absent. Vent under the head or at a very short distance behind the throat. Gill-openings narrow.

In spite of their external appearance, these fishes have nothing to do with the Eels; they are strongly modified, degraded Characinids, as first pointed out by Reinhardt. The few genera and species (about 30) are confined to the fresh waters of Central and South America. No fossils are known. Eight genera may be distinguished:—

A. A cranial fontanelle; maxillary bone larger than the praemaxillary; anterior nostril on the upper surface of the head; vent below the head; body scaly: Sternopygus, Eigenmannia, Sternarchus, Rhamphosternarchus, Rhamphichthys, Steatogenys.

B. No cranial fontanelle; maxillary bone very small; anterior nostril on the upper lip; vent on the throat.

a. Body scaly: Carapus.

b. Body naked; an electric organ: Gymnotus.

The mouth is small or very small, and the modifications of the snout in the genera Sternarchus and Rhamphichthys recall those noticed among the Mormyridae. The air-bladder is divided into an anterior and a posterior part, united by a slender duct. The vertebrae vary in number from 70 (Sternopygus) to 240 (Gymnotus). Gymnotus is unique in this sub-order in having as many as 8 pterygials (actinosts) to the pectoral fin, as in Anguilla.

{580}[Illustration: FIG. 351.—Outlines of heads, showing shape of snout and position of vent (v). A, Sternarchus albifrons; B, Sternarchus macrostoma; C, Rhamphosternarchus curvirostris; D, Rhamphosternarchus tamandua.]

The best known member of this family is the so-called Electric Eel (Gymnotus electricus), of the Orinoco, Amazons, and intermediate river-systems. It grows to a length of 8 feet and the thickness of a man's thigh, and is much feared for the electric shocks it is able to discharge. The "Tremblador," as it is called by the Spanish-speaking inhabitants of the Orinoco district, is found only in marshes and in comparatively shallow parts of rivers, to the great annoyance of travellers who have to ford at such points, beasts of burden being frequently knocked down by the electric shock. Specimens have often been exhibited alive in this country; two brought to London in the year 1842, neither of them weighing more than one pound, had by 1848 reached the weights of 40 and 50 pounds respectively. About four-fifths of the length of the fish is occupied by the tail, which contains the electric organ; this is formed by modified muscular tissue, and consists of two huge masses, longitudinal bands or columns, of cells filled with a jelly-like substance, occupying the whole of the caudal region below the vertebral column and separated by a narrow median septum; a smaller body, of similar structure, extends along each side at the base of the anal fin. The whole apparatus is supplied with a great number of nerves branching from the spinal nerves. The electrical apparatus is exercised by the will of the fish, even to {581}a distance, but this faculty is exhausted by continuous employment, and is recovered during repose. Although apparently not exempt from exaggeration and fable, Humboldt's account in Observations de Zoologie, p. 497, is recommended for further information on the habits and modes of capture of Gymnotus.

FAM. 3. CYPRINIDAE.—Mouth usually more or less protractile, toothless, bordered by the praemaxillaries and the maxillaries, or, more frequently by the praemaxillaries only. Parietal bones united in a sagittal suture, or separated by a fontanelle; opercular bones well developed; symplectic present. Lower pharyngeal bones falciform, subparallel to the branchial arches, provided with teeth arranged in one, two, or three series, and often remarkably specialised. Ribs mostly sessile; no parapophyses in the thoracic region; epipleurals and epineurals, mostly free, floating. Pectoral fins inserted very low down, folding like the ventrals. Body naked or scaly. No adipose dorsal fin.

The brain-case is produced forward to the nasal capsule. The branchiostegal rays are reduced to 3; the branchiostegal membrane is usually more or less extensively grown to the isthmus. The suborbital branch of the sensory canals is usually produced on the operculum, as in the Characinidae. The ventral rays number 7 to 12, rarely 5 or 6. Pyloric appendages to the stomach are absent.

Freshwater fishes feeding on vegetable substances or small animals, and dispersed over the whole world with the exception of South America, Madagascar, Papuasia, and Australasia. The species are exceedingly numerous, about 1300 being known, referable to four sub-families, as proposed by Sagemehl.

(i.) CATOSTOMINAE.—Margin of upper jaw formed in the middle by the small praemaxillaries and on the sides by the maxillaries, which are hidden in thick fleshy lips; no barbels; pharyngeal teeth in a single row, very numerous, comb-like; air-bladder large, divided into two or three parts by transverse constrictions, not surrounded by a bony capsule. Mostly from North America; two species from China and one from Eastern Siberia. Fossil in the Lower Tertiary of North America.

Principal genera:—Sclerognathus, Carpiodes, Catostomus, Moxostoma.

{582}(ii.) CYPRININAE.—Maxillaries not bordering the mouth; barbels absent, or one or two pairs; pharyngeal teeth in one to three rows, in small number, often very large, and working against a sclerous plate attached to a ventral process of the basi-occipital, which extends under the anterior vertebrae. Air-bladder usually large and divided into an anterior and a posterior part, rarely tripartite, not surrounded by a bony capsule. The great bulk of the family, represented in every part of its range. Remains of several of the existing genera have been found in Oligocene and later beds of Europe, Sumatra, and North America.

Principal genera:—Cyprinus, Catla, Catlocarpio, Osteochilus, Labeo, Discognathus, Psilorhynchus, Capoëta, Barbus, Gobio, Pseudogobio, Saurogobio, Rhinogobio, Oreinus, Schizothorax, Ptychobarbus, Gymnocypris, Diptychus, Aulopyge, Ceratichthys, Pimephales, Campostoma, Cochlognathus, Exoglossum, Meda, Lepidomeda, Rhinichthys, Rohteichthys, Leptobarbus, Rasbora, Luciosoma, Nuria, Amblypharyngodon, Cyprinion, Semiplotus, Xenocypris, Leuciscus, Tinca, Leucosomus, Chondrostoma, Achilognathus, Rhodeus, Danio, Pteropsarion, Hypophthalmichthys, Abramis, Nematabramis, Aspius, Leucaspius, Alburnus, Barilius, Bola, Neobola, Chelaethiops, Chela, Culter, Pelecus, Parapelecus, Cachius, Opsariichthys, Scombrocypris, Squaliobarbus, Luciobrama.

(iii.) COBITIDINAE.—Maxillaries not bordering the mouth; barbels three to six pairs; pharyngeal teeth in one row, in moderate number. Anterior part of the air-bladder divided into a right and left chamber separated by a constriction, and enclosed in a bony capsule, the posterior part free, or absent. Loaches, characterised externally by a low, elongate body, without or with minute scales. Europe, Asia, Abyssinia. Miocene of Oeningen.

Principal genera:—Botia, Lefua, Diplophysa, Nemachilus, Misgurnus, Cobitis, Lepidocephalichthys, Acanthophthalmus, Eucirrhichthys, Apua.

(iv.) HOMALOPTERINAE.—Maxillaries not bordering the mouth, which is inferior; barbels three or four pairs; pharyngeal teeth in one row, in moderate number. Air-bladder rudimentary, divided into two lateral halves, encased in a bony capsule. Mountain forms with depressed head and horizontally expanded paired fins. China, India, Further India, Malay Peninsula and Archipelago.

Genera:—Homaloptera, Helgia, Glaniopsis, Gastromyzon.

The recently described Gyrinochilus, from Borneo, resembling {583}Homaloptera in habit, with two gill-clefts on each side, an upper and a lower, a tadpole-like mouth without barbels, and a small, free air-bladder, should probably be regarded as the type of a fifth sub-family.

Many of the genera of the Cyprininae are partly founded on the shape and the disposition of the pharyngeal teeth, which, adapted to various requirements, may be conical, hooked, spoon-shaped, molariform, etc., etc. The importance attached to the disposition of these teeth in one, two, or three series for the definition of genera, has been rather exaggerated.

The Cyprinids constitute the majority of the freshwater fishes in Europe, Asia, and North America; they are comparatively few (about 100 species) in Africa, where they coexist with the Characinids. Some, like the Carp (Cyprinus carpio) and the Tench (Tinca vulgaris), are sluggish, except during the breeding season, when they show great excitement and indulge in leaps out of the water; others, like the Bleak (Alburnus lucidus) are constantly on the move in large shoals near the surface; whilst others again, like the M‘Biriki of Lake Tanganyika (Barbus tropidolepis), behave after the manner of Salmon and Trout, {584}travelling long distances, against rapids and over waterfalls, to reach their breeding places at the heads of rivers. During the breeding season, the males of many species assume a more brilliant livery, or develop pearl-like or spiny excrescences on various parts of the head, or also on the body and fins. Cyprinids are oviparous, with the exception of a small Barbel from Natal, discovered and described by Prof. Max Weber as Barbus viviparus.

A most striking instance of symbiosis is offered by a little Carp-like fish of Central Europe, the "Bitterling" (Rhodeus amarus). The genital papilla of the female acquires a great development during the breeding season, becoming produced into a tube nearly as long as the fish itself; by means of this ovipositor the comparatively few and remarkably large eggs, measuring 3 millimetres in diameter—the fish being only 60 to 80 millimetres long—are introduced through the gaping valves, between the branchial laminae of pond mussels (Unio and Anodonta) where, after being inseminated, they undergo their development, the fry leaving their host about a month later, having attained a length of 10 or 11 millimetres. The mollusc reciprocates by throwing off its embryos on the parent fish, in the skin of which they remain encysted for some time, the period of reproduction of the fish and mussel coinciding.

Some members of this family grow to a very large size,—4 to 6 feet; such is the case with the Carp, a native of Asia, introduced into England towards the beginning of the seventeenth century; the Catla (Catla buchanani) of India, Burma, and Siam; the Mahaseer (Barbus mosal) of the mountain streams of Asia, the scales of which may be as large as the palm of a hand; and Hypophthalmichthys molitrix of China and Manchuria, remarkable for the low position of the eyes, the fusion of the gill-rakers into thin plates of spongious appearance, which must act as a most efficient sifting apparatus, and the presence of an involuted problematic superbranchial organ to each branchial arch.

Among well-known aberrations produced by artificial selection may be mentioned the "Leather Carp," a race in which the scales are either lost or much reduced in number, and enlarged {585}along the lateral line and the back, and the Gold-Fish, a variety of Cyprinus carassius, remarkable for its golden or bright red colour, or its perfect albinism, as well as its monstrous form the Telescope Fish, with enormously projecting eyes, and enlarged, horizontally spread caudal fin. This family has also yielded numerous more or less well-established examples of hybridism, congeneric and digeneric, originally described as distinct species, the produce of which is believed to be in some cases fertile for at least one generation.

The crystalline silvery colouring matter of various Cyprinids is said to have been employed from time immemorial for ornamental purposes by the Chinese. The well-known and important industry of "Essence Orientale" and artificial pearls, carried on in France and Germany with the scales of the Bleak, was not introduced before the middle of the seventeenth century.

The Loaches, Cobitidinae, which form a very natural sub-family, are small fishes, few species growing to a foot in length, mostly living in small streams and ponds. Many delight in the mud at the bottom, in which they move like Eels. In some cases, the branchial respiration appears to be insufficient, and the intestinal tract acts as an accessory breathing organ. The air-bladder, which is partially encased in a bony capsule, may be so reduced as to lose its hydrostatic functions and becomes transformed into a sensory organ, its outer exposed surface being connected with the skin by a meatus between the bands of muscle, and conveying the thermo-barometrical impressions to the auditory nerves; hence the name of "Wetterfisch," by which Loaches are known in some parts of Germany.

The Homalopterinae are more or less perfectly adapted to life in rapid streams, the most remarkable in this respect being {586}Gastromyzon of North Borneo, in which the pectoral and ventral fins are much expanded to form, with the belly, a sucker by which the fish adhere to the stones of mountain torrents, showing a remarkable analogy to Exostoma among the Silurids.

FAM. 4. SILURIDAE.—Mouth non-protractile, bordered by the praemaxillaries and the maxillaries, or by the praemaxillaries only, the maxillaries being often rudimentary and supporting the base of a barbel; jaws usually toothed. Parietal bones usually confluent with the supraoccipital, forming a single large plate (parieto-occipital); symplectic and suboperculum absent. Pharyngeal bones normal, with small teeth. Ribs attached to the lower surface of long parapophyses; epipleurals absent. Pectoral fins inserted very low down, folding like the ventrals, often armed, like the dorsal, with a strong bony spine. Body naked or with bony plates. An adipose dorsal fin often present. One to four pairs of barbels.

The skull and the opercular apparatus show a reduction in the number of elements as compared with the Characinids and Cyprinids, such as the absence of the metapterygoid, the often rudimentary, rod-like condition of the palatine, and the fusion of the parietals with the supraoccipital. The scapular arch is solidly united to the skull and is often very massive, and the occiput may be connected with the base of the dorsal fin by a buckler formed by the expansion of the first and second inter-neural bones. The pterygials or supports of the pectoral rays are large and reduced to two or three. Teeth are rarely present {587}on the maxillary bones (Diplomystes, Eutropiichthys), being usually confined to the praemaxillaries and dentaries; they often occur on the palate. The branchiostegal rays vary from 4 to 17. The lips are sometimes much developed, and may form a sucking disk, as in Euchilichthys and Exostoma. As in the Cyprinids, the pungent spines which may arm the fins have nothing in common with the true spines of Acanthopterygians; they result from the co-ossification, with age, of successive articles; but, contrary to the condition in the Cyprinids, the axis of the spine is single, not double. The ventral rays vary from 6 to 16, 6 being the most frequent number. Some of the exterior vertebrae may be solidly fused together, and also with the occipito-nuchal buckler. Prof. Ramsay Wright has shown, by a study of the development, that the complex which follows the first vertebra, which is more or less rudimentary, if distinct, represents the fusion of the 2nd, 3rd, and 4th vertebrae, without even nerve-foramina denoting its compound origin; the first, strongly developed, transverse process represents that of the 4th vertebra. The air-bladder is usually large and trilocular, but additional septa may greatly complicate its structure, and external diverticula may also exist; it may be more or less reduced and entirely or partially enclosed in a bilateral bony capsule formed by the transverse processes of the vertebrae, and sometimes (Nematogenys) ankylosed to the skull. In a few genera, like Cetopsis, the air-bladder seems to be altogether absent: it is reduced to two small oval sacs encased in the large compound anterior vertebra. As in Loaches, the air-bladder is often in immediate contact with the skin behind the shoulder-girdle. The intestinal tract may be simple and short (carnivorous forms) or extremely long and convoluted (Callichthys); as in Cyprinids, pyloric appendages are absent.

Cat-Fishes, as Silurids are usually called, are a large family embracing some 1000 species, spread over the fresh waters of all parts of the world, but mostly from between the tropics. Only a few are marine (Plotosus, Arius, Galeichthys).

This family may be divided into eight subfamilies

{588}(i.) CLARIINAE.—Dorsal and anal fins very long, extending to the caudal; gill-membranes free, or narrowly united to the isthmus. Asiatic-African genera: Clarias, Heterobranchus, Plotosus. Asiatic-Australian: Copidoglanis. Asiatic: Cranoglanis. African: Clariallabes, Allabenchelys, Gymnallabes, Channalabes. Australian: Cnidoglanis.

(ii.) SILURINAE.—Dorsal fin very short or absent, anal very long; gill-membranes free. Europaeo-Asiatic: Silurus. Asiatic: Silurodon, Silurichthys, Saccobranchus, Wallago, Belodontichthys, Cryptopterus, Callichrous, Hemisilurus, Ailiichthys, Ailia, Schilbichthys, Laïs, Pseudeutropius, Pangasius, Osteogeniosus, Helicophagus, Silondia. African: Eutropius, Schilbe, Siluranodon, Physailia, Parailia. Australian: Eumeda, Neosilurus.

(iii.) BAGRINAE.—Dorsal fin short, followed by a more or less elongate adipose fin; anal short or moderate; gill-membranes free. Asia, Africa, America, Australia: Arius. Asia and America: Amiurus. Asiatic: Macrones, Pseudobagrus, Liocassis, Bagroides, Bagrichthys, Rita, Acrochordonichthys, Acysis, Olyra, Hemipimelodus. African: Bagrus, Clarotes, Chrysichthys, Gephyroglanis, Auchenoglanis, Notoglanidium, Anoplopterus, Galeichthys. American: Diplomystes, Paradiplomystes, Aelurichthys, Genidens, Noturus, Callophysus, Pimelodus, Pimelodina, Nanoglanis, Heptapterus, Nematogenys, Pariolius, Pirinampus, Conorhynchus, Notoglanis, Callophysus, Sorubim, Piramutana, Bagropsis, Sciades. Australian: Nedystoma, Pachyula.

(iv.) DORADINAE.—A short-rayed dorsal fin and an adipose, the latter sometimes replaced by a second rayed dorsal; anal short or moderate; gill-clefts more or less widely interrupted below. African: Synodontis, Chiloglanis, Atopochilus, Euchilichthys, Mochocus, Doumea, Phractura, Paraphractura, Andersonia, Trachyglanis, Belonoglanis. Asiatic: Bagarius, Glyptosternum, Gagata, Pseudecheneis, Exostoma, Sisor, Breitensteinia, Sosia, Chaca. South American: Doras, Oxydoras, Leptodoras, Physopyxis, Glanidium, Centromochlus, Wertheimeria, Cetopsis.

(v.) MALOPTERURINAE.—No rayed dorsal fin, an adipose; anal short; gill-clefts interrupted below. African: Malopterurus.

(vi.) CALLICHTHYINAE.—Dorsal, anal, and adipose fins short; body completely cuirassed; praemaxillaries much reduced, the border of the upper jaw formed mainly by the maxillaries. South American: Callichthys, Corydoras.

{589}(vii.) HYPOPHTHALMINAE.—Dorsal fin short, behind the ventrals, anal long; gill-clefts wide or interrupted below. South American: Ageniosus, Trachelyopterus, Auchenipterus, Epapterus, Tetranematichthys, Hypophthalmus, Helogenes.

(viii.) TRICHOMYCTERINAE.—Dorsal fin short, far back, behind the ventrals; no adipose fin; anal short; operculum and interoperculum armed with erectile spines. South American: Trichomycterus, Eremophilus, Stegophilus, Vandellia, Acanthopoma.

Our knowledge of the distribution in time of the Silurids is still very scanty, and throws no light on the derivation of the group. Arius, and two genera apparently related to it, Rhineaster and Bucklandium, have left remains in the Eocene of Europe and North America, and traces of various recent genera have been found in later Tertiary deposits in Europe, Asia, and North and South America.

The habits of the Silurids are extremely diversified, and the shape of the body varies accordingly. The body may be very short and the head enormous and excessively depressed, for instance in the Indo-Burmese Chaca lophioides, which, as its name implies, resembles the Fishing-Frog or Angler; stout and Cottus-like in some South American Pimelodus; Loach-like in Trichomycterus and Stegophilus; more or less Eel-shaped in Clarias and its allies, etc.; the extreme of slenderness obtains in the African Channalabes, the body being excessively elongate (over 100 vertebrae), the ventral fins absent, and the pectorals rudimentary or absent. Among other remarkable forms may be mentioned the Indian Sisor, which resembles Aspredo, and in which the upper caudal ray is much thickened and greatly prolonged; Pseudecheneis, living in rapids of the Himalayas and Khasia hills, provided with a transversely plaited ventral disk between the pectoral fins; the African Phractura and Andersonia, resembling Loricaria; and the likewise African Belonoglanis, comparable to a Needle-Fish. The spines which so frequently arm the dorsal and pectoral fins may be barbed or serrated, and constitute formidable defensive weapons; in the South American Ageniosus valenciennesi, the maxillary bone is transformed into a strong, barbed, erectile spine, replacing the barbel. Stings of even the smaller Cat-Fish are at least as painful as that of a bee, and this is probably due to some poisonous property of the dermal secretion of the Fish.

{590}[Illustration: FIG. 356.—Harmout, Clarias anguillaris (after Valenciennes). ¼ nat. size.]

Cope believed an orifice at or above the axil of the pectoral fin in Noturus to be the opening of the duct of a poison-gland; "from it may frequently be drawn a solid gelatinous style ending in a tripod, each limb of which is dichotomously divided into short branches of regular length." I think this condition of things has nothing to do with a poison-organ, and is merely a repetition of what is observed in Loaches and in the Characinid Xenocharax, where I have found a gelatinous substance filling the short duct by which the membrane of the air-bladder is placed in communication with the skin and the sensory organ of the lateral line. Most Silurids can live in very foul water, taking in air from the surface, and spend a comparatively long time out of the water, without being possessed of any special apparatus for atmospheric respiration. A few genera, however, are provided with an accessory breathing organ: in Clarias, Heterobranchus, and allies, there is a dendritic superbranchial organ, in Saccobranchus a long air-sac, extending from the first branchial cleft along the side of the body, as described above, p. 295; and these Fish can live for days on land. Clarias lazera has been observed, in Senegambia, to spend several months of the dry season in burrows, from which it emerges at night to crawl about in search of food. Many Silurids, but especially Doras and Synodontis, are known to produce sounds in and out of the water by means of a special mechanism of the air-bladder and the processes of the vertebrae above it, combined with the movements of the pectoral spine grinding in the glenoid cavity. In South America, Doras has been observed to move rapidly on land, projecting itself forward on the pectoral spines by the elastic spring of the tail, travelling long journeys over land, from one drying pond to another, spending whole nights on the way; these migrations sometimes take place {591}in numerous bands, baskets of the small Fish being filled by the Indians who come across them. The African Synodontis are much in the habit of floating or swimming leisurely on the surface with the belly in the air, as was well known to the ancient Egyptians, who have frequently depicted the Fish in this anomalous position. A curious fact in connexion with this habit is that S. membranaceus and S. batensoda, in which it has most frequently been observed, show an inversion of the ordinary mode of coloration, the lower parts being dark brown or black and the upper pale silvery grey. The electric Cat-Fish (Malopterurus electricus), is also a native of Africa, occurring all over the tropical parts of that continent and also in the Lower Nile, growing to a length of three feet. Its flesh is more esteemed than that of other Silurids. It avoids light and is slow in its movements. The electrical apparatus differs absolutely from that of all other Fishes, being derived from the integument, belonging to the glandular system, and surrounding the whole body with a thick coat of grease or gelatinous substance; the apparatus is governed by a single nerve on each side proceeding from a huge ganglionic cell at the anterior extremity of the spinal cord. The shocks given by Madopterurus are very powerful, and the Fish is called "Raad" by the Arabs, a name which means "thunder." Kept in an aquarium with other Fishes, even of the same species, the "Raad" soon kills its companions.

{592}[Illustration: FIG. 358.—Callichthys littoralis, from South America. ⅔ nat. size.]

In this family the eggs and young are usually looked after by the parents. Aristotle observed that the male of the European Silurus glanis watches over and defends the eggs. In one of the commonest North American Cat-Fishes, Amiurus nebulosus, a species which has been largely introduced into some parts of Europe of late, now thriving in many ponds and more or less polluted streams of the Continent, the eggs are deposited near the banks of weedy ponds and rivers without currents, in concealed places beneath logs, stumps, or even in pails or other receptacles, failing which both parents join in excavating a sort of nest in the mud, a work often requiring two or three days of incessant labour. The male watches over the eggs, and later leads the young in great schools near the shore, seemingly caring for them as the hen for her chickens. The Doras and the Callichthys of South America, according to Hancock and Vipan, build regular nests of grass or leaves, sometimes placed in a hole scooped out in the bank, in which they cover their eggs and defend them, male and female sharing in this parental duty. In the likewise South American Corydoras (Callichthys paleatus), as observed by Carbonnier, a lengthy courtship takes place, followed by an embrace, during which the female receives the seminal fluid in a sort of pouch formed by the folded membranes of her ventral fins; immediately after, five or six eggs are produced and received in the pouch, to be afterwards carefully placed in a secluded spot. This operation is repeated many times, until the total number of eggs, about 250, have been deposited. In {593}accordance with these pairing habits, the pectoral spines of the male, which are used in amplexation, are longer and stronger than those of the female. These Fish are monogamous, and both parents remain by the side of the nest, furiously attacking any assailant. Dr. R. Semon has made observations in Queensland on the habits of Arius australis, which builds nests in the sandy bed of the Burnett River. These nests consist of circular basin-like excavations, about 20 inches in diameter, at the bottom of which the eggs are laid, and covered over by several layers of large stones. A still more efficient protection is afforded their progeny by the marine and estuarine species of Arius, Galeichthys, and Osteogeniosus, the male, more rarely the female, carrying the eggs in the mouth and pharynx; these eggs, few in number, are remarkably large, measuring as much as 17 or 18 millimetres in diameter in Arius commersonii, a Fish of three or four feet in length. According to Babuchin, Malopterurus also is said by the Nile fishermen to shelter its fry in the mouth.

Some of the Silurids attain to a very large size. Among these is the type of the family, Silurus glanis, the "Wels" of the Germans, its only European representative, which occurs over a great part of Europe, but is absent from the British Isles, France, the Spanish Peninsula, and Italy. It is most abundant in the Danube basin, where it sometimes reaches a length of 10 feet or more and a weight of 400 lbs. It is the largest strictly fresh-water Fish of Europe. Among the smallest species, we have to mention the "Candiru" of Brazil, Vandellia cirrhosa, 60 millimetres in length and 3 or 4 in diameter, which is believed to enter and ascend the urethra of people bathing, being attracted by the urine; the Fish, having once made its way into the urethra, cannot be pulled out again, owing to the erectile spines which arm its gill-covers. The natives of some parts of the Amazons are in great dread of this Fish, and protect themselves when entering the water by wearing a sheath formed of a small, minutely-perforated cocoanut-shell suspended from a belt of palm-fibres. According {594}to Reinhardt the allied Stegophilus insidiosus, a small colourless Fish, 30 to 40 mm. long, from Brazil and Argentine, lives parasitically in the gill-cavity of large Cat-Fishes (Platystoma). Dr. F. Silvestri has noticed that it sucks the blood in the gills of Platystoma coruscans, a Silurid growing to a length of 6 feet.

FAM. 5. LORICARIIDAE.—Distinguished from the preceding by the sessile ribs and the absence of the transverse processes in the praecaudal vertebrae, which have bifid neural spines. The air-bladder is always much reduced, and enclosed in a right and a left bony capsule formed by the skull and the anterior vertebrae. Gill-openings narrow clefts. The mouth is inferior, with more or less developed circular lips and feeble dentition; it is used as a sucker, by which the Fish fixes itself to any hard object with such strength that it cannot be pulled off without great difficulty. The teeth are usually slender and bicuspid. The food consists of very small prey and more or less putrefied organic substances, the intestine being usually extremely elongate and much convoluted. The habits of these Fish are very little known, but the fact that the males of many species have the pectoral fins much stronger than the females renders it probable that they pair like {595}Callichthys. There are other sexual differences in many species of Plecostomus, Chaetostomus, and Loricaria, as the presence of dermal tentacles on the snout (see Fig. 359), or of hair-like bristles on various parts of the head and fins in the males, which are usually of larger size.

About 200 species are known, all from the tropical and subtropical parts of Central and South America. The largest species (Chaetostomus gigas) measures 2½ feet; many are of very small size. The genera may be referred to two sub-families:—

(i.) ARGINAE.—Body naked; ribs strong. Arges, Stygogenes, Astroblepus.

(ii.) LORICARIINAE.—Body cuirassed by bony plates; ribs very slender. Plecostomus, Liposarcus, Chaetostomus, Cochliodon, Pterygoplichthys, Rhinelepis, Acanthicus, Otocinclus, Hypoptopoma, Loricaria, Acestra.

The "Prenadillas" of the Andes, Arges and Stygogenes, were believed to live in subterranean waters within the bowels of active volcanoes, and to be ejected with streams of mud and water during eruptions, a story that has been repeated by Humboldt. The fact is that they live in small torrents at great altitudes (up to 10,700 feet), and are swept down during periods of disturbance caused by the eruption of the volcano. The members of the sub-family Loricariinae vary much in the shape of the body, which may be short and stout, or more or less slender, the extreme in the latter respect being attained by the species of the genus Acestra.

{596}FAM. 6. ASPREDINIDAE.—This family is also closely related to the Siluridae. The ribs are sessile as in the Loricariidae, but inserted very low down on the centra, which higher up bear strong transverse processes. The opercular bone is entirely absent. The gill-opening is reduced to a foramen in front of the pectoral fin. The head is extremely depressed and the mouth terminal; the tail is very slender; the body is naked. The air-bladder is large and free, the intestinal canal short. Four genera from South America: Aspredo, Bunocephalus, Bunocephalichthys, Dysichthys. Species 18. Aspredo, of the Guianas, the largest form, reaching to about a foot in length, is remarkable for the manner in which the female carries her eggs. The skin of the lower parts assumes a spongy condition about the breeding season, and the eggs, after being deposited, become attached to the lower surface of the head, belly, and paired fins, forming a single layer; each egg becomes connected with the skin of the mother by a cup-shaped, pedunculate base, supplied with blood-vessels and coated with a layer of epithelium, the formation of which is still unexplained.

{597}CHAPTER XXII

TELEOSTEI (CONTINUED): SYMBRANCHII—APODES—HAPLOMI—HETEROMI—CATOSTEOMI— PERCESOCES—ANACANTHINI

SUB-ORDER 3. SYMBRANCHII

Eel-shaped Fishes without paired fins, with the pectoral arch free or suspended from the skull, and with the anterior vertebrae distinct, without Weberian ossicles. Gill-openings confluent into a single, ventral slit. Air-bladder absent.

The structure of the skull conforms to that of typical Malacopterygians. The praemaxillary and maxillary are well developed, the latter placed behind the former, and forming but a very small part of the oral border; the symplectic is present; the parietals form a long sagittal suture, and separate the frontals from the supraoccipital. The vertebrae are very numerous, the praecaudal bearing very strong parapophyses, to which short, slender ribs are attached. The skin is naked (Symbranchidae) or covered with minute scales (Amphipnoidae), and the vertical fins are rudimentary, reduced to mere dermal folds.

Like the Apodes, which they resemble in general appearance, these Fishes are no doubt derived from some low type with abdominal ventral fins, but whether from the Malacopterygii or the Haplomi we have as yet no data from which to conclude. Only two families are known.

FAM. 1. SYMBRANCHIDAE.—Post-temporal well developed, forked, attached to the skull. Inhabitants of the fresh or brackish waters of South-Eastern Asia, Tropical America, Australia, and Tasmania. Three genera are known: Symbranchus, with two species from India and the Malay region, and one from Central and South America; Monopterus, with a single species {598}from China, Japan, and the Malay region; and Chilobranchus, with two species from Australia and Tasmania. Although the South American Symbranchus has been observed to live in marshes which periodically dry up, the Fish burying itself in the mud like a Lepidosiren, the branchiae are fully developed on the four branchial arches. In Monopterus, of similar habits, the branchial laminae are rudimentary, and on three arches only. No accessory breathing organ is known to exist.

FAM. 2. AMPHIPNOIDAE.—Post-temporal absent, the shoulder-girdle free from the skull. The Cuchia, Amphipnous cuchia, the sole representative of this family, an inhabitant of the fresh and brackish waters of India and Burma, growing to two feet in length, is remarkable for the presence of a respiratory air-sac on each side of the neck behind the head, communicating with the gill-cavity. Of the three branchial arches the second alone possesses gill-filaments; the third supports, in their place, a thick and semi-transparent tissue; the principal organs of respiration are two small bladders, resembling the posterior portions of the lungs of snakes, which the animal has the power {599}of filling with air immediately derived from the atmosphere. Although covered over by the common integuments, these bladders present externally, when inflated, two protuberances of a round shape. Of the whole volume of blood contained in the branchial artery, one-third passes through the gills and respiratory bladders, whilst the other two-thirds are conveyed directly from the heart to the aorta without being exposed to the action of the air. This amphibious Fish, when in the water, constantly rises to the surface for the purpose of respiration, and it is often found lying in the grassy sides of ponds after the manner of Snakes.

SUB-ORDER 4. APODES.

Air-bladder, if present, communicating with the digestive tract by a duct. Praemaxillaries absent; the maxillaries, if present, separated on the median line by the coalesced ethmoid and vomer. Pectoral arch, if present, not connected with and remote from the skull; mesocoracoid arch absent. Fins without spines, the ventrals absent. Anterior vertebrae distinct, without Weberian ossicles.

The Apodes or Eels are elongate, serpentiform Fishes with naked skin, or with minute scales imbedded in the skin, the opercular bones small and completely hidden under the integument; narrow or minute gill-openings; the vertical fins, if present, confluent behind or separated by the projecting tip of the tail. The pterygo-palatine arch is often reduced or absent, and there is no distinct symplectic; the supraoccipital bone is small, separated from the frontals by the parietals, which meet on the middle line. The vertebrae are very numerous (up to 225), and the praecaudals bear strong parapophyses, to which short, slender ribs may be attached; epineurals are sometimes present. The five families into which this sub-order is divided show remarkable degrees of simplification of the skull, through reduction or loss of either the maxillary or the pterygo-palatine arches.

There has been much difference of opinion in the determination of the bones of the upper jaw in these Fishes. Cuvier regarded the lateral bones of the upper jaw as praemaxillaries, Owen and Richardson as palatines (at least in Muraenas), whilst Peters {600}and most recent authors have identified them throughout the order as maxillaries. The conclusion I have come to from the examination of numerous skulls belonging to various genera, is that the praemaxillaries have disappeared in all, whilst the maxillaries have persisted in the true Eels (Anguillidae) and disappeared in the Muraenidae, their place being taken by the fused palato-ectopterygoids, which may even join the mandibular suspensorium. The vestigial bone, regarded by Jacoby as the pterygoid in Muraena helena, may be identified as the meta-pterygoid, and therefore does not disprove the homology, here suggested, of the other elements of the palate.

FAM. 1. ANGUILLIDAE.—Maxillaries present, separated on the median line by the ethmo-vomer; palato-pterygoid bone present, connected with the hyomandibular and quadrate; gill-clefts separate, opening into the pharynx by wide slits; tongue present; vent far removed from the head.

Spread over all the seas of the temperate and tropical zones, often descending to the greatest depths, a few entering fresh waters. Many are known to undergo very striking metamorphoses, the pellucid, strongly compressed larvae (Leptocephalus) having long been a puzzle to naturalists.

{601}Nearly 150 recent species are known, of which some 50 are deep-sea forms, occurring down to 2500 fathoms. Scanty fossil remains, referable to recent genera or scarcely different from them, are known from the Eocene of Europe. The Cretaceous genus Urenchelys, from England and the Lebanon, is interesting as representing a more generalised type, the hindmost vertebrae bearing a pair of expanded hypural bones, showing the diphycercal Eels to have been derived from Fishes with a normal caudal fin.

The genera are numerous. The following are the principal:—Anguilla, Simenchelys, Ilyophis, Conger, Coloconger, Congromuraena, Uroconger, Heteroconger, Muraenesox, Nettastoma, Nettophichthys, Saurenchelys, Nettenchelys, Myrus, Myrophis, Derichthys, Chilorhinus, Muraenichthys, Liuranus, Ophichthys, Moringua.

In the first four genera, small, more or less lineal rudimentary scales are embedded in the skin, arranged in small groups, which are placed obliquely at right angles to one another, forming a curious pattern; but these scales are so small that they escape the notice of the superficial observer, hence Eels have been improperly included among the Fishes forbidden as food by the Mosaic prescriptions. In the other genera, including the exclusively marine Conger of our coasts, scales are really absent.

The Common Eel (Anguilla vulgaris) has a very wide distribution, being found over the greater part of Europe, North Africa, Temperate Asia, and perhaps also North America east of the Rocky Mountains, Mexico, and the West Indies. Its record from Australia and New Zealand is probably due to the imperfection of our knowledge of the specific characters. It is not found in the Black Sea nor in the rivers flowing into it, owing, no doubt, to the sulphurous nature of the bottom of the sea, to which, as we now know, these Fish would have to resort for breeding.

The mode of propagation of the Eel long remained a mystery, from the fact that individuals found in fresh water never show ripe genital glands. The idea had been entertained of their being hermaphrodite, and internal parasites had also given rise to the belief in their viviparous nature. The genital glands of the female were first investigated by Rathke in 1838, but it was not until 1874 that those of the male were discovered by Syrski, and shortly after fully described by L. Jacoby, who, in his final contribution to the subject, concluded that Eels need salt water for the development of their organs of generation, and that this development takes place, not near the coast, but further out in deep water.

{602}[Illustration: FIG. 363.—Larva of Common Eel, Leptocephalus brevirostris of Kaup. (After Kaup.)]

As a rule it is not until the fifth or sixth year that the Eels go to the sea for the purpose of propagation, which takes place at great depths—at least 200 fathoms. Males have been observed to precede the females. The breeding season over, the Eels do not return to fresh waters, but are believed to die soon after. The eggs were discovered by Raffaele in 1888 in the Gulf of Naples, and shortly after Grassi and Calandruccio finally settled the question of the breeding and development of the Fish from observations made in the Mediterranean. Their conclusions are thus summed up:—"The Common Eel matures in the depths of the sea, where it acquires larger eyes than are ever observed in individuals which have not yet migrated to deep water. The abysses of the sea are its spawning places; its eggs float in the sea water. In developing from the egg, it undergoes a metamorphosis, it passes through a larval form denominated Leptocephalus brevirostris." What length of time the development requires is not yet fully established, since the Leptocephali are rarely found at the surface, most of the specimens studied by Grassi and Calandruccio having been obtained from the stomach of the Sun-Fish (Orthagoriscus mola) in the Straits of Messina; but it is believed that the young Eels or "elvers," which ascend our rivers in such prodigious numbers in spring and summer ("Eel-Fares") are already one year old. Some individuals apparently spend their whole life in fresh waters, but they are barren. A specimen was kept in confinement in the family of the French naturalist Desmarest for upwards of 40 years, growing to a length of 4½ feet, being already of large size at the time of {603}its capture. Eels are extremely voracious, and endowed with an extraordinary tenacity of life; they can live for many hours out of the water, and are often met with at night creeping through the grass of meadows from one pond or stream to another.

One of the most remarkable among the deep-sea Eels is the Snub-nosed Eel (Simenchelys parasiticus), which has been found in great numbers off Newfoundland and the Azores, at depths of 200 to 900 fathoms. The maxillary and mandibular bones are very short and massive, provided with large obtuse teeth; the head is short and bulldog-like in aspect, the mouth small and bordered by a thick circular lip. Some specimens have been observed to burrow in the muscles of living Halibut and other large Fishes, after the manner of Myxine.

FAM. 2. NEMICHTHYIDAE.—Distinguished from the preceding by the position of the vent, which is close to, or at no great distance from, the gill-openings. The rays of the vertical fins are connected by thin membrane instead of being imbedded in thick skin, as in most Eels; in some of the genera the jaws are excessively prolonged, needle-like, sometimes recurved. Deep-sea Eels of small size, represented in the Atlantic, Pacific, and Indian Oceans by about 10 species, referred to 6 genera: Dysomma, Dysommatopsis, Nemichthys, Spinivomer, Serrivomer, Gavialiceps.

FAM. 3. SYNAPHOBRANCHIDAE.—Maxillaries narrowly separated on the median line, their extremity strongly attached by ligament to the mandible; pterygo-palatine arch absent. Gill-openings externally confluent into a single ventral slit. Deep-sea Fishes, resembling the true Eels in the general form and in the presence of linear scales placed at right angles, but differing in the absence of the pterygo-palatine arch, as in the Saccopharyngidae. Eight species of Synaphobranchus are known, from the Atlantic, Pacific, and Indian Oceans, at depths of 200 to 2000 fathoms.

FAM. 4. SACCOPHARYNGIDAE.—Maxillaries narrowly separated on the median line, extremely elongate; mouth enormous; pterygo-palatine arch absent; hyomandibular arch slender and movably articulated to the cranium, the two bones (hyomandibular and quadrate) of which it is composed being capable of being swung in all directions; branchial arches far behind the skull; no branchiostegal rays or pharyngeal bones.

Extraordinary-looking deep-sea Fishes allied to the Eels, of which they appear to be a further degraded type, the muscles {604}being feebly developed and the skeleton imperfectly calcified. The mouth, furnished with rather long but feeble, or even minute teeth, and the pharynx and stomach are capable of great distension, these Fish being able to get outside a prey very much larger than themselves; the eyes are situated far forward on the head; the tail is extremely slender and elongate. Four genera are known, each with a single species, from the Atlantic: Saccopharynx, Eurypharynx, Macropharynx, and Gastrostomus. The depths at which they have been obtained vary between 389 and 1467 fathoms, but three out of the four known specimens of Saccopharynx were brought to the surface by having swallowed a Fish too large for the capacity of the stomach. The length of the largest specimen is about 6 feet, of which the tail constitutes nearly three-fourths.

FAM. 5. MURAENIDAE.—Maxillaries absent, replaced by the palato-pterygoid, the mouth bordered by the latter and the {605}ethmo-vomer; palato-pterygoid bone separated from hyomandibular arch; branchial openings into the pharynx narrow slits; no tongue.

The body is naked, pectoral fins are usually absent, and the gill-cleft is a small round opening. The opercular bones are much reduced in size, and the pectoral arch may be totally absent.

Voracious marine Fishes, inhabiting tropical and subtropical waters, being especially abundant about coral reefs. Some 120 species are known, many reaching a very large size, and being also remarkable for their variegated coloration. The genera are mostly founded on the dentition, which shows much diversity; the following are the principal:—Myroconger, Enchelycore, Muraena, Thyrsoidea, Lycodontis, Pythonichthys, Echidna, Channomuraena. The Muraena of the ancient Romans, Muraena helena, of the Mediterranean, Eastern Atlantic, and neighbouring parts of the Indian Ocean, occurring exceptionally as far north as the English coast, grows to 4 feet, and its flesh was more esteemed than that of any other Fish, large numbers being reared in specially constructed reservoirs near the sea, and fed on the corpses of slaves. Channomuraena vittata, from the coast of Cuba, is known to attain a length of 8 feet, and Thyrsoidea macrura, from the Indian Ocean and the Malay Archipelago, to upwards of 10 feet.

SUB-ORDER 5. HAPLOMI.

Air-bladder, if present, communicating with the digestive tract by a duct. Opercle well developed. Pectoral arch suspended from the skull; no mesocoracoid arch. Fins usually without, rarely with a few spines; ventrals abdominal, if present. Anterior vertebrae distinct, without Weberian ossicles.

The absence of the mesocoracoid arch distinguishes the Haplomi from the Malacopterygii, with which they are united by various authors. They lead to the Percesoces through the Cyprinodontids, and to the Lower Acanthopterygians, such as the Berycidae, through the Scopelids, Stephanoberycids, and Percopsids, as is evidenced by the structure of the mouth and the forward position, in some of the genera, of the ventral fins, which, however, are never attached to the pectoral girdle. Most of the forms which are here included inhabit either fresh water or the deep sea.

{606}SYNOPSIS OF THE FAMILIES.

I. Parietals separating the frontals from the supraoccipital; post- temporal simple; praecaudal vertebrae with autogenous parapophyses.

Margin of the upper jaw formed by the praemaxillaries and the maxillaries; basis cranii simple; no adipose dorsal fin 1. Galaxiidae.

Margin of the upper jaw formed by the praemaxillaries only; basis cranii double; adipose dorsal fin present 2. Haplochitonidae.

II. Frontals in contact with the supraoccipital.

A. Praecaudal vertebrae without parapophyses.

1. Margin of the upper jaw formed by the praemaxillaries and the maxillaries.

Body without or with minute scales, usually with rows of scutes; adipose dorsal fin usually present 3. Enchodontidae.†

Body scaly; post-temporal forked; no adipose dorsal fin; ventrals with 6 to 11 rays 4. Esocidae.

Body scaly; post-temporal incompletely ossified; pectoral fin without pterygials; no adipose dorsal fin; ventrals with 3 rays only 5. Dalliidae.

2. Maxillaries excluded from the oral border.

a. Adipose dorsal fin usually present; ventral fin with 7 to 10 rays.

Post-temporal forked; dorsal fin formed of articulated rays 6. Scopelidae.

Post-temporal simple; dorsal fin very long, formed of slender, non-articulated, simple or bifid rays 7. Alepidosauridae.

b. No adipose dorsal fin; head and mouth enormous, dentition feeble; body naked; ventral fins, if present, with 5 rays 8. Cetomimidae.

B. Praecaudal vertebrae with well-developed parapophyses; maxillaries excluded from the oral border.

1. Dorsal and anal fins without spines; scales cycloid, or with erect spines; no adipose fin.

Mouth not protractile; ventral fins far forward, with 7 to 17 rays 9. Chirothricidae.†

Mouth not protractile; ventral fins remote from the pectorals, with 9 rays 10. Kneriidae.

Mouth protractile; ventral fins, if present, with 5 to 7 rays 11. Cyprinodontidae.

Mouth scarcely protractile; ventral fins rudimentary or absent; vent close to the gills 12. Amblyopsidae.

Mouth slightly protractile; ventral fins with 5 or 6 rays 13. Stephanoberycidae.

2. Dorsal and anal fins with true spines; scales ctenoid; an adipose dorsal; ventral fins with 9 rays 14. Percopsidae.

† Fossil only.

{607}FAM. 1. GALAXIIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter behind the former, and toothless. Parietals in contact with each other, and separating the frontals from the supraoccipital; opercular bones all well developed. Basis cranii simple. Ribs inserted on strong, autogenous parapophyses; epipleurals and epineurals. Post-temporal simple, attached to the epiotic; post-clavicle present. Body naked. Vertical fins far back; no adipose dorsal fin. Pectoral fins inserted very low down. Ventrals, if present, with seven rays. Air-bladder present. Ova falling into the cavity of the abdomen before exclusion.

The genus Galaxias has an interesting distribution, the species of which it is made up occurring in the fresh waters of the southern hemisphere, viz. 8 in New Zealand and neighbouring islands, 7 in New South Wales, 3 or 4 in South Australia, 1 in West Australia, 2 in Tasmania, 7 in South America, from Chili southwards, and 1 at the Cape of Good Hope. One species (G. attenuatus) is even believed to be identical in New Zealand, Tasmania, South Australia, the Falkland Islands, and South America. This conclusion is probably correct from the fact, which may account for the distribution of the whole genus, {608}that it is not confined to fresh waters, but occurs also in the sea. Specimens were observed by Mr. Rupert Vallentin in the Falkland Islands, where the Fish is known to the inhabitants as "Smelts," in shoals in the shallow water along the shore; and, according to Mr. F. E. Clarke, the same species, in New Zealand, periodically descends to the sea, where it spawns, from January to March, and returns from March to May. A marine species has recently been discovered at the Chatham Islands. In New Zealand, the Galaxias were called "Trout" by the settlers before the introduction of Salmonids, whilst the fry of G. attenuatus are eaten as "Whitebait." The largest species reach the length of a foot. Neochanna, from New Zealand, differs from Galaxias in the absence of ventral fins; it has been found in burrows, which it excavates at a distance from water.

FAM. 2. HAPLOCHITONIDAE.—Small fresh-water Trout-like Fishes, agreeing in most respects with the Galaxiidae, to which they are unquestionably closely related, differing only in the greater extent of the praemaxillaries, which exclude the maxillary from the oral border, in the double basis cranii (the prootics uniting under the brain, leaving a canal between them and the parasphenoid), in the shorter parapophyses, which, like the neural arches of the praecaudal vertebrae, are autogenous, and in the presence of a small adipose dorsal fin, opposed to the anal.

Two genera: Haplochiton, naked, with a single species from Chili, the southern extremity of South America, and the Falkland Islands, and Prototroctes, covered with small scales, of which one species inhabits Queensland, another South Australia, and a third New Zealand. In Haplochiton, the urogenital orifice of both sexes is produced into a cylindrical tube, which lies concealed in a groove in front of the anal fin.

FAM. 3. ENCHODONTIDAE.—Margin of the upper jaw formed by the praemaxillaries and the maxillaries, the latter sometimes toothed like the former. Frontals in contact with the supraoccipital; basis cranii simple. Ribs sessile; praecaudal vertebrae without transverse processes. Rayed dorsal fin never much extended; sometimes an adipose fin behind it. Scales delicate or absent, but occasional longitudinal series of scutes occur, the dorsal series, when present, being unpaired.

Cretaceous Fishes allied to, and apparently more generalised than, the Esocidae and Scopelidae. Numerous remains from {609}Europe and North America, referred to 7 genera: Enchodus, Eurypholis, Palaeolycus, Halec, Cimolichthys, Prionolepis.

FAM. 4. ESOCIDAE.—Margin of the upper jaw formed by the praemaxillaries and maxillaries, the latter behind the former, and toothless. Supraoccipital in contact with the frontals, separating the small parietals; opercular bones all well developed; basis cranii simple. No parapophyses, except to the hindermost praecaudal vertebrae; epipleurals and epineurals. Post-temporal forked, the upper branch attached to the epiotic, the lower to the exoccipital; post-clavicle present. Vertical fins far back; no adipose dorsal fin. Pectoral fins inserted very low down; ventrals with 6 to 11 rays. Air-bladder present.

As in the Haplochitonidae, the neural and haemal arches are bones distinct from the centra, and although parapophyses are not developed, the ribs are not inserted on the centra, but on distinct bases wedged into the latter. Teeth are present on the vomer, palatine, and pharyngeal bones.

A small family of carnivorous freshwater Fishes, including the Pike (Esox), of predaceous habits, unsurpassed in greediness and voracity, and the small and insignificant-looking Umbra, distinguished by the more anterior position of the dorsal fin, the larger scales, and the moderately large gape, with feeble villiform teeth. The range of the Esocidae is restricted to the cold and temperate parts of the northern hemisphere. Besides the well-known Esox lucius of Europe, Northern Asia, and the northern parts of North America, growing to a length of 4 feet, and the Maskinongy (E. nobilior) of north-eastern North America, reaching twice that length, the first genus comprises three smaller species from the Eastern United States. Remains of Esox have been found in {610}various freshwater deposits in Europe as far back as the Oligocene. E. lepidotus, of which very perfect specimens have been found in the Upper Miocene beds of Oeningen in Baden, differs from the living species in its much larger scales and in the greater approximation of the ventral and anal fins, two characters in which it approaches Umbra. Only two species of the latter are known: U. crameri ("Hundsfisch"), from the stagnant waters of Austria-Hungary, and U. limi ("Mud-Fish"), living in swamps and ditches in Canada and the north-eastern United States, often remaining imbedded in the mud of prairie sloughs and bog-holes.

FAM. 5. DALLIIDAE.—The genus Dallia, with a single species inhabiting the streams and ponds of Alaska and Siberia, is related to Umbra, but differs in the very thin and papery skeleton, with the post-temporal imperfectly ossified and the pectoral fin without pterygials or actinosts. The dorsal fin is far back and opposite to the anal, as in the Pike. The ventral fins are composed of three rays only, and the pectorals, which have a somewhat fleshy base, have as many as 36. The scales are extremely small, and partly imbedded in the skin. The Black-Fish, D. pectoralis, abounds in Sphagnum ponds, feeding on plants and worms, and forming the chief food of the natives of some parts of Northern Alaska, where, with the exception of the Salmonids, it is the only freshwater Fish. Turner, its discoverer, says its vitality is {611}extraordinary: Black-Fishes will remain frozen in baskets for weeks, and when thawed are as lively as ever, one having been swallowed in a congealed condition by a dog, thawed out by the heat of the stomach, and vomited up alive.

FAM. 6. SCOPELIDAE.—Praemaxillaries much elongate, and completely excluding the maxillaries from the oral border. Supraoccipital in contact with the frontals, sometimes partly covered by the parietals; opercular bones all well developed. Basis cranii simple. Ribs sessile; no parapophyses on the praecaudal vertebrae; epipleurals and epineurals. Post-temporal forked, the upper branch in contact with the epiotic or the supraoccipital, the lower with the opisthotic; post-clavicle present. An adipose dorsal fin often occurs; luminous spots often present on head and body. Ventral fins with 7 to 10 rays. Air-bladder sometimes absent.

A large family (over 100 known living species), mostly of pelagic and deep-sea Fishes. A great number of fossil types have been described.

Recent genera:

A. Without photophores: Saurus, Saurida, Bathysaurus, Harpodon, Scopelarchus, Aulopus, Odontostomus, Omosudis, Sudis, Paralepis, Bathypterois, Benthosaurus, Chlorophthalmus, Ipnops. B. With photophores: Scopelus, Dasyscopelus, Neoscopelus, Scopelengys, Nannobrachium, Scopelosaurus.

Fossil genera:

A. Cretaceous: Sardinioides, Acrognathus, Leptosomus, Sardinius, Dactylopogon, Nematonotus, Microcoelia, Opisthopteryx, Apateodus, Rhinellus. B. Eocene, Oligocene, and Miocene: Omiodon, Scopeloides, Parascopelus, Anapterus.

{612}[Illustration: FIG. 371.—A, Scopelus crocodilus (after Goode and Bean). B, Bathypterois dubius (after Collett). C, Ipnops murrayi, with dorsal view of head (after Goode and Bean).]

{613}The members of this Family vary much in form, and among them are to be found some of the most curious adaptations to bathybial existence. One of the best known is Harpodon nehereus, which, when newly taken, is brilliantly phosphorescent all over the body; in a salted and dry condition it is the "Bombay-duck," a delicacy eaten with curries, and exported in large quantities from the west coast of India. It is not known to occur at any great depth, and is not even restricted to the sea, being very abundant in the rivers and estuaries of Bengal and Burma; whilst an allied species, H. squamosus, is found in the Indian Ocean at depths of 120 to 300 fathoms. In Bathypterois, the eyes are very small; some of the rays of the paired fins being excessively prolonged, acting as tactile organs, and compensating the reduction in the eyes. Sir John Murray has observed about B. longipes: "When taken from the trawl [from 2650 fathoms] they were always dead, and the long pectoral rays were erected like an arch over the head, requiring considerable pressure to make them lie along the side of the body; when erected they resembled Pennatulids like Umbellula." In Ipnops, which resembles in general form the large-eyed Chlorophthalmus gracilis, the upper surface of the broad spatulate snout is occupied by a luminous organ longitudinally divided into two symmetrical halves, and the eyes are absent, unless, as first supposed, this extraordinary organ be a modification of them; but Professor Moseley's examination seems to have proved beyond doubt that it is a special form of phosphorescent organ, the object of which would be to attract other creatures to the wide gape of a Fish which, living in the abysses of the sea and deprived of organs of sight and touch, would have great difficulty in procuring its food. Odontostomus, with a very large eye which can be turned upwards and sidewards, and enormous compressed curved teeth, barbed at the tip and depressible backwards, is one of the few Scopelids in which scales are completely absent.

The numerous species (about 50) of Scopelus and their allies are moderate-sized or small pelagic and deep-sea forms found in nearly all the seas, some coming to the surface at night, whilst others are confined to great depths; they are remarkable for the series of phosphorescent spots (photophores) on the body, and in some species also on the head, where they may form large patches on the snout. The arrangement of these photophores is a very {614}definite one, and it has been used for the division of these Fishes into genera or sub-genera. The ventral fins have a more forward position than in most other members of the Family.

FAM. 7. ALEPIDOSAURIDAE.—Characters as in the preceding, but supratemporal simple, attached to the opisthotic, and dorsal fin very long, formed of slender, non-articulated, simple or bifid rays, extending along nearly the whole length of the back, followed by a small adipose fin. The air-bladder is absent and the body scaleless. The skeleton is feebly ossified; the dentition is very powerful, some of the teeth on the palate and mandible being very strongly enlarged. 4 or 5 species are known, from considerable depths in the Atlantic and Pacific Oceans, referable to one genus, Alepidosaurus or Plagyodus. A. ferox, from the Atlantic, reaches a length of 4 feet.

FAM. 8. CETOMIMIDAE.—The affinities of the recently discovered genera Rondeletia and Cetomimus, deep-sea Fishes from the North Atlantic, at depths of 1000 to 1600 fathoms, are still uncertain, as the skeleton could not be examined; they are probably most nearly related to the Scopelidae. The head is enormous, with very wide gape, that of Cetomimus being suggestive of that of a Right Whale; the teeth are small and coarsely granular; the gill-openings are very wide; the body is more or less compressed and scaleless; the dorsal and anal fins are opposed to each other; no adipose dorsal fin. In Rondeletia, the eyes are moderately large, and ventral fins, with 5 rays, are present; in Cetomimus, the eyes are very small, and ventral fins are absent.

{615}FAM. 9. CHIROTHRICIDAE.—Praemaxillaries delicate and styliform, completely excluding the maxillaries from the upper border of the mouth; jaws with feeble dentition or toothless; opercular apparatus complete. Praecaudal vertebrae with robust parapophyses, to which ribs are attached. Ventral fins far forwards.

These Fishes, of which three fossil genera are known from the Cretaceous of Germany and Syria, appear to be related to the Scopelidae, from which the strong parapophyses distinguish them. Chirothrix is remarkable for its excessively enlarged ventral fins with about 17 rays; these fins were taken for the pectorals by the early describers. In Telepholis and Exocoetoides, the ventral fins are smaller than the pectorals, and formed of 7 or 8 rays only; the dorsal region, in the former, is protected by a covering of small, thin, rounded or polygonal dermal scutes, each bearing a median tubercle.

FAM. 10. KNERIIDAE.—Margin of the upper jaw formed by the praemaxillaries; mouth toothless, not protractile. Parietals separated by the supraoccipital. Pharyngeal bones toothless. Praecaudal vertebrae with parapophyses. Body covered with small scales. Ventrals with 9 rays. No adipose dorsal fin. Air-bladder present.

{616}The genus Kneria comprises two species from the fresh waters of tropical Africa, one from Angola, the other from East Africa. Small Loach-like Fishes, two to four inches long, with the upper jaw projecting beyond the mouth, which is inferior and transverse; no barbels; gill-membranes entirely grown to the isthmus, the gill-opening being a rather narrow vertical slit; dorsal and anal fins short, the former opposite, or nearly opposite, to the ventrals; the snout of the male(?) of K. angolensis is described as beset with small spine-like excrescences; the intestinal tract makes several convolutions.

FAM. 11. CYPRINODONTIDAE.—Mouth protractile, the maxillaries excluded from the oral border; teeth in the jaws and on the pharyngeal bones; pterygo-palatine arch weak or rudimentary; opercular bones all well developed. Basis cranii simple. Praecaudal vertebrae with strong parapophyses, bearing the ribs; epipleurals inserted on the ribs. Post-temporal forked. Ventrals, if present, with 5 to 7 rays. No adipose dorsal fin. Air-bladder sometimes absent.

From a physiological point of view, this Family may be divided into carnivorous forms, with short digestive tract, and phytophagous or limnophagous ones, in which the intestine forms numerous coils. To the first division belong the living genera Cyprinodon, Characodon, Tellia, Haplochilus, Fundulus, Rivulus, Cynolebias, Orestias, Empetrichthys, Jenynsia, Pseudoxiphophorus, Belonesox, Gambusia, Anableps, among existing forms, and the fossil genera Prolebias (Oligocene and Miocene) and Pachylebias (Miocene); to the second, the living genera Poecilia, Mollienesia, Platypoecilus, and Girardinus. Procatopus, a near ally of Haplochilus, recently discovered in South Cameroon, is remarkable for having the ventral fins inserted far forward, below the pectoral fins.

These are small or very small Fishes, only a few reaching a length of about a foot, confined to fresh or brackish waters, recognisable externally by the flat head with protractile mouth, the usually large scales, and the absence of a well-developed lateral {617}line. The teeth vary much in shape: cardiform, villiform, or compressed, and bi- or tri-cuspid; the palate is either toothless, or teeth are present on the vomer. About 200 species are known, mostly from the American continent, only about 30 being known from other parts of the world, viz. Southern Europe, Southern Asia and Japan, and Africa. In many species the sexes are dissimilar, the female being larger and less brilliantly coloured, with smaller fins; the anal fin of the male may be modified into an intromittent organ by means of which internal fertilisation takes place, the ova developing in a sort of uterus, which the young leave in a more or less advanced stage of growth. The most curious of the Cyprinodontids is the genus Anableps, of Central and South America, surface-swimming Fishes, the strongly projecting eyes of which are divided by a horizontal band of the conjunctiva into an upper part adapted for vision in the air, and a lower for vision in the water, and the pupil is divided into two parts by a constriction; the larger species grows to the length of a foot.

{618}FAM. 12. AMBLYOPSIDAE.—Mouth scarcely protractile, the maxillaries excluded from the oral border; teeth small, in jaws and palate, and on the pharyngeal bones. Praecaudal vertebrae with very strong parapophyses, bearing the ribs on their upper surface; epipleurals inserted on the ribs. Ventral fins rudimentary or absent. Vent jugular, close to the gill-clefts. Air-bladder present.

Small ovoviviparous Fishes, closely related to, and evidently derived from, the Cyprinodontids, measuring from 1 to 5 inches, inhabiting ditches and small streams, or confined to subterranean waters of limestone caves, in the United States east of the Rocky Mountains. Six species, referable to three genera, are known. In Chologaster, the eyes are well developed and the body is coloured. C. cornutus inhabits the lowland streams and swamps of the South Atlantic States, from Virginia to Florida; C. agassizii is found in the underground streams of Kentucky and Tennessee; and C. papilliferus occurs under stones in the springs of south-western Illinois. Amblyopsis and Typhlichthys, which are evidently derived from the former, or from forms closely related to it, have the eyes rudimentary and more or less concealed under the skin, and the body is colourless. Amblyopsis spelaea is widely distributed in the caves east of the Mississippi, {619}both north and south of the Ohio River; it is common in the River Styx of the Mammoth Cave. Typhlichthys subterraneus is found with the latter species in the caves east of the Mississippi, but is confined to the south side of the Ohio River, whilst T. (Troglichthys) rosae is found in the caves west of the Mississippi River. Of Amblyopsis spelaea, the late Professor Cope has observed: "If these Amblyopses be not alarmed, they come to the surface to feed, and swim in full sight, like white aquatic ghosts. They are then easily taken by the hand or net, if perfect silence is preserved, for they are unconscious of the presence of an enemy except through the medium of hearing; this sense, however, is evidently very acute, for at any noise they turn suddenly downwards, and hide beneath stones, etc., on the bottom." Dr. Garman thinks, on the contrary, that such a sense can hardly be developed in recesses where we are accustomed to think any sounds other than those made by the rippling or dripping water are almost unknown, and that it is through the sense of touch, and not through hearing, that the Fish is disturbed. In fact, the head is provided with a great number of tactile papillae, arranged in transverse ridges, provided with nervous filaments, which evidently compensate the loss of the visual organ.

FAM. 13. STEPHANOBERYCIDAE.—This Family has hitherto been placed near the Berycidae, among the Acanthopterygii, but there are no spinous rays in the dorsal and anal fins; and the ventrals, formed of one simple and four or five branched rays, are abdominal. The genus Stephanoberyx, with two species from the Atlantic, at depths of 535 to 2949 fathoms, is characterised by a large, thick, cavernous head, with thin bony spine-bearing ridges, a large mouth bordered by the protractile praemaxillaries, behind which are the large maxillaries, a short dorsal and a short anal, opposed to each other behind the ventrals, and the body covered with feebly imbricated scales, each bearing in the centre one or several erect spines. The largest specimen measures 6 inches. Malacosarcus, a small Fish from the Pacific, at depths of 2350 and 2425 fathoms, is very closely allied to Stephanoberyx, but its scales are very thin and cycloid. The striking resemblance which the head {620}bears to that of the Berycid Melamphaes may be merely a case of convergence, and it must be borne in mind that this appearance is approached by some species of Scopelus, with which both Malacosarcus and Melamphaes were originally confounded. The praecaudal vertebrae are provided with parapophyses. I have ascertained on a specimen of Stephanoberyx monae that the air-bladder is connected with the dorsal side of the stomach by a short and comparatively wide duct.

FAM. 14. PERCOPSIDAE.—Margin of the upper jaw formed by the praemaxillaries; mouth small, not protractile, toothed; palate toothless. Supraoccipital in contact with the frontals, separating the small parietals. Basis cranii simple. Most of the praecaudal vertebrae with parapophyses, on the upper surface of which the ribs are inserted; no epipleurals. Post-temporal forked; post-clavicle present; scapular foramen in the scapula, on which three hour-glass-shaped pterygials are inserted, a fourth being inserted on the coracoid. Dorsal fin with two true spines; anal with one or two; ventrals far forward, with 9 rays; pectorals inserted rather high. A small adipose dorsal fin. Body covered with strongly ctenoid scales. Air-bladder present (with open duct).

This is a most interesting group of Fishes, from the resemblance which they bear to the Perches, and they have therefore been raised to the rank of a sub-order, Salmopercae, by Jordan and Evermann, who regard them as "archaic fishes, relics of some earlier fauna, and apparently derived directly from the extinct transitional forms through which the Haplomi and Acanthopteri have descended from allies of the Isospondyli [Malacopterygii]." On the other hand, an analysis of their characters shows them to belong to the Haplomi, of which they {621}may be regarded as highly specialised members, having evolved in the direction of the Acanthopterygii.

Only two genera are known, each with a single species: Percopsis, from the rivers and streams of Canada and the north-eastern United States, and Columbia, more recently discovered in the sandy or weedy lagoons along the Columbia River. These Fishes are of small size, not exceeding 6 inches in length. Their eggs are unusually large.

SUB-ORDER 6. HETEROMI.

Air-bladder without open duct. Opercle well developed; parietal bones separating the frontals from the supraoccipital. Pectoral arch suspended from the supraoccipital or the epiotic, the post-temporal small and simple or replaced by a ligament; no mesocoracoid. Ventral fins abdominal, if present.

The Halosauridae and Notacanthidae are deep-sea Fishes of obscure affinities. In the abdominal position of the many-rayed ventral fins and in the absence of the mesocoracoid arch they agree with the Haplomi; but if, as the investigations of Günther lead us to believe, there is really no open communication between the air-bladder and the digestive tract, they {622}should be removed from this physostomous sub-order. The two families have many characters in common, such as the attachment and structure of the pectoral arch, which is devoid of a post-clavicle, the position of the pectoral fins high up the sides, the strong parapophyses inserted very low down on the centra of the vertebrae, the extent of the parietal bones, which meet in a sagittal suture and separate the frontals from the supraoccipital. The recent discovery of a third family, the Lipogenyidae, which, in the structure of the dorsal fin, is exactly intermediate between the two others, has lessened the gap between the Lyomeri (Halosauridae) and Heteromi (Notacanthidae) of Gill, which I have proposed to unite in a suborder under the latter name.

These Fishes are no doubt derived from forms in which a separate caudal fin existed; such a type must have been near the Dercetidae, as defined by A. S. Woodward, which may provisionally be placed here.

An imperfectly known Fish from the Chalk of Mount Lebanon, Pronotacanthus sahelalmae, appears to bear some affinity to Notacanthus, and has been placed in the same family; but its characters are not sufficiently defined to refer it without doubt to this division.

There is a fifth family which may enter this sub-order: the Fierasferidae, the structure of which has been exquisitely described and figured by Emery. Hitherto placed with or near the Ophidiidae, they differ widely from them, as well as from all Acanthopterygians, in the conformation of the skull, the supraoccipital being separated from the frontals by the parietals, which form a long median suture. This is a feature which has only been observed in Fishes with abdominal ventral fins, and although the total absence of those fins in Fierasfer deprive us of an important criterion in deciding on its affinities, I am inclined to regard this family as derived from an "abdominal" type. The conformation of the pectoral arch has much in common with that of the Halosaurs, and, notwithstanding the interpretation that has been given to the bones at the back of {623}the cranium in the latter type, the same may be said, in a general way, of the skull.

As pointed out by Emery, the very anterior position of the vent in the Fierasferidae is directly related to the curious mode of life of these Fishes, and the analogous condition obtained in various families, such as the Gymnotidae, Nemichthyidae, Amblyopsidae, shows it to be a character of relatively small systematic importance.

SYNOPSIS OF THE FAMILIES.

A. Vent posterior.

a. A distinct caudal fin; ordinary scales small or wanting, but enlarged scutes along the side 1. Dercetidae.†

b. Tail tapering to a point; scales cycloid.

No spines; dorsal fin short, anal very long 2. Halosauridae.

Fins with spines, dorsal short, anal long 3. Lipogenyidae.

Dorsal fin formed of a series of spines, anal long, formed partly of spines and partly of soft rays 4. Notacanthidae.

B. Vent immediately behind the gill-opening; no caudal fin; scales absent 5. Fierasferidae.

FAM. 1. DERCETIDAE.—Body much elongate; ordinary scales small or wanting, but two or more continuous series of enlarged scutes along each side; mouth large, praemaxillaries apparently forming the greater part of the upper border of the mouth, which is toothed; opercular apparatus complete. Dorsal fin more or less extended, without spines; anal short, caudal separate; ventrals with not less than 7 or 8 rays.

Dercetis, Leptotrachelus, Leptecodon, Pelargorhynchus, and Stratodus, from the Upper Cretaceous of Europe, Syria, and North America.

FAM. 2. HALOSAURIDAE.—Body elongate, covered with cycloid scales, the tail tapering to a point, without caudal fin; head with scales; mouth moderate, bordered by the praemaxillaries and the maxillaries, both toothed; suborbitals large; praeopercle rudimentary. Dorsal fin short, formed of soft rays, above or a little behind the ventrals, which are rather far back, and formed of 9 or 10 rays; anal very long, without spines, extending to the end of the tail. Ovaries transversely laminated, the ova falling into the abdominal cavity. Some 10 living species are known, referred to three genera, inhabiting the Atlantic, Pacific, and Indian Oceans, at depths of 500 to 1400 fathoms.

{624}In Halosaurus the scales of the lateral line, which runs near the lower profile, are scarcely enlarged, and are destitute of luminous organs. Halosaurichthys differs in the union of the ventral fins with each other, as in Notacanthus. In Halosauropsis the scales of the lateral line are strongly enlarged and pouch-like, and bear photophores.

This family is one of great antiquity, being represented in the Upper Cretaceous of Westphalia by Echidnocephalus, which, as shown by A. S. Woodward, appears to have been closely related to Halosaurus.

FAM. 3. LIPOGENYIDAE.—Similar to the preceding in shape and in the position of the dorsal fin, but with a toothless, roundish, inferior, suctorial mouth, and with the short dorsal and the long anal formed partly of spines and partly of soft rays. Head and body covered with minute scales; lateral line nearer the dorsal than the ventral profile. Ventrals with 3 spines and 7 soft rays. A single species, Lipogenys gillii, from the North Atlantic, 865 fathoms.

FAM. 4. NOTACANTHIDAE.—Body elongate, covered with very small cycloid scales, the tail tapering to a point, without caudal fin; head scaly; mouth small, inferior, bordered by the praemaxillaries only; jaws toothed; no suborbitals; praeoperculum small; post-temporal replaced by ligament. Dorsal fin formed of a series of short disconnected spines; anal very long, formed partly of spines and partly of soft rays, extending to the end of the tail. Ventrals with 1 to 5 spines and 7 to 10 soft rays.

{625}Two genera: Notacanthus, with the ventrals connate or confluent and with 6 to 12 dorsal spines; and Polyacanthonotus, with the ventrals separated and 27 to 38 dorsal spines. Nine species, from the Mediterranean, the Atlantic, and the Pacific, at depths of 400 to 1875 fathoms.

FAM. 5. FIERASFERIDAE.—Body elongate or extremely attenuate, naked, the tail tapering to a point or truncate, without distinct caudal fin; mouth small, inferior, bordered by the praemaxillaries; jaws toothed; no suborbitals; praeoperculum well developed. Dorsal and anal fins very long, extending to the end of the tail, and formed entirely of soft rays. Ventral fins absent. Vent situated immediately behind the gill-opening. Air-bladder with a muscular apparatus for dilatation of its anterior part.

A single genus, Fierasfer, with about 10 species, distributed over nearly all the warm and tropical seas, rarely found as far north as the west coast of Ireland. Encheliophis, without pectoral fins, is the larval form of Fierasfer.

Fierasfer spends the greater part of its existence in the interior of Holothurians and other Echinoderms as well as in bivalve Mollusca. It has been observed to enter Holothurians by the posterior or anal aperture, either head first or tail foremost, in the latter case availing itself of the suction which takes place alternately with the expulsion of water by that orifice; it remains near the anus, from which it projects its head in search of food outside its host. It is neither a true parasite nor a commensal or mutualist, in the sense given to these terms by Van Beneden, but simply a lodger, "inquilino," as Emery puts it. Semper, however, regards Encheliophis vermicularis as a true parasite, feeding on the viscera of the Holothurian in which it lives. Putnam has examined eight specimens of a Fierasfer from the Bay of Panama, which were obtained alive from pearl oysters, and also one beautifully {626}enclosed in a pearly covering deposited upon it by the oyster; a similar specimen is preserved in the British Museum.

SUB-ORDER 7. CATOSTEOMI.

Air-bladder, if present, without open duct. Parietal bones, if present, separated by the supraoccipital. Pectoral arch suspended from the skull; no mesocoracoid arch; coracoid usually very large. Ventral fins, if present, abdominal, or pelvis attached to the coracoid bones.

The mouth is small and bordered by the praemaxillaries or by the praemaxillaries and a small portion of the maxillaries. The air-bladder is present, except in the Solenostomidae and Pegasidae.

Following the suggestions of Kner and Steindachner and Cope to their logical conclusion, A. S. Woodward, in his valuable catalogue of the Fossil Fishes in the British Museum, has united the Lophobranchs of Cuvier with the Hemibranchs of Cope, a course which seems fully justified, and has received {627}further support from the recent investigations of Swinnerton, who has proposed to unite the two groups under the new name of Thoracostei. The name Phthinobranchii has also been suggested by O. P. Hay for the same association. The structure of the Lophobranchs (Solenostomidae and Syngnathidae) shows that these fishes are only extremely specialised forms of the group of which the Sticklebacks are the well-known type, and the character of the "tufted" gills alone is surely not of sufficiently great importance to warrant the retention of the Lophobranchii as a division equivalent to the sub-orders adopted in the present classification. Besides, as recently pointed out by A. Huot, there is no fundamental difference, but only one of degree, between the so-called tufted gill and the normal type; each "tuft" corresponds to one branchial lamella, and at a certain stage of development the disposition of the branchial lamella is the same in a Syngnathus and in an ordinary Teleostean. I have recently attempted to show that the Lamprididae are related to the Hemibranchii, although sufficiently distinct to warrant the establishment of a division, named Selenichthyes.

SYNOPSIS OF THE FAMILIES.

I. Praeoperculum and symplectic distinct; branchial apparatus fully developed; gills pectinated; mouth terminal, toothless; post-temporal forked, free; pelvic bones connected with scapular arch; ventral fins with 15 to 17 rays; ribs long, sessile; fins without spines (SELENICHTHYES) 1. Lamprididae.

II. Praeoperculum and symplectic distinct, latter much elongate; branchial apparatus more or less reduced; gills pectinate; post-temporal simple, immovable; mouth terminal (HEMIBRANCHII).

A. Mouth toothed.

1. Pelvic bones usually connected with scapular arch; spinous dorsal represented by isolated spines.

Snout conical or but slightly tubiform; ventral fins with 1 spine and 1 or 2 soft rays; ribs slender, free; anterior vertebrae not enlarged 2. Gastrosteidae.

Snout tubiform; ventral fins with 1 spine and 4 soft rays; {628} ribs flattened, fused with the lateral bony shields; anterior vertebrae not enlarged 3. Aulorhynchidae.

Snout tubiform; ribs slender, free; first vertebra enlarged 4. Protosyngnathidae.

2. Pelvic bones not connected with scapular arch; ventrals without spine, with 5 or 6 rays; snout tubiform; first vertebra very elongate, formed by the fusion of several.

Isolated dorsal spines; body scaly 5. Aulostomatidae.

No dorsal spines; body naked 6. Fistulariidae.

B. Mouth toothless; snout tubiform; two short dorsal fins, the first with a few spines; ventral fins with 3 to 5 rays; anterior vertebrae elongate.

Body covered with bony shields and small rough scales 7. Centriscidae.

Body completely cuirassed by bony shields which are fused with the endoskeleton 8. Amphisilidae.

III. Praeoperculum absent; symplectic much elongate; branchial apparatus more or less reduced; gill-lamellae reduced in number and enlarged, forming rounded lobes; post-temporal simple, immovably attached to the skull; mouth toothless, at the end of a tubiform snout; body covered with bony plates (LOPHOBRANCHII).

Two dorsal fins; ventral fins present, with 7 rays; gill-openings wide; exoskeleton of large star-like plates 9. Solenostomidae.

A single dorsal fin; no ventral fins; gill-openings very small; exoskeleton in the form of rings 10. Syngnathidae.

IV. Praeoperculum and symplectic absent; gills pectinated; mouth inferior, toothless; body entirely covered with bony plates; ventral fin with 2 or 3 rays (HYPOSTOMIDES) 11. Pegasidae.

FAM. 1. LAMPRIDIDAE.—Body short and deep, with minute scales. Snout short; mouth toothless, bordered by the praemaxillaries and, to a small extent, by the maxillaries; opercular bones well developed. Gills four, pectinated; branchial apparatus fully developed. Post-temporal bone forked. Vertebrae very numerous (21 + 25), without transverse processes; ribs strong, long. Fins without spines; dorsal and anal elongate. Pectoral fins with very short pterygials folding downwards against the body. Pelvic bones connected with the coracoids, which are very large, and do not form a suture at their ventral extremity. Ventral fins with 15 to 17 rays.

The Opah or King-Fish (Lampris luna), the sole representative of this family, is remarkable for its large size (growing to a length of four feet) and its vivid colours. Its flesh is rich, and intermediate between that of the Salmon and that of the Tunny. It is a pelagic fish of wide distribution, known from the North {629}Atlantic and Mediterranean and from distant points in the Pacific; specimens are occasionally captured on our coasts. It feeds on other fish, but little is known of its habits and nothing of its development.

The affinities of the Lamprididae are very doubtful. Lampris has usually been placed with the Acanthopterygians, a view which is still upheld by Gill. I now agree with this high authority in regarding the bone which I took for an infraclavicle as a much developed coracoid, and the bone termed by me the coracoid as a pterygial. But it has also been shown, by Starks, that such a thing as an infraclavicle does not exist in the Stickleback, the bone so-called being only a part of the coracoid; and as in most of the Sticklebacks the pelvic bones join the latter, the resemblance between them and Lampris remains. As I have previously pointed out, the absence of spines in the fins, and the position of the ventral fins, together with the great number of rays in the latter, which is only met with in the lower Teleosteans, are characters which necessitate the removal of Lampris from the Acanthopterygians, and I cannot find a better place for them than near the Gastrosteidae.

The whole question of the arrangement of the Physoclists with abdominal ventrals (Catosteomi and Percesoces) is, I feel, much in need of revision, and it may be found advisable to break up this group into a greater number of sub-orders, in which case the Selenichthyes would stand by themselves; the Hemibranchii and Lophobranchii would be united under the former name, as proposed by Woodward, or under that of Thoracostei (Swinnerton) or Phthinobranchii (Hay). The position in the system of the Pegasidae is still somewhat doubtful. This family is regarded by some authors as related to the mail-cheeked Acanthopterygians.

FAM. 2. GASTROSTEIDAE.—Body more or less elongate, naked or protected by bony shields, tapering to a slender caudal peduncle. Head moderate, with short or elongate and tubiform snout; mouth small, terminal, toothed; opercular bones well developed; suborbitals in contact with praeoperculum, protecting the cheek. Gills four, pectinated. Praecaudal vertebrae with strong transverse processes and slender, free ribs. Spinous dorsal represented by isolated spines. Pectoral fins with short {630}pterygials. Pelvic bones usually connected with scapular arch. Ventral fins with one spine and one or two soft rays.

Four genera: Gastrosteus, Apeltes, Eucalia, Spinachia.

The little Three-spined and Two-spined Sticklebacks (Gastrosteus aculeatus and G. pungitius), which include many varieties that have been regarded as distinct species, are among the best known of our British Fishes. They are remarkable for the perfect indifference with which they can be transported from fresh into salt water, and vice versa, and for the elaborate nests which the males build in fresh or brackish water, and over which they watch with the greatest vigilance after the female has deposited her relatively large eggs. These nests are made of weeds and twigs fastened together by threads secreted by the kidneys of the male. The {631}larger fifteen-spined Stickleback (Spinachia vulgaris) is entirely marine; its nests are to be found on our coasts in sheltered rock-pools, and they are made chiefly of sea-weeds and Hydrozoa. Sticklebacks are short-lived, and are believed to breed only once.

The Gastrosteidae are restricted to the northern hemisphere, being more abundant in the higher latitudes, extending to Iceland, Greenland, and Bering Straits; the southernmost points of their distribution are Algeria in the Old World, and Lower California in the New.

A very large number of species have been described, but probably only about a dozen deserve to stand.

FAM. 3. AULORHYNCHIDAE.—The genera Aulorhynchus and Auliscus, each with one species from the Northern Pacific, much resemble Spinachia in outward form and in the equal size of the anterior vertebrae, but the snout is still more produced, tubiform, and the ventral fins are formed of one spine and four soft rays. The difference which justifies their separation as a distinct family resides in the disposition of the ribs, which are flattened and ankylosed to the lateral bony shields.

FAM. 4. PROTOSYNGNATHIDAE.—This family appears to be intermediate between the Gastrosteidae and the Aulostomatidae, agreeing with the former in possessing slender, free ribs, with the latter in having the first vertebrae elongate, though to a {632}less degree than in Aulostoma. Its only representative is Protosyngnathus sumatrensis, from a Tertiary freshwater formation in Sumatra, which has been referred, without adequate grounds, to Aulorhynchus or Auliscops.

FAM. 5. AULOSTOMATIDAE.—Allied to the Aulorhynchidae, differing in the ventral fins devoid of spines, formed of 5 or 6 rays, widely removed from the pectoral arch, the very elongate, saddle-shaped anterior vertebra formed by the fusion of several, the large supratemporals produced backward over the anterior vertebra, the very elongate pterygials of the pectoral fin, and the compressed body covered with small ctenoid scales. Ribs are rudimentary or absent. No suborbitals. The snout is long, tubiform; the small terminal mouth bears bands of minute teeth, and the lower jaw has a small barbel at the symphysis. A single genus, Aulostoma, with two species from the Atlantic coasts of tropical America, and two from the Eocene and Miocene of Europe. A. coloratum grows to a length of 26 inches.

FAM. 6. FISTULARIIDAE.—Body greatly elongate, naked. First vertebra much elongate, formed by the fusion of several; strong transverse processes to the ribs in front and behind, those of two vertebrae suturally united; ribs rudimentary or absent. Supratemporal much produced posteriorly, extending over the anterior vertebrae; suborbitals absent; snout forming a long tube, which terminates in a narrow mouth with minute teeth. Spinous dorsal entirely absent. Pterygials of pectoral fin very elongate. Ventral fins very small, with 6 soft rays, inserted far behind the pectoral girdle.

The Flute-mouths, Fistularia, which Dr. Günther describes as "gigantic marine Sticklebacks living near the shore, from which they are frequently driven into the open sea," are represented by three species, from the tropical and subtropical parts of the Atlantic and Indo-Pacific. The middle rays of the forked caudal fin are produced into a long filament. The largest species, F. tabaccaria, reaches a length of 6 feet. The same genus is represented by two species in the Upper Eocene and Oligocene of Europe, and Urosphen, from the Upper Eocene, is regarded as allied to it.

{633}FAM. 7. CENTRISCIDAE.—Body moderately elongate, partially enclosed in a bony armour, which is distinct from the endoskeleton. Anterior vertebrae elongate, with strong parapophyses ankylosed to the exoskeleton; no ribs. Suborbitals absent; snout forming a long tube, with small, terminal, toothless mouth. Two dorsal fins, the anterior with a very strong spine. Pterygials of pectoral fin very small. Ventral fins small, with 4 or 5 rays, the pelvic bones in contact with the postclavicles.

Centriscus, with five species in the Atlantic and Pacific Oceans, represents this family at the present day. C. scolopax has occasionally been found on the English coast. Isolated spines from the Pliocene of Tuscany have been referred to the same genus. Rhamphosus, from the Eocene of Monte Bolca, is believed to have been allied to Centriscus.

FAM. 8. AMPHISILIDAE.—Near the preceding, but body extremely compressed and completely enclosed in a thin bony armour which is fused with the endoskeleton; the caudal region, much abbreviated, is free and relegated to the ventral surface, the body terminating in the two dorsals, of which the first bears a strong spine. The ventral fins are far back, very small, formed of 3 or 4 rays.

Amphisile is represented by three or four recent species in the Indian and Pacific Oceans, and two are known from Upper Eocene and Oligocene beds in Europe. Dr. Arthur Willey has observed these fishes in the Southern Pacific. A. strigata "lives in small shoals of about half-a-dozen individuals, and swims about with rapidity in a vertical position, cleaving the water with its razor-shaped body."

FAM. 9. SOLENOSTOMIDAE.—Body moderately elongate, with large star-like ossifications. Anterior vertebrae elongate, without transverse processes; no ribs. Snout much produced, tubiform; mouth, small, terminal, toothless; no praeoperculum; symplectic elongate; gill-opening wide; gill-lamellae small rounded lobes. Two short dorsal fins, the rays of the anterior not articulated, flexible spines. Pterygials of pectoral fin very small. Ventral fins large, with 7 rays, behind the pectoral arch. No air-bladder.

The unique genus, Solenostomus, with three or four species from the Indian and Pacific Oceans, may be regarded as in many respects intermediate between the Centriscidae and the Syngnathidae. In the female the inner side of the ventral fins coalesces {634}with the integuments of the body, forming a large pouch for the reception of the eggs.

Solenorhynchus, from the Upper Eocene of North Italy, probably belongs to this family, but its form is much more elongate, and the exoskeleton is in regular rings.

FAM. 10. SYNGNATHIDAE.—Body more or less elongate, protected by an exoskeleton forming rings. Anterior vertebrae not elongate; parapophyses strong, ankylosed to the exoskeleton; no ribs. Snout much produced, tubiform; mouth small, terminal, toothless; no praeoperculum; symplectic elongate; gill-cleft reduced to a very small opening near the upper posterior angle of the gill-cover; gill-lamellae small rounded lobes. A single dorsal fin. Pectoral fins, if present, with very small pterygials; ventrals absent. Caudal fin often absent; tail sometimes prehensile.

This family embraces about 175 marine species, and is represented over the greater part of the world. Principal genera: Siphonostoma, Syngnathus, Penetopteryx, Ichthyocampus, Nannocampus, Osphyolax, Urocampus, Doryichthys, Coelonotus, Stigmatophorus, Nerophis, Protocampus, Gastrotoceus, Solenognathus, Hippocampus, Acentronura, Phyllopteryx.

Remains are found in the Upper Eocene and Miocene beds of Europe, and have been referred to Siphonostoma and Syngnathus, and to the extinct genus Calamostoma. It is probable that Pseudosyngnathus, from the Upper Eocene of Monte Bolca, is the type of a distinct family.

The best known members of this family are the Needle-Fish or Pipe-Fish (Siphonostoma and Syngnathus) and Sea-Horse (Hippocampus) of our coasts. The latter, like Amphisile, swims with the body in a vertical position. In most species the male takes charge of the eggs, in a pouch under the tail (Siphonostoma, {635}Syngnathus, Penetopteryx, Nannocampus, Stigmatophorus, Hippocampus), in a groove under the tail (Phyllopteryx), or in a groove on the abdomen (Doryichthys, Coelonotus, Nerophis, Gastrotoceus).

An Australian species of Syngnathus has been described by E. P. Ramsay under the name of S. intestinalis, from its living inside Holothurians, in the manner of Fierasfer, and G. Lunel has observed a Doryichthys to offer a similar instance of inquilinism.

One of the most remarkable types of Syngnathids is Phyllopteryx, from Australia. The spines and knobs of the head and body are furnished with dermal appendages, which closely imitate the fucus among which they live.

FAM. 11. PEGASIDAE.—Body short or moderately elongate, encased in an exoskeleton forming rings. Anterior vertebrae not elongate; no ribs. Snout produced beyond the mouth, which is small, inferior, and toothless; no praeoperculum, no symplectic; gill-opening very small; gills pectinated. A single dorsal fin. Pectoral fins large, horizontal; ventrals reduced to one or two filamentous rays, behind the scapular arch. Air-bladder absent.

{636}Five or six species, referable to two genera, Pegasus and Parapegasus, make up this family. They are very small fishes, inhabiting the coasts of China, Japan, Arabia, the Malay Archipelago, and Australia. Pegasus is remarkable among all fishes in having the five anterior rays of the pectoral fin transformed into strong spines.

SUB-ORDER 8. PERCESOCES.

Air-bladder, if present, without open duct. Parietal bones separated by the supraoccipitaL Pectoral arch suspended from the skull; no mesocoracoid arch. Ventral fins, if present, abdominal, or at least with the pelvic bones not solidly attached to the clavicular arch.

This group connects the Haplomi with the Acanthopterygii, the Scombresocidae being somewhat related to the Cyprinodonts, whilst the Anabantidae show distinct affinity to the Osphromenidae in the following sub-order. Other families, previously included among the Scombriform Acanthopterygians, are placed here on the assumption that the loose attachment of the pelvic bones to the clavicles is a primitive character, and not the result of degeneration, such as occurs in some cases among true Acanthopterygians. Although this sub-order is perhaps only an artificial association, it must be borne in mind that, notwithstanding the very wide divergence which exists between the first and last families, and however dissimilar their members may appear to be at first sight, a gradual passage may be traced connecting the most aberrant types.

SYNOPSIS OF THE FAMILIES.

I. Ventral fins, if present, inserted far behind the pectorals; no spines to the fins.

Ribs attached to the extremity of much-developed parapophyses; lower pharyngeal bones completely united; pectoral fins inserted very high up 1. Scombresocidae.

Ribs mostly sessile; lower pharyngeal bones distinct; pectoral {637} fins nearer the ventral than the dorsal line 2. Ammodytidae.

II. Ventral fins, if present, more or less approximated to the pectorals.

A. Two well-developed dorsal fins, the anterior small and formed, at least in part, of spinous rays.

1. Ribs attached to strong parapophyses.

Pelvic bones free or connected with the clavicles by ligament; pectoral fins inserted high up 3. Atherinidae.

Pelvic bones suspended from the postclavicles; pectoral fins inserted very high up; teeth very feeble or absent 4. Mugilidae.

Pelvic bones suspended from the postclavicles; pectoral fins low down, with detached lower rays 5. Polynemidae.

Pelvic bones connected with the clavicles by ligament; pectoral fins nearer the ventral than the dorsal line; dentition powerful, cardiform; scales minute or absent 6. Chiasmodontidae.

2. Anterior ribs sessile; pelvic bones not connected with the scapular arch; pectoral fins nearer the ventral than the dorsal line 7. Sphyraenidae.

B. Spinous dorsal, if present, connected with the soft.

1. Anterior vertebrae without parapophyses; scales on head, if present, small.

Oesophagus with lateral sacs which are beset with papillae internally; spinous dorsal long; scales rhomboidal, in oblique transverse series; pelvic bones free 8. Tetragonuridae.

Oesophagus with lateral sacs which are beset with toothed papillae internally; spinous dorsal, if distinct, shorter than the soft dorsal; scales moderate or small, cycloid, often deciduous 9. Stromateidae.

No sacs in the oesophagus; fins without spines; scales very small or absent 10. Icosteidae.

2. All, or all but the anterior two vertebrae with parapophyses; scales on head large; a superbranchial cavity.

No spines to the fins 11. Ophiocephalidae.

Strong spines to the dorsal, anal, and ventral fins 12. Anabantidae.

FAM. 1. SCOMBRESOCIDAE.—Maxillary entering the border of the upper jaw; dentition moderately strong or feeble. Lower pharyngeal bones united. Praecaudal vertebrae with strong parapophyses supporting the ribs. Body covered with cycloid scales. Pectoral fins inserted very high up; ventral fins widely separated from the pectorals, without spines, with 6 rays. Dorsal fin opposed to the anal, and likewise formed entirely of soft rays. Air-bladder generally present, sometimes cellular.

The shape of the head and body vary greatly, and the pectoral fin may reach an extraordinary wing-like development. The dorsal fin may be followed by a series of finlets, as in many of the Scombridae. Most of the Scombresocidae, of which about 200 species are known, are marine; some are carnivorous, {638}others (Hemirhamphus) mainly herbivorous, feeding on green algae. Nearly all are in the habit of making great leaps out of the water, this tendency culminating in the Flying-Fish (Exocoetus), which skip or sail through the air in a manner the explanation of which has given rise to much controversy. According to the latest evidence the sole source of motive power is the action of the strong tail while in the water; no force is acquired while the fish is in the air. The pectorals are not used as wings but as parachutes. There is every passage between the small pectoral fin of a Saurie (Scombresox) or a Hemirhamphus and the swallow-like wings of the most developed Exocoetus. The genus Hemiexocoetus is a very remarkable connecting form. The Gar-Pike (Belone), of which one species is common on our coasts, have both jaws produced into a long slender beak; the bones are green. In Hemirhamphus the lower jaw only is prolonged; some of the species, living in fresh water, are viviparous, the anal fin being modified into a copulatory organ, as in many Cyprinodonts.

Scombresocidae occur in all the tropical and temperate seas. Belone, Scombresox, and Hemirhamphus are found in Upper Eocene and Miocene beds of Europe, and, as stated above, Protaulopsis should perhaps be referred to this family.

{639}FAM. 2. AMMODYTIDAE.—Maxillary excluded from the border of the upper jaw; mouth protractile; dentition feeble or absent. Lower pharyngeal bones separate. Praecaudal vertebrae without parapophyses. Body covered with very small cycloid scales. Pectoral fins nearer the ventral than the dorsal line; ventral fins, if present, widely separated from the pectorals, without spine, with 6 rays. Dorsal and anal fins more or less elongate, formed of soft rays. Air-bladder absent.

The existing genera, Ammodytes, with 8 species, from the temperate coasts of the northern hemisphere, and Hypoptychus, from northern Japan, with a single species, are deprived of ventral fins, and their exact relations remained obscure until the structure of the Oligocene Cobitopsis revealed their affinity to the Scombresocidae, or at least their pertinence to the present suborder. The Greater Sand-Eel or Launce (Ammodytes lanceolatus) and the Lesser Sand-Eel (A. tobianus) are common on our coasts, and are remarkable for the manner in which, by means of their sharp-pointed snout, they bury themselves with great rapidity in the sand, darting in and out like arrows.

FAM. 3. ATHERINIDAE.—Maxillary excluded from the border of the upper jaw; dentition more or less developed. Body covered with cycloid or ctenoid scales. Ribs attached to strong parapophyses. Pectoral fins inserted high up; ventral fins more or less approximated to the pectorals, with one spine and five soft rays; pelvic bones connected with the clavicular symphysis by a ligament. Two well-separated dorsal fins, the anterior small and formed, at least in part, of spinous rays. Air-bladder present.

Carnivorous Fishes, mostly marine and of small size, much valued as food, and distributed along the coasts of most tropical and temperate seas; some inhabit fresh waters. A silvery lateral band, or "stole," is usually present. About 65 species are known, referred to 14 genera: Atherina, Iso, Chirostoma, Thyrina, Atherinella, Labidesthes, Atherinopsis, Atherinops, Telmatherina, Neatherina, Pseudomugil, Rhombatractus, Aida, Melanotaenia.

{640}Represented in the Upper Eocene of Europe by several species of Atherina and by the extinct genus Rhamphognathus.

FAM. 4. MUGILIDAE.—Maxillary excluded from the border of the upper jaw; dentition feeble or absent. Body covered with cycloid scales. Ribs attached to the extremity of strong parapophyses. Pectoral fins inserted high up; ventral fins more or less approximated to the pectorals, with one spine and five soft rays; pelvic bones suspended from the post-clavicles. Two well-separated dorsal fins, the anterior formed of a small number of spines. Air-bladder present.

These Fishes are closely related to the preceding, of which they are a further specialisation, the pharyngeal bones having a complicated structure, much reducing the oesophageal opening, and the vertebrae being reduced in number (24 to 26 instead of 32 to 60). They feed on organic matter contained in mud, and inhabit the fresh waters and coasts of the temperate and tropical regions. The species number about 100. Principal genera: Mugil, Myxus, Anostomus, Joturus. Grey Mullets (Mugil) are represented on our coasts by three species, valued as food, one of which (M. capito) has a remarkably wide range, occurring from Scandinavia to the Cape of Good Hope. Remains referred to the same genus occur in the Miocene and Oligocene.

FAM. 5. POLYNEMIDAE.—Maxillary excluded from the border of the upper jaw; dentition feeble. Body covered with ctenoid {641}scales. Ribs attached to the extremity of strong parapophyses. Pectoral fin inserted low down, with a lower portion consisting of free rays; the upper portion, or fin proper, attached to the scapula, the lower to a fenestrate bone which appears to be formed by coalesced pterygials (Fig. 391, pt.). Ventral fin more or less approximated to the pectoral, with one spine and five soft rays; pelvic bones suspended from the post-clavicles. Two well-separated dorsal fins, the anterior formed of a small number of spines. Air-bladder, if present, very large.

The vertebrae number 24 (10 + 14).

Three closely allied genera: Polynemus, Pentanemus, and Galeoides, with about 25 species, from the shores of tropical seas, often entering rivers. Some attain a length of 4 feet, and are valued as food or for the isinglass yielded by their air-bladder. The free pectoral filaments are organs of touch, and can be moved independently of the fins.

FAM. 6. CHIASMODONTIDAE.—The deep-sea genera, Chiasmodon, Pseudoscopelus, and Champsodon, which have been placed either with the Gadidae, the Trachinidae, or the Berycidae, may be referred to the Percesoces, as the pelvic fins have only a ligamentous connexion with the pectoral arch. Unfortunately, the skeleton has only been examined in Champsodon; it is remarkably similar to that of the Atherinidae. As in Atherinichthys, the posterior extremity of the air-bladder is protected by a bony sheath formed by the expanded ring-like haemal processes of the anterior caudal vertebrae. Vertebrae 32 (16 + 16). The {642}scales are absent or very small and spinulose, the mouth large, with cardiform teeth; spinous dorsal short, soft dorsal and anal elongate. Chiasmodon and Pseudoscopelus have a complicated system of sensory organs on the body, which in the latter suggest the photophores of Scopelids. Champsodon vorax is a fish of extreme voracity, swallowing prey much larger than itself. Only four species of this family are known.

FAM. 7. SPHYRAENIDAE.—Maxillary excluded from the border of the upper jaw; dentition very strong. Body covered with cycloid scales. Anterior ribs sessile, the rest inserted on parapophyses. Pectoral fin nearer the ventral than the dorsal outline; ventral fin more or less approximated to the pectoral, with 1 spine and 5 soft rays; pelvis not connected with the pectoral arch. Two well-separated dorsal fins, the anterior formed of a small number of spines. Air-bladder large. Vertebrae 24.

Carnivorous Pike-like Marine Fishes from the tropical and sub-tropical seas, often found at the mouths of rivers. The "Barracudas" form a single genus, Sphyraena, with about 20 species, the largest of which grow to 8 feet and are dangerous to people bathing; many are valued as food, but some are reported to be poisonous, at least at certain seasons. Remains of several species are known from the Eocene and later periods in Europe and North America.

FAM. 8. TETRAGONURIDAE.—Maxillary excluded from the border of the upper jaw; dentition feeble. Oesophagus with lateral sacs which are beset with papillae internally; a series of gill-raker-like knobs below the pseudobranchiae. Body covered with rhomboidal, striated scales in oblique transverse series, those of every single transverse series coherent. Ribs mostly sessile. Pectoral fin nearer the ventral than the dorsal outline. Ventral fin with 1 spine and 5 soft rays, near the pectoral, but pelvis free from the pectoral arch. A long continuous dorsal fin, its anterior portion formed of numerous short spines. Air-bladder absent. Vertebrae 58.

This family includes a single, rather rare fish, Tetragonurus cuvieri, from the Mediterranean and neighbouring parts of the Atlantic and the South Pacific. It is said to descend to great depths at certain seasons, and to feed on Medusae; its flesh is poisonous. Young specimens have been observed by Emery to live in the respiratory cavity of large Salpae.

{643}FAM. 9. STROMATEIDAE.—Although including a number of forms very unlike Tetragonurus in external appearance, there is no doubt that this family, hitherto placed near the Scombridae, is very closely allied to the preceding, agreeing with it in the presence of lateral oesophageal sacs bearing internally papillae (which are besides beset with setiform teeth), and, in most genera, in the presence of a series of knobs, more or less similar to gill-rakers, below the pseudobranchiae. The pelvic bones are sometimes free from the pectoral arch, as in the Tetragonuridae, sometimes more closely attached, but only by ligament, and movable. The principal difference resides in the scales, which are always cycloid and usually very small and more or less deciduous, and in the spinous dorsal being shorter than the soft, or even quite rudimentary. The ventrals are sometimes absent. The air-bladder is present or absent. The number of vertebrae varies from 24 to 46.

Marine Fishes, pelagic or deep-sea, feeding on Crustaceans, Medusae, or the fry of other fish. About 45 species are known, referable to 10 genera: Nomeus, Cubiceps, Psenes, Seriolella, Psenopsis, Centrolophus, Lirus, Stromateus, Peprilus, and Stromateoides. Many of the species have a wide distribution, but are rare in collections. The Black-Fish (Centrolophus niger) and its close ally C. britannicus, and the Rudder-Fish (Lirus perciformis and L. medusophagus), have occurred, at rare intervals, on the British coasts. The Stromateidae were represented by several species in the Cretaceous (Platycormus and Homosoma).

The widely distributed Nomeus gronovii, so remarkable for its enormous ventral fins, folding in a ventral groove, has been observed in New South Wales to be only found on the coast when the Siphonophores called "Portuguese Men-of-War" or Physalia are driven ashore, the fish swimming beneath them, as the young Caranx are in the habit of doing under Medusae. As observed by Waite, the benefit of such a partnership must primarily be with the fish, for it is a voluntary agent, whereas the Physalia has no power of locomotion. "If the fish secures safety from its enemies by entering the area embraced by the deadly tentacles of the Physalia, which attain a length of 10 to {644}12 feet, it must be immune to their influence: a remarkable condition, considering that small fish have often been seen in their stomachs and entangled in their tentacles." This observer adds: "It is probable that, in addition to protection, the fish derives its food from association with the Physalia, much as does the Remora in accompanying a shark. The Physalia doubtless paralyses many more animals than it can consume—the residue falling to the lot of the fishes, which may be present to the number of ten."

FAM. 10. ICOSTEIDAE.—The so-called "Rag-Fishes," in which the skeleton is quite soft and cartilaginous, are aberrant deep-sea forms evidently related to the Stromateidae; they lack the oesophageal teeth and the processes of the last gill-arch, but Icosteus at least has the gill-raker-like knobs below the pseudobranchiae. The pelvis is widely separated from the clavicles. Spines are absent in the fins, and the body is naked or covered with small cycloid scales. Vertebrae in large number (up to 70).

Icosteus, Icichthys, and Acrotus, each with a single species, from the Pacific coast of North America.

FAM. 11. OPHIOCEPHALIDAE.—Maxillary excluded from the border of the upper jaw. Head and body covered with cycloid scales. Anterior ribs sessile, the remainder inserted on the parapophyses. Pectoral fin low down; ventral fin, if present, near the pectoral, with 6 soft rays; pelvic bones connected with the clavicular symphysis by ligament. Dorsal and anal fins long, without spines. Air-bladder present, much elongate.

These Fishes are provided with an accessory superbranchial cavity, and are able to breathe atmospheric air. All are {645}inhabitants of fresh waters and are carnivorous. Only two genera are known: Ophiocephalus, with about 25 species from Eastern Asia and 3 from Tropical Africa, and Channa, distinguished by the absence of ventral fins, with 3 species from Ceylon and China.

FAM. 12. ANABANTIDAE.—Differ from the preceding, to which they are closely related, in having part of the dorsal and anal fins and the outer ventral ray spinous, and a shorter, Perch-like body covered with ctenoid scales. The accessory superbranchial organ is still more developed, with thin bony laminae, which are more or less folded and covered with a mucous membrane. These Fishes can live a long time out of water, and the name Anabas scandens, or Climbing Perch, recalls the fact that its first observers in India ascribed to it the habit of climbing up low trees by means of the spines with which its {646}gill-covers and ventral fins are armed. This species, which attains a length of 8 inches, is found in estuaries and fresh waters of India, Ceylon, Burma, and the Malay Peninsula and Archipelago; 3 other species occur in the Malay Archipelago, and 11 in Africa.

SUB-ORDER 9. ANACANTHINI.

Air-bladder without open duct. Parietal bones separated by the supraoccipital; prootic and exoccipital separated by the enlarged opisthotic. Pectoral arch suspended from the skull; no mesocoracoid arch. Ventral fins below or in front of the pectorals, the pelvic bones posterior to the clavicular symphysis and only loosely attached to it by ligament.

Fins without spines; caudal, if present, without expanded hypural, perfectly symmetrical, and supported by the neural and haemal spines of the posterior vertebrae and by basal bones similar to those supporting the dorsal and anal rays. This type of caudal fin must be regarded, as I have pointed out, as secondary, the Gadidae being, no doubt, derived from Fishes like the Macruridae, in which the homocercal fin had been lost. The scapular foramen or fenestra is nearly always between the scapular and coracoid bones, as in the Trachinidae and several allied families, not in the coracoid, as in the other Acanthopterygians. The first two vertebrae have no epipleurals.

Mr. C. Tate Regan, who has recently given a good definition of the Anacanthini, divides them into three families.

{647}FAM. 1. MACRURIDAE.—Mouth more or less inferior, protractile; teeth small, none on palate. Anterior vertebrae without transverse processes, with the ribs sessile, the rest with strong transverse processes supporting the ribs, which themselves bear epipleurals. Gill-membranes free from isthmus or narrowly attached; 6 or 7 branchiostegal rays; gills 3½ or 4; pseudobranchiae rudimentary or absent. Ventral fins below the pectorals, with 7 to 12 rays. Body short, tail elongate and tapering to a point, without caudal fin. A short anterior dorsal, with a single simple ray, and a long dorsal and anal meeting together at the end of the tail, formed entirely of articulated rays—the two dorsals sometimes continuous (Lyconus).

Deep-sea Fishes with very large eyes and small or rather large mouth, usually covered with rough spiny scales; a mental barbel is present, except in Lyconus, and the muciferous cavities of the skull are strongly developed, the bones being remarkably thin. About 120 species are known, some of which have a wide distribution. Macrurids have been found in all the seas where deep-sea dredging has been practised—the greatest depth at which they have been obtained being 2650 fathoms. Principal genera: Macrurus, Gadomus (with perforate scapula) Coryphaenoides, Hymenocephalus, Malacocephalus, Lionurus, Trachyrhynchus, Steindachneria, Bathygadus, Lyconus, Macruronus. A larval form of this family has received the name of Krohnius; it is remarkable for the filamentous prolongation of the ventral rays, which recalls the larval Trachypterus.

FAM. 2. GADIDAE.—Mouth moderate or large, more or less protractile. Anterior vertebrae without transverse processes, with the ribs sessile, the rest with strong transverse processes, {648}usually supporting ribs, which themselves bear epipleurals. Gill-membranes free from isthmus or narrowly attached; 6 to 8 branchiostegal rays; gills 4, a slit behind the fourth; no pseudobranchiae. Ventral fins jugular, with 1 to 9 soft rays. Body more or less elongate, covered with small cycloid scales. Dorsal and anal fins elongate, formed of articulated rays, sometimes divided into two or three distinct portions. Caudal fin more or less distinct, supported by the unmodified or but slightly modified neural and haemal spines of the last vertebrae, which are perfectly symmetrical (diphycercal or isocercal type).

A mental barbel is often present, as in the Macruridae, and the suture between the frontal bones has disappeared in most of the members of this very natural family. About 120 species are distinguished, mostly marine, many being adapted to life at great depths. All are carnivorous. They inhabit chiefly the northern seas, but many abyssal forms occur between the tropics and in the southern parts of the Atlantic and Pacific. Principal genera: Gadus, Merluccius, Holargyreus, Lotella, Physiculus, Phycis, Haloporphyrus, Tripterophycis, Lota, Molva, Onus, Bregmaceros, Antimora, Raniceps, Brosmius.

Several species, referred to Gadus and Brosmius, have been described from the Miocene. Nemopteryx, which is allied to Gadus, is from the Oligocene.

The fishes of this family are among the most important from an economic point of view. It will suffice to allude merely by name to the following among the European forms:—The Cod-Fish (Gadus morrhua), the largest species, reaching a length of {649}4 feet and a weight of 100 lbs., the Haddock (G. aeglefinus), the Whiting (G. merlangus), the Bib or Pout (G. luscus), the Pollack (G. pollachius), the Coal-Fish (G. virens), the Hake (Merluccius vulgaris), and the Ling (Molva vulgaris). Species of Merluccius occur also on the coasts of Chili and of New Zealand. The Rocklings (Onus or Motella) are of small size; several species are of common occurrence in our tide-pools. The Burbot (Lota vulgaris) is a freshwater fish, exceeding a length of 3 feet, of excellent quality, but unfortunately local and rare in this country.

FAM. 3. MURAENOLEPIDIDAE.—Closely related to the Gadidae, from which they differ in not having a separate caudal fin, in the gill-openings being narrow and below the base of the pectorals, in the increased number (ten) of the pectoral pterygials, and in the peculiar scales, similar to those of the Anguillidae. Ventrals with 5 rays. A mental barbel.

A single genus, Muraenolepis, from the coast of Kerguelen Island.

{650}CHAPTER XXIII

TELEOSTEI (CONTINUED): ACANTHOPTERYGII—OPISTHOMI—PEDICULATI—PLECTOGNATHI

SUB-ORDER 10. ACANTHOPTERYGII.

Air-bladder usually without open duct. Opercle well developed; supraoccipital in contact with the frontals. Pectoral arch suspended from the skull; no mesocoracoid. Ventral fins thoracic or jugular, more or less firmly attached to the clavicular arch. Gill-opening usually large, in front of the base of the pectoral fin.

The character from which this sub-order, the most comprehensive of the whole class, derives its name, viz. the presence of non-articulated, more or less pungent, rays in the dorsal and anal fins, is by no means universal, exceptions to the rule being numerous. The mouth is usually bordered by the premaxillaries to the exclusion of the maxillaries, and if these should, by exception, enter the oral edge, they are always toothless. The ventral fins are sometimes inserted at some distance behind the base of the pectorals (Haplodactylidae, Platycephalidae), in which case, however, this is due to the elongation of the pelvic bones, which are solidly attached to the clavicular arch. The sub-order is broken up into divisions, which follow in somewhat arbitrary order, the natural affinities being opposed to a linear arrangement; the annexed diagram is intended to remedy this defect.

SYNOPSIS OF THE DIVISIONS.

I. No suborbital stay, or process extending from the suborbital bones towards the praeoperculum; basis cranii double in the symmetrical forms. Primary shoulder-girdle composed of a perforate scapula {651} and a coracoid; of the four or five pterygials, or basal bones of the pectoral fins, only one or two are in contact with the coracoid; ventral fins thoracic.

Rays of the caudal fin not strongly forked at the base; hypural usually with a basal spine or knob-like process on each side; epipleural bones usually inserted on the parapophyses or on the ribs; dorsal fin usually with strong spines; caudal peduncle rarely much constricted I. PERCIFORMES.

Rays of the caudal fin strongly forked at the base, embracing a considerable portion of the hypural, which always bears a basal spine; epipleural bones usually inserted on the centra or on the parapophyses, rarely on the ribs; dorsal spines feeble or detached; caudal peduncle much constricted; scales usually very small or absent II. SCOMBRIFORMES.

Rays of the caudal fin not strongly forked at the base; ventral {652} fins with one spine and six to eight soft rays, or cranium asymmetrical III. ZEORHOMBI.

II. No suborbital stay; basis cranii double; scapula absent, the pterygials inserted on the coracoid; ventral fins thoracic IV. KURTIFORMES.

III. No suborbital stay; basis cranii simple; scapula and coracoid more or less reduced, sometimes vestigial; pterygials large, only one or two in contact with the coracoid; ventral fins thoracic V. GOBIIFORMES.

IV. No suborbital stay; basis cranii simple; a perforate scapula; three pterygials in contact with the coracoid; ventral fins thoracic; a suctorial laminated disk on the upper surface of the head VI. DISCOCEPHALI.

V. A suborbital stay, the second suborbital bone more or less produced on the cheek or joining the praeoperculum; ventrals thoracic VII. SCLEROPAREI.

VI. No suborbital stay; ventrals usually jugular or mental; if thoracic, structure of the pectoral arch differing from that ascribed to the first five divisions of this Synopsis.

Pectoral fin with vertical or subvertical base; anal fin usually elongate, rarely small VIII. JUGULARES.

Pectoral fin with horizontal or sub-horizontal base; body exceedingly compressed; dorsal fin with all the rays simple; anal fin absent or very small IX. TAENIOSOMI.

DIVISION I.—PERCIFORMES.

No bony stay for the praeoperculum. Basis cranii double. Spinous dorsal usually well developed. None of the epipleural bones attached to the centra of the vertebrae in the praecaudal region. Pectoral arch with well-developed scapula and coracoid, the former pierced by a foramen or fenestra; pterygials longer than broad, more or less regularly hour-glass-shaped, four or five in number, one or two of which are in contact with the coracoid. Ventral fins thoracic.

This large group, consisting chiefly of marine forms, has members in all parts of the world, with the exception of the Arctic and Antarctic regions, and was already represented by numerous Berycidae and a few Serranidae and Scorpididae in the Upper Cretaceous. The division into families is a task of considerable difficulty, and the necessities of a linear arrangement result in the breaking up of some natural sequences. Thus it appears highly probable that the Scorpididae, themselves derived, together with the Serranidae, from the Berycidae, lead to the Carangidae in the division Scombriformes, whilst a nearly perfect passage can be traced between the Acanthuridae of this division and the Balistidae among the Plectognaths.

{653}SYNOPSIS OF THE FAMILIES.

I. Gills four, a slit behind the fourth.

A. Two nostrils on each side.

1. Ventrals with 1 spine and 6 to 13 soft rays. 1. Berycidae.

2. Ventrals with not more than 5 soft rays.

a. Lower pharyngeal bones not completely united, showing at least a median suture.

α. Gill-membranes nearly always free from isthmus.

* Ventrals little if at all behind the pectorals.

† Third vertebra without transverse processes or with sessile ribs.

§ A more or less developed subocular shelf, or inner lamina of the suborbitals supporting the eye-ball, sometimes reduced to a mere process of the second suborbital.

‖ Ribs inserted on the transverse processes, when these are developed.

Body covered with very large bony scales; ventrals with a very strong spine and 2 or 3 very short soft rays 2. Monocentridae.

Dorsal very short, with few graduated, adnate spines, anal very long 3. Pempheridae.

Spinous dorsal usually well developed, soft dorsal usually not much more developed than the anal; palate usually toothed 11. Serranidae.

Dorsal and anal fins elongate and formed mostly of articulated soft rays, the spines feeble and few 12. Pseudochromididae.

Dorsal and anal fins much elongate, without distinct spines; body band-like 13. Cepolidae.

Teeth in the jaws fused to form a beak 14. Hoplognathidae.

Soft dorsal and anal much elongate; a separate spinous dorsal 15. Sillaginidae.

Soft dorsal much longer than the anal; a separate spinous dorsal 16. Sciaenidae.

‖‖ Ribs mostly sessile, behind the parapophyses; body deep; mouth moderately large and protractile.

Post-temporal forked, distinct from skull 25. Scorpididae.

Post-temporal completely ankylosed to the skull; mouth very protractile 26. Caproidae.

§§ No subocular shelf.

‖ Ribs mostly sessile, behind the parapophyses; anal spines 3 to 14.

Teeth conical; palate toothed; mouth feebly protractile 4. Centrarchidae.

Teeth incisor-like; fins densely scaled 5. Cyphosidae.

Teeth conical; palate toothless 6. Lobotidae.

Maxillary very slender; mouth very protractile 7. Toxotidae.

No entopterygoid; mouth very protractile 8. Nandidae.

‖‖ Ribs inserted on the transverse processes when {654} these are developed; not more than 3 anal spines.

Mouth not or but feebly protractile; spinous dorsal usually longer than the soft; anal with 1 or 2 spines 9. Percidae.

Mouth moderately protractile; palate toothed; spinous dorsal not longer than the soft; anal with 2 or 3 spines 10. Acropomatidae.

Mouth very protractile; palate toothless; praemaxillary with an upwardly directed lateral process 17. Gerridae.

Mouth moderately protractile; palate toothed; anal longer than soft dorsal; body scaly 18. Lactariidae.

Mouth moderately protractile; palate toothless; anal much longer than soft dorsal; body naked 19. Trichodontidae.

†† Transverse processes developed on the third vertebra and bearing the ribs; palate usually toothless.

No subocular shelf; teeth small 22. Pristipomatidae.

A subocular shelf; teeth often large, either cutting in front or molar-like on the sides 23. Sparidae.

A subocular shelf; teeth very small or absent; a pair of barbels on the throat 24. Mullidae.

** Ventrals rather far behind the base of the pectorals; lower pectoral rays unbranched, often thickened; no subocular shelf.

Anal fin nearly as long as the soft dorsal 20. Latrididae.

Anal fin much shorter than the soft dorsal 21. Haplodactylidae.

β. Gill-membranes attached to the isthmus.

* Scales well developed; vertebrae 24 or more.

A subocular shelf; mouth small; palate toothless 27. Chaetodontidae.

No subocular shelf; mouth small; palate toothless 28. Drepanidae.

Subocular shelf more or less developed; a superbranchial respiratory organ 31. Osphromenidae.

** Scales minute; mouth small; vertebrae 22 or 23.

Post-temporal not distinctly forked; vertebrae with strong transverse processes; ventrals with 1 spine and 2 to 5 soft rays 29. Acanthuridae.

Post-temporal forked; vertebrae without transverse processes; ventrals with 2 spines and 3 soft rays between them 30. Teuthididae.

b. Lower pharyngeals completely united into one bone, without median suture 32. Embiotocidae.

B. A single nostril on each side; lower pharyngeal bones more or less completely united, but with persistent suture; no subocular shelf; palate toothless 33. Cichlidae.

II. Gills three and a half; lower pharyngeals completely united into one bone, without median suture; palate toothless.

A single nostril on each side; teeth conical or incisor-like; a subocular shelf 34. Pomacentridae.

Two nostrils on each side; anterior teeth usually strong and canine-like; teeth on pharyngeal bones conical or tubercular; no subocular shelf 35. Labridae.

Two nostrils on each side; anterior teeth more or less coalesced into a beak; teeth on pharyngeal bones flat, tessellated; no subocular shelf 36. Scaridae.

{655}FAM. 1. BERYCIDAE.—One or several of the suborbitals emitting an internal lamina supporting the eye; entopterygoid present. Anterior vertebrae without transverse processes; all or most of the ribs inserted on the transverse processes where these are developed. Two nostrils on each side. Gill-membranes free from isthmus; 4 to 10 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae. Lower pharyngeal bones separate. Ventral fins with 1 spine and 6 to 13 soft rays.

This family is remarkable for the retention of two archaic characters: the large number of rays to the ventral fins and the duct between the air-bladder and the digestive tract; the latter character is, however, not universal, and has only been found in two genera (Beryx and Holocentrum). The scaling of the body varies greatly, and so does the development of the spines in the vertical fins. Several genera (Melamphaes, Anoplogaster, Trachichthys, etc.) have the head studded with large muciferous cavities which are covered with a thin skin. The vent is usually situated far behind the ventral fin, but in Paratrachichthys, a genus closely allied to Trachichthys, it occupies a more anterior position, between the ventrals, whilst in Aphredoderus it shifts still further with age, opening on the throat in the adult.

The Berycidae were abundantly represented in Cretaceous deposits by Beryx and other genera more or less closely related to living forms, and they appear to have been the precursors of other Perciform Fishes. About 70 species, referred to 13 genera, are known to live at the present day, mostly at great depths, in {656}the seas nearly all over the world. But one freshwater form is known, Aphredoderus sayanus, the little Pirate Perch of North America, growing to 5 inches in length. The largest marine forms (Beryx and Gephyroberyx) measure from 1 to 2 feet.

Recent genera: Beryx, Polymixia, Aphredoderus, Melamphaes, Plectromus, Scopelogadus, Anoplogaster, Caulolepis, Trachichthys, Paratrachichthys, Gephyroberyx, Myripristis, Holocentrum.

Fossil genera: Sphenocephalus, Acrogaster, Pycnosterinx, Hoplopteryx, from the Upper Cretaceous. Asineops, from the Eocene of North America, is supposed to be allied to Aphredoderus. Beryx is represented by several species in the Upper Cretaceous, and Holocentrum occurs in the Eocene and Miocene.

FAM. 2. MONOCENTRIDAE.—The single genus Monocentris, with two species, one from the seas of Japan, China, and India, and one from the South Pacific, is very nearly related to the Berycidae, but differs in the absence of ribs on the anterior six vertebrae, in the very large bony scales, forming together a coat of mail, and in the structure of the ventral fin, which is reduced to a strong spine and two or three very short soft rays. The spines of the dorsal fin are very strong and isolated.

FAM. 3. PEMPHERIDAE.—The resemblance which the fishes united under this family bear to Beryx is very striking, and applies to the skeleton as well as to the external characters. But the ventral fins are formed of one spine and five soft rays, as in most Acanthopterygians. Bathyclupea agrees with Beryx in {657}being possessed of an open duct to the air-bladder. About twelve species are known, referable to four genera: Pempheris, Parapriacanthus, Neopempheris, from the Indian, Pacific, and tropical Atlantic Oceans, and the deep-sea Bathyclupea, from the Indian and Caribbean Seas, at depths of 145 to 419 fathoms.

FAM. 4. CENTRARCHIDAE.—No subocular lamina of the suborbitals, or subocular shelf; entopterygoid present; palate toothed; teeth conical. Praecaudal vertebrae with transverse processes from the third or fourth to the last; ribs mostly sessile, behind the transverse processes. Two nostrils on each side. Gill-membranes free from isthmus; 5 to 7 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae more or less developed, often rudimentary or absent. Lower pharyngeal bones separate. Soft portion of dorsal fin not more developed than the anal. Carnivorous freshwater fishes, some entering brackish water. Many are known to build nests. Mostly inhabitants of North America, the best known being the Sun-Fishes (Lepomis), and Black Bass (Micropterus), several species of which have recently been introduced into continental Europe. Principal genera: Pomoxys, Centrarchus, Ambloplites, Chaenobryttus, Micropterus, Lepomis, Elassoma, Kuhlia. Thirty-two species are known.

FAM. 5. CYPHOSIDAE.—Herbivorous fishes, agreeing in their essential osteological characters with the preceding, differing in the incisor-like outer teeth and densely-scaled fins. Some 14 species are known, from the Pacific and Indian Oceans, referable to 4 genera: Cyphosus (Pimelepterus), Hermosilla, Sectator, Medialuna.

{658}FAM. 6. LOBOTIDAE.—As in Centrarchidae, but transverse processes of vertebrae very short, and palate toothless. Two genera: Lobotes, with two species from the warm parts of the Indian and Pacific Oceans, the Mediterranean, and the Atlantic coast of America, and Datnioides, with two species from the estuaries of the Ganges and the rivers of Burma, Siam, and the Malay Peninsula and Archipelago.

FAM. 7. TOXOTIDAE.—No subocular shelf; entopterygoid present; palate toothed; mouth very protractile; maxillary very slender. Ribs sessile, behind parapophyses which commence from the third vertebra. Two nostrils on each side. Gill-membranes free from isthmus; 7 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones separate. Ventral fins with 1 spine and 5 soft rays.

A single genus, Toxotes, with 5 species from the fresh waters and coasts of the East Indies, N. Australia, Polynesia, and New Zealand. Toxotes jaculator derives its name from its habit of capturing insects flying near the surface of the water by shooting drops of water at them, a habit which it continues in captivity.

FAM. 8. NANDIDAE.—No subocular shelf; no entopterygoid; palate toothed; mouth very protractile. Praecaudal vertebrae with parapophyses from the 7th or 8th; ribs mostly sessile, behind the parapophyses. Two nostrils on each side. Gill-membranes free from isthmus; 6 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae absent. Lower pharyngeal bones separate. Soft portion of dorsal fin not more developed than the anal. Ventral fins with 1 spine and 5 soft rays.

Small carnivorous freshwater fishes, of which 14 species are known, referable to 6 genera: Nandus, Catopra, and Badis from South-Eastern Asia, Polycentropsis from West Africa, Polycentrus and Monocirrus from South America.

FAM. 9. PERCIDAE.—No subocular lamina of the suborbitals; entopterygoid present. Anterior vertebrae without transverse processes; all or most of the ribs inserted on the transverse processes when these are developed. Two nostrils on each side. Mouth not or but feebly protractile. Gill-membranes free from the isthmus; 6 to 8 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae more or less developed, often rudimentary, rarely absent. Lower pharyngeal bones separate. Soft portion of dorsal fin not very much more developed than {659}the anal; latter with 1 or 2 spines only. Ventrals with 1 spine and 5 soft rays.

Embrace about 90 species from the freshwaters of the Northern Hemisphere, referable to 12 genera: Perca, Lucioperca, Percina, Etheostoma, Boleosoma, Ulocentra, Diplesium, Ammocrypta, Crystallaria, Aspro, Percarina, Acerina. The British representatives of this family are the Perch (Perca fluviatilis) and the Pope (Acerina cernua). The largest forms are the Pike-Perches or Sander (Lucioperca) of Eastern Europe, Western Asia, and North America, which reach a length of 4 feet and are highly valued for the table. The American Darters (Etheostoma and allies), on the other hand, are among the smallest fishes, but many are remarkable for their brilliant coloration.

FAM. 10. ACROPOMATIDAE.—An ill-defined group of marine fishes, some deep-sea, placed here provisionally as annectant between the Percidae and the Serranidae (Pomatominae), differing from the latter in the absence of a subocular shelf. Spinous dorsal short.

About 28 species, mostly from the Pacific Ocean, distributed in 9 genera: Propoma, Xenichthys, Xenocys, Synagrops, Malacichthys, Acropoma, Melanostoma, Epigonus (Telescops), Dinolestes.

FAM. 11. SERRANIDAE.—Second suborbital with an internal lamina supporting the globe of the eye; entopterygoid present; palate usually toothed. Anterior vertebrae without transverse processes; all or most of the ribs inserted on the transverse processes where these are developed. Two nostrils on each side. Gill-membranes free from isthmus; 6 or 7 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae usually present. Lower pharyngeal bones usually separate. Soft portion of dorsal usually not much more developed than the anal. Ventral fins with 1 spine and 5 (rarely 4) soft rays.

One of the largest families of fishes. The principal genera may be grouped as follows:—

SERRANINAE.—Percichthys, Percilia, Lateolabrax, Niphon, Morone, Percalates, Ctenolates, Macquaria, Siniperca, Coreoperca, Acanthistius, Trachypoma, Centrogenys, Polyprion, Oligorus, Stereolepis, Dinoperca, Liopropoma, Aulacocephalus, Plectropoma, Epinephelus, Cromileptes, Paranthias, Serranus, Centropristes, Chelidoperca, Gilbertia, Caesioperca, Caprodon, Anthias, Callanthias, Pseudoplesiops, Plesiops, Trachinops.

{660}GRAMMISTINAE.—Grammistes, Rhypticus. PRIACANTHINAE.—Priacanthus, Pseudopriacanthus. CENTROPOMINAE.—Lates, Psammoperca, Centropomus. POMATOMINAE.—Pomatomus, Scombrops. AMBASSINAE.—Ambassis. CHILODIPTERINAE.—Chilodipterus, Apogon. LUTJANINAE.—Lutjanus, Glaucosoma, Therapon, Hoplopagrus, Etelis, Aprion, Aphareus, Odontonectes. CIRRHITINAE.—Cirrhites, Cirrhitichthys. PENTACEROTINAE.—Pentaceros, Pentaceropsis, Histiopterus.

The number of recent species amounts to about 550, the great majority of which are marine.

The earliest fossil form is Prolates, from the Upper Cretaceous of France. Morone, Serranus, Percichthys, Anthias, and Apogon are represented in Eocene and later strata.

The range of the family is almost cosmopolitan; few of the Marine Perches descend to any great depth. Some of the species of Stereolepis and Epinephelus grow to a length of 6 to 10 feet. Several species of Serranus (S. cabrilla, S. scriba, S. hepatus), inhabiting the Mediterranean and neighbouring parts of the Atlantic, and some Lutjanus are normally hermaphrodite. Some Chilodipterus and Apogon are remarkable for their nursing habits, the male sheltering the eggs in his mouth.

The curious genera Anomalops and Photoblepharon, of each of which a single species is known from the Malay Archipelago and the South Pacific, have been made the types of a family, ANOMALOPIDAE, the systematic position of which remains uncertain since the osteological characters have not been examined. {661}They are remarkable for the movable flap below the eye, bearing a luminous organ, the nature of which has recently been investigated by Max Weber.

FAM. 12. PSEUDOCHROMIDIDAE.—Closely allied to the Serranidae, and connected with them through Plesiops and its allies. Dorsal and anal fins elongate and formed mostly of articulated soft rays, the spines being feeble and few.

A. With two lateral lines: Pseudochromis, Cichlops.

B. With a single lateral line: Opisthognathus, Latilus, Caulolatilus, Lopholatilus, Malacanthus, Bathymaster, Rathbunella.

Marine, mostly of small size, inhabiting the Atlantic, Indian, and Pacific Oceans. About 30 species. One of the largest and best-known members of this family is the Tile-Fish (Lopholatilus chamaeleonticeps), living upon the bottom of what is known as the Gulf Stream slope, off the coast of New England, where it was first observed in 1879. Here the water is normally comparatively warm, coming as it does from the superheated region of the Gulf of Mexico. During a series of unusually severe gales in 1882, this mass of water was pushed aside, as it were, and replaced by colder water. As a result, millions and millions of these fishes were killed, and their dead bodies literally covered the surface of the sea for hundreds of square miles. It was feared that the Tile-Fish was exterminated; this was not so, however, and the fish has reappeared in tolerable abundance within the last few years.

FAM. 13. CEPOLIDAE.—Agree in essential characters with the preceding, but body band-like with very numerous vertebrae {662}(15 + 54), and very elongate dorsal and anal fins formed of soft rays, of which all except the first three dorsal and the first anal are articulated and branched.

Although these fishes have hitherto been placed near the Blenniidae, the Gobiidae, or the Trachypteridae, they are nothing but extremely elongate Perches, and they stand in the same relation to the Serranidae as the Trichiuridae to the Carangidae and Scombridae. They hardly deserve to rank as a family distinct from the Pseudochromididae.

Two genera, Cepola and Acanthocepola, with 10 species, from the Mediterranean and North-Eastern Atlantic, the Indian Ocean, and the Western Pacific. The Band-Fish (Cepola rubescens), which is common in the Mediterranean, is sometimes found on the British coasts; it grows to a foot and a half in length, and is remarkable for its bright red colour.

FAM. 14. HOPLOGNATHIDAE.—Characters of Serranidae, but teeth fused to form a beak as in Tetrodon; palate toothless.

Hoplognathus, with 4 species, from the Pacific Ocean.

FAM. 15. SILLAGINIDAE.—As in Serranidae, but soft dorsal and anal much elongate, as in Pseudochromididae, from which the Sillaginidae differ in the separate spinous dorsal. Palate toothed. Connecting the Serranidae and the Sciaenidae.

Small Marine Fishes from the Indian and Pacific Oceans, ascending rivers. A single genus, Sillago, with about 10 species.

{663}FAM. 16. SCIAENIDAE.—Also closely related to the Serranidae. Dorsal fin with a short spinous and a long soft portion; anal much shorter than the latter. Palate usually toothless.

A large family of about 150 species, mostly marine. Principal genera: Arripis, Sciaena, Corvina, Otolithus, Ancylodon, Nebris, Larimus, Pogonias, Haplonotus, Umbrina, Eques.

Many of these fishes reach a large size, and the flesh of nearly all is esteemed. The Meagre (Sciaena aquila) is sometimes taken on our coast. The Drum (Pogonias chromis) so called from the sounds which it produces, in common with many other Sciaenids, is remarkable for having the lower pharyngeal bones united, as is also the case in the North American freshwater genus Haplonotus. The air-bladder is usually large and complicated, provided with more or less numerous appendages.

FAM. 17. GERRIDAE.—Agree in the character of the vertebral column with the Serranidae, but differ in the absence of a subocular shelf; the very protractile mouth usually descends when protruded and the praemaxillary emits an upward lateral process; palate toothless; lower pharyngeal bones usually large and more or less completely coalesced.

About 60 species of carnivorous, mostly small, fishes, from the tropical seas, referable to 3 genera: Gerres, Equula, Gazza.

FAM. 18. LACTARIIDAE.—Intermediate between Serranidae and Trichodontidae. No subocular shelf; palate toothed; branchiostegal rays 7; scales small, cycloid, deciduous; spinous dorsal short; anal longer than the soft dorsal; scapula with two foramina.

Lactarius delicatulus, from the coasts of Southern Asia.

FAM. 19. TRICHODONTIDAE.—Agree in the character of the vertebral column with the Serranidae, but have no subocular shelf; body naked, and anal much longer than the soft dorsal; palate toothless; only 5 branchiostegal rays.

Two genera, each with a single species, from the North Pacific, Trichodon and Arctoscopus, bearing some resemblance to the Trachinidae, with which they have usually been associated.

FAM. 20. LATRIDIDAE.—Marine Fishes intermediate between the Serranidae and the Haplodactylidae, agreeing with the former in the extent of the anal fin, which is nearly as long as the soft dorsal, and with the latter in the absence of a subocular shelf and the posterior position of the ventrals. A single genus, Latris, with 3 or 4 species, from the coasts of Australia and New Zealand.

{664}FAM. 21. HAPLODACTYLIDAE.—No subocular shelf; entopterygoid present; palate usually toothless. Vertebrae with transverse processes from the third or fourth; all the ribs attached to the transverse processes when these are present; anterior epipleurals strong. Two nostrils on each side. Gill-membranes free from the isthmus; 5 or 6 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones separate. Soft portion of the dorsal fin much more developed than the anal. Ventral fins with 1 spine and 5 soft rays, inserted far back behind the pectorals, the lower rays of which are simple and more or less thickened.

This family embraces the genera Haplodactylus, Chilodactylus, Chironemus, and Threpterius, with some 30 species from the seas of the Southern Hemisphere and Japan. They feed chiefly on crustaceans, molluscs, and other invertebrates living among sea-weed.

FAM. 22. PRISTIPOMATIDAE.—No subocular shelf; entopterygoid present; palate toothless. Vertebrae with transverse processes from the third; all the ribs attached to the transverse processes. Two nostrils on each side. Gill-membranes free from isthmus; 5 to 7 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones separate. Ventral fins with 1 spine and 5 soft rays.

Pristipoma, Haemulon, Diagramma, and Pentapus, distributed over all the tropical and subtropical seas, a few entering fresh waters. About 130 species are known.

FAM. 23. SPARIDAE.—Second suborbital with an internal lamina supporting the globe of the eye; entopterygoid present; palate usually toothless; teeth often either cutting in front, or molar-like on the sides. Vertebrae with transverse processes from the second or third; all the ribs attached to the transverse processes. Two nostrils on each side. Gill-membranes free from isthmus; 5 to 7 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones separate. Soft portion of dorsal fin not much more developed than the anal. Ventral fins with 1 spine and 5 soft rays.

The Sea-Breams embrace some 200 species, distributed over the coasts of nearly the whole world. Some are herbivorous, but the majority are carnivorous.

Principal genera: Scolopsis, Dentex, Synagris, Caesio, Maena, Oblata, Melambaphes, Girella, Doydixodon, Cantharus, Box, {665}Crenidens, Pachymetopon, Dipterodon, Sargus, Charax, Lethrinus, Sphaerodon, Sparus, Pagrus, Pagellus.

Abundantly represented in Eocene and Miocene beds by remains of Sargus, Sparus, Pagrus, Pagellus, and by the extinct genera Ctenodentex, Sparnodus, and Trigonodon. Some species grow to a length of three feet, such as the "Sheep's-Head" of North America, one of the best salt-water fishes of the United States, and the "Schnapper" (Sparus unicolor), of Australia, also much esteemed. Some of the Atlantic and Mediterranean species of Box, Sargus, Charax, Sparus, and Pagellus are known to be normally, or at least very frequently, hermaphrodite.

FAM. 24. MULLIDAE.—The "Red Mullets" are very nearly related to the Sparidae, with which they agree in the structure of the vertebral column and the presence of a subocular shelf. They differ in the very weak dentition, the presence of a pair of hyoid barbels, the reduced number (4) of branchiostegal rays, and the double perforation of the scapula. Two short dorsal fins, remote from each other, the anterior with weak spines.

Small marine and brackish-water fishes, feeding on animalcules and decomposing matter; inhabitants of nearly all the tropical seas and extending to Northern Europe. About 50 species are known, referred to 5 genera: Upeneoides, Upeneichthys, Mullus, Mulloides, and Upeneus.

{666}The British species are Mullus barbatus and M. surmuletus, remarkable for their beautiful pink or red colour, and much valued on the market, although no longer held in the high estimation for which they were noted by the Romans.

FAM. 25. SCORPIDIDAE.—Second suborbital with an internal lamina supporting the globe of the eye; entopterygoid present; palate toothed. Ribs sessile, behind the parapophyses when these are present. Two nostrils on each side. Gill-membranes free from isthmus; 7 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones separate. Ventral fins, if present, with 1 spine and 5 soft rays.

This family embraces 12 species from the coasts of Africa, Southern Asia, Australia, and New Zealand, referable to 5 genera: Scorpis, Atypichthys, Atyposoma, Henoplosus, Psettus. The fish here figured (Psettus sebae, Fig. 408) is remarkable for the excessive depth of the body, which is greater than in any other species.

Aipichthys, one of the few Acanthopterygian types known to have existed in the Cretaceous period, appears to belong to the family Scorpididae as here defined, and not to the Carangidae.

FAM. 26. CAPROIDAE.—Characters of Scorpididae, but supratemporal completely ankylosed to the skull.

The Boar-Fish (Capros aper) of the Atlantic and Mediterranean is occasionally found on our southern coasts, and is highly remarkable for the hair-like bristles with which its scales are {667}covered, an extreme exaggeration of the "Ctenoid" type. The mouth is very protractile, and the vertebrae are only 22 or 23 in number. Antigonia, with a single species found at remote points in the Atlantic, Pacific, and Indian Oceans, is probably allied to Capros, with which it is believed to be connected through the fossil genus Proantigonia, from the Upper Miocene of Croatia.

FAM. 27. CHAETODONTIDAE.—Closely allied to and evidently derived from the more generalised types of the Scorpididae, differing in the attachment of the gill-membranes to the isthmus. Post-temporal more or less firmly united with the skull, sometimes indistinctly bifurcate. Mouth small; palate toothless; soft portions of vertical fins usually covered with scales; ribs usually strong and blade-like; body short and deep.

A large group of about 200 marine carnivorous fishes from the tropics, mostly of small size, remarkable for their singular forms and markings and brilliant coloration. They are particularly abundant about volcanic rocks and coral reefs.

{668}An Atlantic species of Ephippus (E. faber) is extremely remarkable, when adult, for an enormously enlarged globular bony mass on the back of the head, formed by hypertrophy of the frontal and supraoccipital bones.

Principal genera: Ephippus, Parapsettus, Scatophagus, Chaetodon, Chelmo, Heniochus, Holacanthus, Pomacanthus, Platax.

Chaetodon, Holacanthus, Pomacanthus, Scatophagus, Ephippus, and Platax were represented in the Eocene of Europe.

FAM. 28. DREPANIDAE.—The genus Drepane, with a single species from the Indian Ocean, is very closely related to the Chaetodontidae, but it lacks the subocular shelf, and it is distinguished externally by the very elongate, falciform pectoral fin.

FAM. 29. ACANTHURIDAE.—A more or less developed subocular shelf; entopterygoid present. Mouth very small, not or but slightly protractile, the maxillary more or less firmly attached or ankylosed to the praemaxillary; teeth conical, bristle-like, or incisor-like. Palate toothless. Vertebrae 22 or 23, the praecaudals with strong transverse processes commencing from the first; ribs and epipleurals inserted on the transverse processes. Post-temporal not distinctly forked, ankylosed to the skull. Two nostrils on each side. Gill-membranes broadly attached to the isthmus; 4 or 5 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones separate. Body covered with minute, often rough scales. Dorsal and anal fins elongate, with more or less strong spines. Ventrals with 1 spine and 2 to 5 soft rays.

A family of about 80 species, mostly herbivorous, from the tropical seas, referred to 6 genera: Zanclus, Ctenochaetus, Acanthurus, Colocopus, Prionurus, Naseus. They form a connecting link between the Chaetodontidae and the Plectognathi.

Remains from the Eocene of Europe have been referred to Zanclus, Acanthurus, and Naseus, and to the extinct genera Aulorhamphus and Apostasis.

FAM. 30. TEUTHIDIDAE.—No subocular shelf; entopterygoid present. Mouth very small, beak-like, not protractile, with incisor-like teeth; maxillary ankylosed to the praemaxillary. Palate toothless. Two nostrils on each side. Gill-membranes broadly attached to the isthmus; 5 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones separate. Supratemporal forked. Vertebrae {669}23, with sessile ribs and no parapophyses, the epipleurals inserted on the ribs. Body covered with very small scales. Vertical fins elongate, with strong spines, 6 or 7 in the anal. Ventrals with 2 spines and 3 soft rays between them.

A single recent genus, Teuthis, with about 30 species, herbivorous fishes from the Indian and Western Pacific Oceans. According to Bottard the sting from the spines of these fishes is much dreaded. Archaeoteuthis, from the Oligocene of Switzerland.

FAM. 31. OSPHROMENIDAE.—Second suborbital with a more or less developed internal lamina; entopterygoid present; palate toothed. Most of the praecaudal vertebrae with transverse processes, to which the ribs are attached. Two nostrils on each side. Gill-membranes attached to isthmus; 4 to 6 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae absent. Lower pharyngeal bones separate. Vertical fins very variable in extent, the spines sometimes very numerous, sometimes absent. Ventral fins with not more than 5 soft rays, sometimes reduced to a filamentous ray. A superbranchial respiratory organ, situated in a cavity above the gills.

Freshwater fishes having much in common with the Anabantidae, and likewise confined to South-Eastern Asia and Africa. Only 22 species are known, referable to 7 genera: Helostoma, Polyacanthus, Osphromenus, Trichogaster, Luciocephalus, Betta, and Micracanthus. The latter, the only African representative of the family (one species from the Ogowe), hardly differs from the Malay genus Betta. Most of the Osphromenidae are notable as aquarium fishes. The largest species, the Gourami (Osphromenus olfax), growing to a length of 2 feet, from the Malay Archipelago, is one of the best flavoured fishes of the Far East and has been acclimatised in India, the Guianas, and Mauritius. A domesticated variety of the Chinese Polyacanthus opercularis, known as Macropodus viridi-auratus, remarkable for the beauty of its form and colour, readily breeds in our aquariums. Like the Gourami, the male constructs a nest of air-bubbles, strengthened by a buccal secretion, and watches over the eggs and young. The little Betta pugnax, from South-Eastern Asia, derives its name from its excitable nature, which causes specimens to be kept by the Siamese in glass vessels where they engage in fights, special breeds being cultivated for the purpose. According to Cantor, {670}the Siamese in 1840 were as infatuated with the combats of these fishes as the Malays are with their cock-fights, and the licence to exhibit them was farmed, bringing in a considerable annual revenue to the king.

FAM. 32. EMBIOTOCIDAE.—Second suborbital with an internal lamina supporting the globe of the eye; entopterygoid present; palate toothless. Ribs sessile, above and behind the parapophyses, where these are present. Two nostrils on each side. Gill-membranes free from isthmus; 5 or 6 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeals united, with conical or pavement-like teeth. Anal fin, with three spines. Ventral fins with 1 spine and 5 soft rays.

Small or moderate-sized fishes inhabiting California and Japan, mostly marine, one species, however, inhabiting fresh waters, whilst another descends to a great depth. They feed mostly on crustaceans, but one genus (Abcona) is herbivorous. The name "Surf-Fishes," by which they are generally known, refers to the fact that most species are found in the surf along sandy beaches. All are viviparous in the strictest sense of the term, the young remaining for a long time closely packed in a sac-like enlargement of the oviduct analogous to a uterus; they are of relatively large size at birth, and quite similar in form to the parent, whilst at an earlier period they differ in having the vertical fins much more elevated. Twenty-four species are known. Principal genera: Hysterocarpus, Abcona, Cymatogaster, Embiotoca, Ditrema.

FAM. 33. CICHLIDAE.—No subocular shelf; entopterygoid {671}present; palate toothless; lower pharyngeal bones more or less completely united, with median suture. Vertebrae with parapophyses from the third; ribs most frequently sessile or subsessile. A single nostril on each side. Gill-membranes free from isthmus; 5 or 6 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae absent. Dorsal fin, with numerous spines; anal with 3 spines or more. Ventral fins with 1 spine and 5 soft rays.

Fresh or brackish-water fishes, variable in form and dentition, some carnivorous, others chiefly herbivorous. In some American forms (Cichla) the males and females differ during the spawning season, the male developing a hump on the top of the head, which disappears afterwards. The eggs and young are cared for by the parents; either the male or the female, according to the species, sheltering them in the mouth or pharynx. These fishes, often designated as "Chromides," a name which properly pertains to members of the following family, inhabit Africa, Madagascar, Syria, India and Ceylon, and Central and South America, from Texas to Uruguay. About 45 genera are distinguished, based mostly on the number of anal spines and the dentition, which for variety of types is comparable to that of the Characinidae. Of these 45 genera, 30 are African. 150 species are known from Africa (with Syria and Madagascar), 140 from America, and 3 from India and Ceylon. Principal genera—African: Lamprologus, Hemichromis, Paratilapia, Xenotilapia, Tropheus, Tilapia, {672}Asprotilapia, Eretmodus, Plecodus, Pseudetroplus. American: Acara, Heros, Hygrogonus, Cichla, Crenicichla, Chaetobranchus, Geophagus, Symphysodon, Pterophyllum. Indian: Etroplus.

No part of the world surpasses Lake Tanganyika in variety of generic and specific types of Cichlidae, the fish-fauna of this great lake being in great majority made up of members of this family.

Priscacara, from the Eocene of North America, is the only extinct genus which can be referred to this family.

FAM. 34. POMACENTRIDAE.—A subocular shelf; entopterygoid present; palate toothless; teeth conical or incisor-like; lower pharyngeals completely united into one bone. Vertebrae with transverse processes from the fourth or fifth; ribs inserted on the transverse processes, when these are present. A single nostril on each side. Gill-membranes free from the isthmus; 5 to 7 branchiostegal rays; gills 3½; pseudobranchiae present. Dorsal fin elongate, with numerous strong spines; anal with 2 spines only. Ventral fins with 1 spine and 5 soft rays.

Small fishes of the tropical and warm seas, resembling the Chaetodontidae in form and mode of life, likewise usually of brilliant coloration; in structural characters intermediate between the Cichlidae and the Labridae. They feed chiefly on small marine animals, but the species with incisor-like teeth are entirely or mainly herbivorous. Over 150 species are known.

Principal genera: Heliastes, Azurina, Amphiprion, Premnas, Dascyllus, Pomacentrus, Glyphidodon, Microspathodon.

{673}The family is supposed to be represented in the Upper Eocene and Lower Miocene of Italy by the extinct genus Odonteus.

FAM. 35. LABRIDAE.—No subocular shelf; entopterygoid present; palate toothless; anterior teeth of the jaws usually strong and canine-like, lateral teeth often soldered at the base; lower pharyngeals completely united into one bone, with conical or tubercular teeth. Vertebrae with transverse processes from the third; all the ribs attached to the transverse processes. Lips thick. Two nostrils on each side. Gill-membranes free or joined to the narrow isthmus; 5 or 6 branchiostegal rays; gills three and a half; pseudobranchiae present. Dorsal fin elongate, with numerous, usually slender, spines. Ventral fins with 1 spine and 5 soft rays.

The "Wrasses" form a large family of mostly brilliantly coloured marine fishes, inhabiting all the tropical and temperate coasts. Their regime is partially herbivorous, partially carnivorous. About 400 species are known. Principal genera: Labrus, Tautoga, Malacopterus, Ctenolabrus, Chaerops, Platychaerops, Heterochaerops, Labrichthys, Cossyphus, Cirrhilabrus, Chilinus, Epibulus, Anampses, Platyglossus, Novacula, Julis, Gomphosus, Chilio, Coris.

Some of the members of this family have been observed to build nests for the protection of their eggs and young. These nests, in the European Labrus, are made of seaweeds, zoophytes, corals, broken shells, etc., and are the work of both the male and the female. It is also in this family that sleep was first observed in fishes, and this has been fully verified by Möbius {674}on Labrus rupestris in an aquarium, the fish seeking a sleeping place at night and laying itself down to rest on one side.

As first pointed out by Günther, the Labridae found in temperate regions have a higher number (30 to 41) of vertebrae than those inhabiting the tropics (23 to 29), a rule which applies more or less to other families of Acanthopterygians. Remains of Labrus and Julis occur in the Eocene and Miocene of Europe. An allied fossil genus, Labrodon, is represented by numerous species in Tertiary beds of Europe and North America. Phyllodus, Egertonia, Platylaemus, and Pseudosphaerodon, from the Eocene and Miocene, are referred, with doubt, to this family.

FAM. 36. SCARIDAE.—Closely allied to the preceding, with which they have usually been united, but differing in the more or less coalescent teeth, forming often a parrot-like beak, the lower pharyngeals united into a concave or spoon-shaped bone with flat, tessellated teeth; and in the development of transverse processes from the first vertebra. Vertebrae 24 or 25.

Curious, mostly brilliantly-coloured fishes of the tropical seas and the Mediterranean, especially abundant about coral-reefs. "Parrot-Wrasses" feed mostly on vegetable matter, corals, and on hard-shelled Mollusca, for crushing which their dentition is well adapted. The largest reach a length of 4 feet. Some are much valued as food, whilst others are reputed poisonous. About 110 species are known, referable to 8 genera: Cryptotomus (Calliodon), Calotomus, Sparisoma, Scarus, Pseudoscarus, Odax, Coridodax, Siphonognathus. The latter is very aberrant in shape, the head and body resembling those of a Pipe-Fish.

Scarus is reported from the Eocene and Miocene of Europe.

{675}DIVISION II.—SCOMBRIFORMES.

No bony stay for the praeopercle. Spinous dorsal, if distinct, formed of short or feeble, slender spines. Epipleurals usually attached to the centra when ribs are sessile, or to the parapophyses of the vertebrae, rarely to the ribs. Pectoral arch similar to that of the Perciformes, but pterygials sometimes more abbreviated. Ventral fins thoracic. Caudal fin, if well developed, with very numerous rays deeply forked at the base.

Although bound by natural ties, the series of families that cluster round the Mackerel offer so many modifications of structure that it is almost impossible to draw up a diagnosis differentiating every one of its members from the Perciformes, with which they are closely connected, and from which they hardly deserve to be separated. Even after removing many genera which have been united with them by my predecessors, and which will now be found scattered among various groups of the system, no good definition of the Scombriformes can be given. The Mackerel and Horse-Mackerel are taken as the pattern-forms around which more or less aberrant types are located, types yet not so aberrant as to be traced back to these familiar forms through a number of intermediate grades. As {676}regards external features, it may be stated that the dorsal and anal spines, if present, are either weak and slender, or, if strong, short and detached; the caudal peduncle is constricted, and the caudal fin, if well developed, is usually deeply forked, and with the forked bases of the very numerous rays much longer than in most of the Perciformes, embracing at least a considerable portion of the expanded hypural bones, a character by which the Chaetodontidae, Acanthuridae, and several extinct types which have been placed with the Carangidae are at once excluded. All are carnivorous and marine, and many are pelagic and of very wide distribution. No praetertiary members of this division, as here defined, have yet been found.

SYNOPSIS OF THE FAMILIES

I. Praemaxillaries more or less protractile, not beak-like; scales small or absent, sometimes with enlarged lateral scutes; spinous dorsal fin short or replaced by a series of isolated spines; anal usually with one or two spines detached from the rest of the fin.

Praecaudal vertebrae with transverse processes behind which the ribs are attached 1. Carangidae.

Praecaudal vertebrae without well-developed parapophyses, ribs and epipleurals inserted close together on the centra 2. Rhachicentridae.

II. Praemaxillaries not protractile; scales usually small or absent; body more or less elongate; dorsal fin elongate, single or divided, without free spines; no free anal spines.

A. Pseudobranchiae present.

Vertebrae without transverse processes; soft dorsal fin longer than the spinous; pectoral fins high up the sides 3. Scombridae.

Vertebrae without transverse processes; soft dorsal fin shorter than the spinous, if the latter be distinct; pectoral fins low down the sides 4. Trichiuridae.

Vertebrae without transverse processes; snout produced into a spear 5. Histiophoridae.

Vertebrae with transverse processes bearing the ribs; snout produced into a sword; no ventrals 6. Xiphiidae.

Vertebrae without transverse processes; gill-membranes attached to isthmus; dorsal and anal fins formed of unarticulated, widely set rays; dentition very feeble 7. Luvaridae.

B. Pseudobranchiae absent; no well-developed transverse processes to the praecaudal vertebrae; the ribs and the epipleurals inserted close together on the centra; snout short and very deep 8. Coryphaenidae.

III. Praemaxillaries not protractile, or if slightly protractile, scales large; dorsal and anal fins elongate, without distinct spinous division; most of the praecaudal vertebrae with strong haemapophyses, to which the ribs are attached 9. Bramidae.

{677}FAM. 1. CARANGIDAE.—Praemaxillaries more or less protractile. Vertebrae 24 to 26; ribs behind the parapophyses; epipleurals on the parapophyses, rarely on the ribs. Body covered with small scales, or naked, often with enlarged scutes on each side of the body or of the tail; dorsal spines few, or slender or rudimentary; a more or less developed spine adnate to the soft portion of the anal, often preceded by a pair of spines separated from the rest of the fin. Pseudobranchiae usually present. Inhabitants of the seas of the temperate and tropical regions, many of the species having a very wide range. About 150 species are known.

Principal recent genera: Caranx, Chloroscombrus, Selene, Mene, Apolectus, Nematistius, Seriola, Seriolichthys, Naucrates, Trachynotus, Zalocys, Lichia, Paropsis, Chorinemus. Species of Caranx, Mene, and Seriola have been described from the Eocene and Miocene of Europe, in which occur also the fossil genera named Vomeropsis, Archaeus, Carangopsis, Carangodes, Ductor, and Semiophorus.

The family is represented on our coasts by the common Horse-Mackerel, Caranx trachurus. The young of this species keep together in small bands in the neighbourhood of medusae, under which they seek shelter when disturbed. The Pilot-Fish, Naucrates ductor, is a truly pelagic fish of wide distribution, which occasionally appears on our coasts, accompanying large sharks and ships. Much has been written on the marvellous habits of this little fish, which is said to lead the shark like a pilot, directing it to its food, in exchange for which services the pilot enjoys protection from the fear which the proximity of its formidable companion inspires to its enemies among other carnivorous fishes, and an abundance of food from the shark's excrements.

FAM. 2. RHACHICENTRIDAE.—Praemaxillaries slightly protractile. Vertebrae 25 (11 + 14), without well-developed parapophyses; ribs and epipleurals inserted close together on the centra. Body covered with very small scales; a series of short isolated dorsal spines; soft dorsal and anal long; pectorals {678}inserted low down. A single genus, Rhachicentrum (Elacate), with a single species from the coasts of the tropical and warmer parts of the Atlantic and of the Indian Ocean.

FAM. 3. SCOMBRIDAE.—Praemaxillaries large, not protractile, beak-like. Vertebrae 30 to 50, without transverse processes, but some of the hinder praecaudals with haemal arches; ribs inserted on the centra or on the haemal arches when these are present; epipleurals all on the centra. Scales cycloid and usually very small (except in Gastrochisma), sometimes absent. A spinous dorsal fin formed of slender spines, folding into a sheath; soft dorsal longer and broken up into finlets, similar to the anal; pectoral inserted high up the sides. Hypural bones completely embraced by the forked bases of the caudal rays. Pseudobranchiae present.

About 50 species, referred to the following genera:—Scomber, Auxis, Thunnus, Sarda, Cybium, Acanthocybium, Gastrochisma (Lepidothynnus). Numerous fossil representatives in Tertiary beds, belonging to Scomber, Auxis, Thunnus, Cybium, and to the extinct genera Eothynnus, Isurichthys, Palimphyes, Scombrinus, Sphyraenodus, Scombramphodon.

These fishes, elegant in form and often in colour, are among the swiftest of the inhabitants of the sea. Some are migratory, like the Mackerel (Scomber scombrus) of the North Atlantic, whilst others are remarkable for their wide distribution. The Tunny (Thunnus thynnus), for instance, the largest member of the family, reaching a length of 10 feet, inhabits the Atlantic, Pacific, and Indian Oceans, extending as far north as the British {679}seas, Newfoundland, California, and Japan. It supplies important fisheries in France and Italy. The Tunnies are the only fish known to be warm-blooded.

FAM. 4. TRICHIURIDAE.—Praemaxillaries not protractile. Vertebrae 32 to 160, without transverse processes; ribs sessile, on the centra or on the haemal arches when these are present; epipleurals, if well developed, on the centra. Scales very small or absent. Spinous portion of dorsal fin much longer than the soft, the spines more or less feeble. Pectoral fins inserted low down the sides. Pseudobranchiae present.

The members of this family show a great variation in the shape of the body, which, although always strongly compressed, is not unlike that of a Mackerel in the more normal types, such as Thyrsites and Ruvettus, whilst, through a chain of genera, it generally assumes an extremely elongate form; concurrently with this elongation of the body, the dorsal fin loses its differentiation into two portions, the ventrals become reduced and disappear, as in the Scabbard- or Frost-Fish (Lepidopus caudatus), while the caudal fin decreases in size, loses its fork-shape, and is finally lost in Trichiurus, in which the body is ribbon-shaped and tapers to a point.

About 25 species are known, pelagic and widely distributed, many descending to great depths.

Principal living genera: Ruvettus, Thyrsites, Epinnula, Nesiarchus, Nealotus, Promethichthys, Dicrotus, Gempylus, Aphanopus, Lepidopus, Euoxymetopon, Benthodesmus, Eupleurogrammus, Trichiurus.

Remains of several species referred to Thyrsites, Lepidopus, and to the extinct genera Thyrsitocephalus, Hemithyrsites, and Trichiurichthys, have been found in the Oligocene and Miocene of Europe.

FAM. 5. HISTIOPHORIDAE.—Praemaxillaries not protractile; snout produced into a spear-shaped rostrum; a praedentary bone; teeth minute. Body elongate, covered with small or rudimentary scales. Vertebrae 24 or 25, without transverse processes; ribs sessile; no epipleurals. One or two dorsal fins, without a distinctly spinous portion. Pectoral fin low down the side. Pseudobranchiae present.

The Sail-Fishes are large oceanic fishes, endowed with great strength and swiftness, occurring in the tropical and sub-tropical {680}seas. Four or five species are distinguishable, and are referable to two genera: Histiophorus, with a single dorsal fin and 2 or 3 ventral rays, and Tetrapturus, with the dorsal divided into two parts and a single ventral ray.

Fossil Histiophoridae are known from the Eocene and later beds in Europe and America. Dr. A. S. Woodward observes that the known fossils are too imperfect to be referred with certainty to their respective genera. Most of them probably belong to Histiophorus, but at least one genus from the Eocene (Xiphiorhynchus) appears to be well distinguished.

The imperfectly known extinct family PALAEORHYNCHIDAE, from the seas of the Eocene, Oligocene, and Miocene periods, with the genera Palaeorhynchus and Hemirhynchus, is probably closely related to the Histiophoridae. The vertebrae number 50 to 60, and the ribs completely encircle the body. In Palaeorhynchus both jaws are equally produced into an ensiform weapon. Blochius, from the Eocene, with diamond-shaped, slightly overlapping bony scutes on the body, is perhaps also to be placed near this family.

{681}FAM. 6. XIPHIIDAE.—Differs from the preceding in the absence of praedentary bone, and in the vertebrae (26 in number), which in the praecaudal region are provided with short but well-developed transverse processes, to which the short ribs are attached. Ventral fins absent, the pectorals being inserted in the place usually occupied by them. Adult without teeth or scales.

A single species, the Sword-Fish (Xiphias gladius), of nearly world-wide distribution, occurring occasionally on the coasts of Great Britain and Ireland.

FAM. 7. LUVARIDAE.—Mouth small, praemaxillaries not protractile, with very feeble dentition. Gill-membranes attached to the isthmus. Vertebrae 23, without transverse processes; ribs blade-like, inserted on the centra; no epipleurals. Body rough, with minute scales. Dorsal and anal fins elongate, formed of unarticulated, widely set rays. Pectoral fins inserted rather low down; ventrals much reduced, the two halves of the pelvis fused into a single bone. Supraclavicle fused with the forked post-temporal. Hypural bones completely embraced by the forked bases of the caudal rays. Pseudobranchiae present.

Luvarus imperialis, a rare pelagic fish from the Atlantic, Mediterranean, and Pacific, growing to a length of 6 feet, and occasionally captured on our coasts, is the only representative of this family. Very little is known of the habits of this strange fish. The excessive length of the intestines and the feeble dentition point to its feeding partly on vegetable matter, partly on minute animals; the circumstances under which certain specimens were captured tend to indicate that they follow up streams of pelagic life such as engage the attention of the Basking Shark, of similar distribution.

FAM. 8. CORYPHAENIDAE.—Praemaxillaries small, not protractile. Vertebrae 30 to 33, without transverse processes; ribs and epipleurals attached close together on the centra. Body elongate; scales small, cycloid or elongate lanceolate. Dorsal and anal fins much elongate, without distinct spines. Pectoral fins inserted rather low down the side. Pseudobranchiae absent.

The "Dolphins" (Coryphaena), of which only two species can be distinguished, are large pelagic fishes, of carnivorous habits, pursuing the Flying-Fish. They grow to a length of 6 feet, and their flesh is much valued. Their deep head, with short snout, {682}and their long posteriorly attenuate body ending in a large forked caudal fin, give them a peculiar appearance.

FAM. 9. BRAMIDAE.—Praemaxillaries small, not or but feebly protractile; maxillaries large, scaly. Vertebrae 42 to 47, the praecaudal without transverse processes, but mostly with hæmal arches to which the ribs are attached, the epipleurals being inserted on the centra. Body deep; scales moderate or large, strongly imbricate, with processes which, in certain parts at least, serve to connect the rows of scales. Dorsal and anal elongate, some or all of the rays simple, but not forming true spines. Pectoral inserted rather low down the side, freely movable upwards and downwards. Pseudobranchiae present.

Pelagic fishes, often descending to great depths. About 12 species are known, referable to 6 genera: Brama, Taractes, Pterycombus, Pteraclis, Bentenia, and Steinegeria. Taractes, often confounded with Brama, differs from it not only in the larger, keeled scales, but also in the protractile mouth and in the much greater development of most of the ribs, which form curved lamellae of great width. Pteraclis is very remarkable for the enormous, sail-like dorsal and anal fins.

DIVISION III.—ZEORHOMBI.

Aberrant, strongly compressed Perciformes, with very short praecaudal region, modified much as in the Flat-Fishes, culminating in asymmetrical forms, and characterised by the combination of an increased number (7 to 9) of ventral rays, with absence of hypural spine (by which the Berycidae are excluded), or by asymmetry of the skull in the forms in which the spine of the ventral fin has been lost.

Among the symmetrical forms, the existing Zeidae agree with the Berycidae in having more than five soft rays to the ventral fins, and are probably derived, together with the Eocene Amphistiidae, from some common ancestral group still to be discovered in Cretaceous beds. These Zeidae have much in common with the Pleuronectidae, and might be regarded as {683}forming part of the family out of which the latter have sprung, were it not that they have lost the last half-gill. Amphistium is probably more nearly related to the Pleuronectidae, which may have been directly derived from the family of which it is as yet the only known representative.

This division embraces three families only:—

A distinct spinous dorsal fin; anal spines detached from the soft portion; a ventral spine; gills three and a half, four slits between them 1. Zeidae.

Dorsal and anal spines few, continuous with the soft rays; a ventral spine 2. Amphistiidae.†

No spines; cranium twisted in front, with the two orbits on one side; gills 4, a slit behind the fourth 3. Pleuronectidae.

† Extinct.

FAM. 1. ZEIDAE.—No subocular shelf; praemaxillaries strongly protractile. Gill-membranes free from isthmus; 7 or 8 branchiostegal rays; gills 3½; pseudobranchiae well developed. Lower pharyngeal bones separated. Vertebrae 30 to 46, the anterior with sessile ribs, the posterior praecaudals with long neural spines bent forwards and with transverse processes directed downwards, forming haemal arches and bearing the ribs at their extremity; epipleurals much reduced or absent; hypural large, without the basal spine or knob present in most Perciformes and all Scombriformes and Percesoces, bearing fewer than 20 rays. Dorsal and anal fins elongate, the former with a distinct spinous portion, the latter with 1 to 4 spines detached from the soft portion. Pectoral fin supported by 4 pterygials, of which 3 are in contact with the perforated scapular bone; post-temporal forked and solidly attached to the skull. Ventral fin with 1 spine and 6 to 8 soft rays.

Scales small or minute, sometimes hard and rough and firmly joined in vertical series; bony plates may be present along the base of the vertical fins. Air-bladder present.

Twelve species are known from the Atlantic and Pacific Oceans, referable to 5 genera: Grammicolepis, Cyttus, Cyttopsis, Zenion, and Zeus. Oreosoma was founded on a young form of a fish allied to Cyttus. Remains of Zeus occur in the Oligocene, and Cyttoides, from the same period, has been compared with Cyttus.

The well-known John Dory (Zeus faber) is much valued for the table.

{684}FAM. 2. AMPHISTIIDAE.—The only known representative of this family, the Upper Eocene Amphistium paradoxum, originally described as a Pleuronectes, has much in common with the Zeidae, from which it differs in the smaller number of vertebrae (10 + 14), and in the dorsal and anal spines being more reduced, adnate and continuous with the series of soft rays; the scales are more normal and imbricate; ventral fins with 1 spine and 8 soft rays. This fish appears to realise in every respect the prototype of the Pleuronectidae before they had assumed the asymmetry which characterises them as a group.

FAM. 3. PLEURONECTIDAE.—Head asymmetrical, the skull twisted in front, with the two orbits on one side in the adult; the side of the body bearing the eyes and turned upwards in life being coloured, the other side colourless and blind. Mouth more or less protractile. Gills 4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones usually separated, rarely imperfectly united. Vertebrae 24 in the most generalised form (Psettodes), varying from 28 to 65 in others, the praecaudals mostly with more or less developed transverse processes, which may be directed downwards and become converted into haemal arches; ribs and epipleurals present. Caudal fin, if well developed, supported by a large hypural usually without basal spine or knob. Dorsal and anal fins much elongate, without spines, the former often extending on the head. Paired fins often reduced, {685}sometimes absent; if fully developed and normally formed, the bones of the pectoral and pelvic girdles as in the Zeidae. Ventral fins usually with 5 to 7 soft rays.

Scales usually imbricate, cycloid or ctenoid; rarely absent; bony tubercles sometimes present. Air-bladder absent.

Most species, and even genera, are either sinistral or dextral, but this is inconstant in some, including the most generalised genus, Psettodes. The very young are transparent and symmetrical, with an eye on each side, and swim in a vertical position like other Fishes. These larval forms have been described as distinct genera, under the names of Peloria, Bibronia, Charybdia, etc. As they grow, the eye of one side moves by degrees to the other side, where it becomes the upper eye. If at that age the dorsal fin does not extend to the frontal region, the migrating eye simply moves over the line of the profile, temporarily assuming the position which it preserves in Psettodes, Atheresthes, and Platysomatichthys; in other genera, the dorsal fin has already extended to the snout before the migration takes place, and the eye, passing between the frontal bone and the tissues supporting the fin, appears to pass from side to side through the head, as was believed by some of the earlier observers.

Flat-fishes are a large group of some 500 species, mostly marine, a few species related to the Soles being confined to the fresh waters of South America and the Malay Archipelago. They range from the Arctic Circle to the southern coasts of the Southern Hemisphere; many occur at great depths (Citharichthys dinoceros down to 955 fathoms). Well-preserved remains referred to Psetta occur in the Upper Eocene, and a species of Solea is known from the Lower Miocene.

{686}[Illustration: FIG. 418.—Outlines of various Pleuronectids, showing differences of form. A, Psettodes erumei; B, Pleuronectes platessa; C, Psetta maxima; D, Solea vulgaris; E, Cynoglossus lingua.]

{687}A satisfactory classification of the Pleuronectidae is still a desideratum, and cannot be attempted until the osteology of the very numerous forms has been thoroughly studied. Even the division into two principal groups, regarded by some recent authors as families, Pleuronectidae and Soleidae, is based on characters which the examination of a large number of generic types shows not to be constant. Thus the former have been defined as having the praeopercular margin distinct externally, the snout not projecting beyond the mouth, the nostrils of the two sides on the coloured side or those of the blind side high up near the dorsal line of the head; the latter as having the praeoperculum hidden under the skin, the snout projecting more or less beyond the mouth, and the nasal organ of the blind side similarly situated to that of the eyed side. However, the genera Aphoristia and Peltorhamphus, and others among the Soles, show exceptions to this definition.

Principal genera: Psettodes, Atheresthes, Platysomatichthys, Hippoglossus, Hippoglossoides, Hippoglossina, Poecilopsetta, Chascanopsetta, Paralichthys, Pleuronectes, Glyptocephalus, Citharus, Rhomboidichthys, Psetta (Rhombus), Arnoglossus, Zeugopterus, Lepidorhombus, Ammotretis, Rhombosolea, Solea, Achirus, Achiropsis, Soleotalpa, Synaptura, Ammopleurops, Aphoristia, Cynoglossus, Symphurus (Plagusia).

The following are the principal British representatives which are valued as food:—The Halibut (Hippoglossus vulgaris), by far the largest of all Flat-Fishes, growing to a length of 10 feet or more; the Long Rough Dab (Hippoglossoides limandoides); the Plaice (Pleuronectes platessa); the Flounder (P. flesus), which ascends streams; the Dab (P. limanda); the Smear Dab, often called Lemon Sole (Glyptocephalus microcephalus); the Witch (G. cynoglossus); the Megrim or Whiff (Lepidorhombus megastoma); the Turbot (Psetta maxima); the Brill (P. laevis); and the Sole (Solea vulgaris).

DIVISION IV.—KURTIFORMES.

No bony stay for the praeopercle. Dorsal spines feeble, few. Scapula absent, the coracoid supporting four small pterygials. Ventral fins thoracic.

FAM. 1. KURTIDAE.—The genus Kurtus, with a single species, from the Indian and Pacific Oceans, forms an isolated, very {688}aberrant group. The strongly compressed body is covered with minute, rudimentary scales; the dorsal is short, with few, graduated spines, and the anal much elongate, with 2 small spines; the ventrals are formed of 1 spine and 5 soft rays. The vertebral column consists of 24 vertebrae; the ribs of the third and fourth are free and slender, whilst the following are immovably fixed between rings formed by the ossification of the outer membrane of the elongate air-bladder in a manner unique among fishes. The skull is peculiar for its very strong, denticulate, occipital crest, which ends posteriorly in a curved spine bent forwards; the suborbitals are slender and do not emit a subocular lamina. Kurtus indicus does not exceed a length of 5 inches.

DIVISION V.—GOBIIFORMES.

No bony stay for the praeoperculum. Basis cranii simple. Spinous dorsal, if present, formed of few, flexible rays. None of the epipleural bones attached to the centra of the vertebrae in the praecaudal region. Scapula and coracoid more or less reduced or even vestigial; pterygials large, 4 or 5 in number, forming together a thin plate which is in contact with or narrowly separated from the clavicle; one or two of the pterygials in contact with the coracoid. Ventral fins thoracic.

The Gobiidae, which alone constitute this division, are not very remote from the Perciformes, and may have evolved out of a type not very different from the Percidae.

{689}FAM. 1. GOBIIDAE.—Suborbital arch ligamentous or absent. Gill-membranes more or less broadly attached to isthmus; 4 to 6 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae often present. All or most of the praecaudal vertebrae with transverse processes bearing the ribs, to which epipleurals are attached. Post-temporal forked, as in normal Perciformes. Ventral fins with 1 feeble spine and 4 or 5 branched rays, often united to form a sucking disk, a transverse fold of skin at their base completing the cup.

Head usually more or less depressed, body varying from short and stout to elongate and eel-shaped, but never with a very high number of vertebrae, these varying from 24 to 37 (10-14 + 13-24); scales cycloid or ctenoid, or absent; no lateral line; mouth moderate or large, dentition various; soft dorsal and anal fins nearly equally developed, varying from very short to very elongate; usually a large anal papilla.

A large family of some 600 species, the great majority marine, mostly carnivorous and of small size. The largest form (Eleotris marmorata, from the rivers of Siam, Borneo, and Sumatra) grows to nearly 3 feet, whilst the smallest (Mistichthys luzonensis, from the Philippines) measures only 12 to 14 millimetres, and is believed to be the smallest known Vertebrate. Gobiids occur in all the seas outside the Arctic and Antarctic circles, and they have representatives in the fresh waters of all parts of the world.

The genera are numerous but difficult of definition. The following are the principal: Eleotris, Oxymetopon, Vireosa, Rhyacichthys, Gobius, Crystallogobius, Aphia, Gobiosoma, Gobiodon, Benthophilus, Typhlogobius, Luciogobius, Sicydium, Lentipes, Periophthalmus, Boleophthalmus, Amblyopus, Trypauchen, {690}Trypauchenichthys. Oxuderces, which has been made the type of a distinct family, appears to differ from Trypauchen only in the absence of ventral fins. Fossils referred to Gobius have been described from the Upper Eocene and Miocene of Europe, but there is no satisfactory evidence that they really belong to this family.

Gobius, of which several species are of common occurrence on our shores, have attracted special interest from their habits during the much prolonged breeding season. The male, usually more brilliantly coloured than the female, mounts guard over the eggs, which are either simply fixed by the female to the under surface of stones or weeds, or in a sort of nest built and kept in constant repair by him. This nest is usually made of a shell of Cardium, Patella, Haliotis, etc., or of the carapace of a crab, with the convexity turned upwards and covered with sand; the sand underneath is hollowed out, and a round opening at the side, coated by a mucus secreted by the skin of the male fish, gives access to the interior; the eggs, which are elongate and pyriform, are stuck to the inner surface of the shell forming the roof. A curious British form is Aphia pellucida, two inches long which, from its transparent and almost colourless body, has long been erroneously supposed to be the fry of some larger fish. Among exotic forms, mention should be made of the Blind Goby (Typhlogobius californiensis), two inches long, uniform light pink, scaleless, with the eyes very small, reduced to mere vestiges, covered by skin, and functional only in the young, living like a slug under rocks between tide marks on the coast of California; and to the Walking-Fish or Jumping-Fish (Periophthalmus), of which various species are found in great abundance on the mud-flats at the mouths of rivers in tropical Africa, Asia, and North-West Australia, skipping about by means of the muscular, scaly base of their pectoral fins, with the head raised and bearing a pair of strongly projecting versatile eyes close together.

{691}DIVISION VI.—DISCOCEPHALI.

Highly aberrant Acanthopterygians with the anterior dorsal fin modified into a suctorial, transversely laminated oval disk on the head, the skull being very much flattened and with simple basis cranii. The pectoral rays are inserted on the small, perforate, scapula and on four hour-glass-shaped pterygials, three of which are in contact with the coracoid. Ventrals thoracic.

FAM. 1. ECHENEIDIDAE.—Maxillary slender, adnate to the upper surface of the praemaxillary; suborbital arch slender. Pectoral fin inserted high up; supraclavicle much reduced; ventral fin with one spine and five soft rays. Body elongate and covered with small scales; soft dorsal and anal fins elongate and opposed to each other. All the praecaudal vertebrae with very strong parapophyses, the anterior with diapophyses as well; ribs and epipleurals nearly equally developed, both inserted at the extremity of the parapophyses.

In spite of a superficial external resemblance to the genus Elacate, the Sucking-Fish bear certainly no affinity to that genus nor to other Scombriformes, as first observed by Gill. They are probably derived from Perciformes, but from which family it is impossible to suggest. Three genera may be distinguished: Opisthomyzon, from the Upper Eocene of Switzerland, with a very small suctorial disk and 23 or 24 vertebrae; Echeneis, with large disk and 30 vertebrae; and Remora, distinguished from the second by a shorter body with only 27 vertebrae. These remarkable fishes, of which about 10 species are distinguished, are distributed all over the tropical and warm seas, and exceptionally carried as far north as the south coast of England. They feed on other fishes, and attach themselves by means of their cephalic {692}sucker to boats or to sharks, turtles, cetaceans, and other large swift-swimming animals. On the East Coast of Africa they are employed by the natives for catching turtles, to the carapace of which they stick with extraordinary tenacity, being held by a line attached to a metal ring round the caudal peduncle. The largest Sucking-fish grows to a length of three feet.

DIVISION VII.—SCLEROPAREI.

Second suborbital bone more or less produced towards or ankylosed with the praeoperculum ("suborbital stay"). Ventral fins thoracic.

The "Cheek-armoured Acanthopterygians," "Joues cuirassées" of Cuvier, after the exclusion of the Sticklebacks, form a perfectly natural association, evidently derived from the Serranidae, with which the more generalised forms have much in common. From the Perch-like genus, Sebastes, a continuous series can be traced towards the Triglidae, especially through such forms as Apistus, Minous, and Choridactylus, in which one or more of the lower pectoral rays are detached from the rest of the fin. Through the Comephoridae the Scorpaenidae are connected with the Cottidae, whilst the latter merge insensibly into the still more aberrant Cyclopteridae. These conclusions, which are apparent enough from a mere comparison of the external characters, become fortified by a study of the skeletons. The passage between the various groups here accepted as families is so complete that no {693}serious objection could be raised to their union in one great family with a number of minor divisions.

The character from which the Scleroparei derive their name is subject to many modifications. The second suborbital (the third if the praeorbital be regarded as the first) may be merely enlarged and prolonged over the cheek towards the praeoperculum (Sebastes, Anoplopoma), or firmly ankylosed to the latter (Scorpaena, Platycephalus), or form part of the external armature of the head (Trigla, Dactylopterus). The structure of the base of the pectoral fin appears to afford important characters for the definition of the families, as first pointed out by Gill; these {694}characters have, however, not yet been tested on a sufficient number of the very numerous forms grouped under Cottidae, some of which I have already transferred to the Comephoridae.

SYNOPSIS OF THE FAMILIES.

I. Head not completely cuirassed.

A. Ventral fins not widely separated; none of the pectoral pterygials in contact with the clavicle.

Two nostrils on each side; basis cranii double; gill-membranes free from isthmus 1. Scorpaenidae.

A single nostril on each side; basis cranii double; gill-membranes free from isthmus 2. Hexagrammidae.

Two nostrils on each side; basis cranii simple; gill-membranes free or narrowly attached to isthmus 3. Comephoridae.

Two nostrils on each side; basis cranii simple; gill-opening narrow, above base of pectoral 4. Rhamphocottidae.

B. Ventral fins, if present, not widely separated; one or several of the pterygials in contact with the clavicle.

Ventral fins distinct; gill-clefts wide 5. Cottidae.

Ventral fins united into a sucking disk; gill-opening narrow, above base of pectoral 6. Cyclopteridae.

C. Ventral fins widely separated; none of the pterygials in contact with the clavicle.

Ventral fins behind base of pectorals; praecaudal vertebrae without transverse processes 7. Platycephalidae.

Ventral fins a little in front of base of pectorals; praecaudal vertebrae with transverse processes 8. Hoplichthyidae.

II. Head completely cuirassed.

Ventral fins narrowly separated; no pectoral appendages; pterygials short and broad 9. Agonidae.

Ventral fins widely separated; 2 or 3 lowermost rays of pectoral fin detached as feelers; pterygials short and broad 10. Triglidae.

Ventral fins narrowly separated; pectoral fin divided into two portions; pterygials elongate 11. Dactylopteridae.

FAM. 1. SCORPAENIDAE.—Head not or but incompletely cuirassed, usually with spines; basis cranii double; parietal bones often meeting on the median line, over the supraoccipital; two nostrils on each side. Gill-membranes free from isthmus; gills 3½ or 4; pseudobranchiae present. Vertebrae 24 to 37, the anterior praecaudals with sessile ribs bearing epipleurals, the posterior with transverse processes, often directed downwards, or forming haemal arches, bearing the rib and the epipleural. Post-temporal more or less distinctly forked, more or less firmly ankylosed to the skull; scapula and coracoid well developed, in contact with each other or separated by cartilage; pectoral rays inserted {695}on the scapula and on 3 or 4 large, hour-glass or anvil-shaped pterygials, two of which are in contact with the coracoid. Ventral fins close together, with 1 spine and 3 to 5 soft rays. Spinous dorsal strong, usually longer than the soft, sometimes extending on the head; anal usually with 3 spines. Body covered with scales or naked.

A large family of carnivorous marine fishes, some descending to great depths, of nearly world-wide distribution, represented by three extinct genera (Ampheristus, Histiocephalus, Scorpaenoides) in the Eocene and by several species of Scorpaena in later formations. About 250 recent species are known. Principal genera: Sebastes, Setarches, Scorpaena, Pterois, Apistus, Minous, Pelor, Choridactylus, Centropogon, Gymnapistus, Amblyapistus, Pentaroge, Tetraroge, Gnathacanthus (Holoxenus), Agriopus, Synancia, Polycaulus.

Great variety of form obtains in this family, from the Perch-like Sebastes to the extraordinary-shaped Tetraroge, Pelor, and Synancia. Many of its members are excellent examples of mimetic adaptation to the surrounding, resembling the rocks among which they live and being covered with dermal appendages simulating weeds. An interesting example of commensalism has been discovered by A. Alcock in Minous inermis, off the coasts of India, which, wherever found, is always more or less incrusted with the {696}Gymnoblastic Hydroid Stylactis minoi. Many of the Sebastes and their allies are of large size and used as food; some are viviparous, the young being produced in great numbers and very small in size. Scorpaena, Pterois, Pelor, and Synancia are dangerous for the stings from their dorsal spines, which are provided with poison glands.

FAM. 2. HEXAGRAMMIDAE.—Head not cuirassed, without strong spines; basis cranii double; a single nostril on each side. Gill-membranes free from isthmus; gills 4; pseudobranchiae present. Vertebrae 42 to 57, most of the praecaudals with transverse processes bearing the ribs and the epipleurals. Post-temporal forked; scapula and coracoid well developed, in contact with each other; pectoral rays inserted on the scapula and 4 anvil-shaped pterygials, 2 of which are in contact with the coracoid. Ventral fins close together, more or less behind the pectorals, with 1 spine and 5 soft rays. Spinous dorsal of usually rather feeble rays, nearly as long as or longer than the soft; anal elongate, with or without spines. Body covered with small scales.

Carnivorous fishes, mostly of large size, from the rocky coasts of the North Pacific. Some are highly valued as food. Twelve species, referable to 6 genera: Hexagrammus, Pleurogrammus, Agrammus, Ophiodon, Zaniolepis, Oxylebius. Hexagrammus and Pleurogrammus are remarkable in having 4 or 5 lateral lines on each side.

FAM. 3. COMEPHORIDAE.—Head not cuirassed, without spines; basis cranii simple; two nostrils on each side. Gill-membranes free or narrowly attached to isthmus; gills 4; pseudobranchiae present or absent. Vertebrae 42 to 64, some or most of the praecaudals with transverse processes bearing the ribs and the epipleurals. Post-temporal forked; scapula and coracoid well developed, in contact with each other or separated by cartilage; pectoral rays inserted on the scapula and on 4 anvil-shaped or plate-like pterygials, 2 of which are in contact with the coracoid. Ventral fins, if present, close together, with 1 spine and 3 to 5 soft rays. Spinous dorsal of rather feeble rays, as long as or shorter than the soft; anal spines feeble or absent. Body covered with small scales or naked.

{697}Four genera, each with a single species: Anoplopoma (Scombrocottus), from the North Pacific from Unalaska to California; Triglopsis, from deep water in Lakes Michigan and Ontario; Cottocomephorus, from Lake Baikal, and Comephorus from the greatest depths of that lake. As in many bathybial forms, Comephorus is colourless and provided with very large eyes; ventral fins are absent and the skeleton is very thin and papery. As a result of this condition, the second suborbital is not produced over the cheek, a unique exception to the main characteristic of this division; but no doubt can be entertained as to the propriety of referring it to the neighbourhood of Anoplopoma, since the recently discovered Cottocomephorus may be regarded as a connecting link between the two genera. Comephorus is viviparous, and dies after parturition. Jordan regards Triglopsis as a relic of a former Arctic marine fauna.

FAM. 4. RHAMPHOCOTTIDAE.—Head incompletely cuirassed, with spines; basis cranii simple; two nostrils on each side. Gill-opening narrow, above the base of the pectoral; gills 3½. Vertebrae 24. Post-temporal short and flat, ankylosed to the skull; scapula and coracoid well developed, separated by cartilage; pectoral rays inserted on the scapula and on 4 plate-like pterygials, 2 of which are in contact with the coracoid. Ventral fins close together, behind the pectorals, with a rudimentary spine and 3 soft rays. Spinous dorsal shorter than the soft; no anal spines. Body densely covered with small prickly scales.

Rhamphocottus richardsonii, a small fish 3 inches in length, from the north-west coast of North America, is the only representative of this family.

FAM. 5. COTTIDAE.—Head not or but incompletely cuirassed, usually with spines; basis cranii simple; parietal bones often meeting on the median line; two nostrils on each side. Gill-membranes free or attached to isthmus; gills 3½ or 4; pseudobranchiae usually present. Vertebrae 24 to 50, the anterior praecaudals with sessile ribs, the posterior with transverse processes, often directed downwards, or forming haemal arches, bearing ribs and epipleurals. Post-temporal more or less distinctly forked; scapula and coracoid separated from each other {698}by the intervention of the plate-like pterygials, of which one, two, or three are in contact with the clavicle; the coracoid more or less reduced. Ventral fins close together, with 1 spine and 2 to 5 soft rays (absent in Ereunias). Spinous dorsal usually shorter than the soft, sometimes quite indistinct; anal without spines. Body naked, partially scaly, or with prickles or bony plates.

Mostly small carnivorous fishes, the largest (Scorpaenichthys) growing to about 3 feet. Some species inhabit fresh waters, but the majority are marine, a few descending to great depths. Nearly all are from the northern regions, but a genus allied to Cottus (Sclerocottus) is from South Georgia, in the Antarctic region. Fossil Cottidae are known from the Upper Eocene and Miocene (Eocottus, Lepidocottus), and are distinguished from the modern forms in the smaller number of vertebrae (24 or 26 instead of 30 to 50). At least 220 species are known. Principal genera: Jordania, Scorpaenichthys, Icelus, Triglops, Cottus, Cottunculus, Blepsias, Pseudoblennius, Hemitripterus, Synchirus, Ascelichthys, Psychrolutes, Ereunias. The little freshwater "Miller's Thumb" (Cottus gobio) and the larger marine "Bull-heads" (C. bubalis and C. scorpius) are the most familiar British representatives of this family. The eggs are deposited on stones, weeds, or other submerged objects, or in a sort of nest, and are guarded by the male, which in most species is distinguished by a large genital papilla; this, in some forms, acts as an intromittent organ.

FAM. 6. CYCLOPTERIDAE.—Very closely related to the preceding, with which they are connected through Psychrolutes, and it is even doubtful whether they deserve to be separated from them. The only important distinctive characters reside in the structure of the ventrals, which, if present (absent in Paraliparis, a close ally of Liparis), are united to form a sucking disk, and the small size of the gill-cleft. The body is short, tumid, tadpole-like, naked or tubercular; the spinous dorsal, if present, is short. Vertebrae 28 to 60, the skeleton feebly ossified.

Sluggish fishes, feeding on small animals and plants, from the North Atlantic and Pacific Oceans, and the Arctic and Antarctic seas, many descending to great depths (1800 fathoms). About fifty species are distinguished. Principal genera: Cyclopterus, Cyclopterichthys, Liparops, Liparis, Careproctus, Paraliparis.

{699}The common Lump-Sucker of our coasts (Cyclopterus lumpus) is the largest member of the group, growing to a length of 2 feet or more. The male makes pits in the sand between stones, in which the female deposits the eggs; he watches over the eggs and also over the young, which cling to his body with their suckers. The "Sea-Snails" (Liparis), are represented by two species on the British coasts.

FAM. 7. PLATYCEPHALIDAE.—Head not cuirassed, much depressed, with spines; basis cranii simple; two nostrils on each side. Gill-membranes free; gills 4; pseudobranchiae present. Vertebrae 27; ribs all sessile, bearing the epipleurals. Post-temporal forked; scapula and coracoid well developed, in contact with each other; pectoral rays inserted on the scapula and on 4 short and broad pterygials, 2 of which are in contact with the coracoid. Ventral fins widely separated, behind the pectorals, with 1 spine and 5 soft rays. Spinous dorsal shorter than the soft; anal without spines. Body covered with small scales.

The single genus Platycephalus, with some 40 species, inhabits the coasts of the Indian Ocean and the Western Pacific.

FAM. 8. HOPLICHTHYIDAE.—Head incompletely cuirassed, much depressed, with spines; basis cranii simple; two nostrils on each side. Gill-membranes attached to isthmus; gills 4; pseudobranchiae present. Vertebrae about 30, the praecaudals with transverse processes. Post-temporal fused with the skull; scapula and coracoid in contact with each other; pectoral rays inserted on the scapula and on 3 plate-like pterygials. Ventral fins widely separated, a little before the pectorals, with 1 spine and 5 soft {700}rays. Spinous dorsal shorter than the soft; anal without spines. Back and sides with bony, prickly plates.

Hoplichthys, with a single species from the coasts of Japan and China. Bembras, with two species from the coasts of Japan, appears to be related to it, but the skeleton is still unknown; it differs in having the body covered with small scales and the gill-membranes free.

FAM. 9. AGONIDAE.—Head completely cuirassed, usually with spines; basis cranii simple; two nostrils on each side. Gill-membranes free or attached to isthmus; gills 3½; pseudobranchiae present. Vertebrae 35 to 50; ribs sessile. Post-temporal fused with the skull; scapula and coracoid in contact with each other, or separated by a cartilaginous space; pectoral rays inserted on the scapula and 3 or 4 plate-like pterygials. Ventral fins close together, with 1 spine and 2 soft rays. Spinous dorsal shorter than the soft, or absent; anal without spines. Body covered with bony plates.

Small fishes, mostly from the coasts of the Northern Atlantic and Pacific, extending into the Arctic Ocean; one species from the coast of Chili. Bathyagonus occurs in the North Atlantic between 350 and 477 fathoms.

About 40 species are known. Principal genera: Agonus, Agonopsis, Bathyagonus, Aspidophoroides. The "Pogge," or Armed Bullhead (Agonus cataphractus), is the only British species of this family.

FAM. 10. TRIGLIDAE.—Head completely cuirassed, with spines; basis cranii double; parietal bones meeting on the median line; two nostrils on each side. Gill-membranes free; gills 4; pseudobranchiae present. Vertebrae 25 to 40, the anterior praecaudals with sessile ribs, the posterior with transverse processes. Post-temporal fused with the skull; scapula and coracoid separated by a cartilaginous space; pectoral rays inserted on the scapula and on 4 large plate-like pterygials, of which two are in contact with the coracoid; 2 or 3 of the lower pectoral rays detached, forming feelers. Ventral fins widely separated, with 1 spine and 5 soft rays. Spinous dorsal shorter than the soft; anal without spines. Body covered with scales or bony plates.

Marine fishes from all warm and temperate regions, some occurring in deep water. They are remarkable for the {701}finger-like appendages of the pectoral fins, which are employed to feel the ground in search of crustaceans and other small animals on which they feed; also for the grunting sounds which they utter by the contraction of the air-bladder. About 50 species are known, referable to 4 genera: Prionotus, Trigla, Lepidotrigla, Peristedion. Fossil remains referred to Trigla have been found in Miocene and later formations. British species are the Grey Gurnard (Trigla gurnardus), the Red Gurnard (T. cuculus), the Tub or Sapphirine Gurnard (T. hirundo), the Piper (T. lyra), the Long-finned Gurnard (T. obscura), and the Streaked Gurnard (T. lineata).

FAM. 11. DACTYLOPTERIDAE.—Head completely cuirassed; basis cranii simple; parietal bones meeting on the median line; two nostrils on each side. Gill-cleft broadly separated by scaly isthmus; gills 4; pseudobranchiae present. Vertebrae 20-22 (8-9 + 12-13), the first very elongate and formed by the fusion of three or four; ribs sessile, no transverse processes. Post-temporal fused with the skull; no supraclavicle; scapula and coracoid well developed, in contact with each other; pectoral rays divided into two parts, inserted on the scapula and on 4 elongate pterygials, of which 3 are in contact with the coracoid. Ventral fins close together, with 1 spine and 4 soft rays. Spinous dorsal shorter than the soft; anal without spines. Body covered with hard, rough scales.

The "Flying Gurnards," of which four species are known, belonging to a single genus (Dactylopterus), are inhabitants of the tropical and warm parts of the Atlantic and the Indian Ocean and Archipelago. They are remarkable, when adult, for the {702}wing-like portion of the pectoral fins, by which they are able to move in the air like Exocoetus, but for shorter distances, and, unlike them, the wings are moved rapidly, the mode of flight resembling that of many forms of grasshoppers; the young, however, have comparatively short pectorals, and were formerly regarded as belonging to a distinct genus (Cephalacanthus).

DIVISION VIII.—JUGULARES.

No bony stay for the praeoperculum. Ventral fins jugular or mental. Gill-openings in front of the pectoral fin, the base of which is vertical or subvertical.

In a recently published note I have alluded to the group of Physoclistous fishes for which I proposed to revive the old name Jugulares, pointing out that some of the forms previously grouped together as Trachinidae agree with the Gadidae, not only in the jugular position of the ventral fins, but also in the condition of the scapula and coracoid. Mr. Regan has since been able to show that the Gadidae and Macruridae possess certain characters in common by which they may be separated not only from the other Jugulares, but even from the Acanthopterygians, and, as mentioned above (p. 646), the Müllerian Sub-order Anacanthini may be maintained, after excluding the Pleuronectidae. That the Blenniidae are akin to Lycodes and its allies has long been admitted, and authors who have placed them in different divisions of their systems have had to confess the difficulty of referring certain genera to the one family rather than to the other. The fact that Lycodes and many forms previously associated with the Ophidiidae agree with the Macruridae and Gadidae in the diphycercal vertebral column and in the absence of spines to the fins is merely, it seems to me, the result of degradation; they probably form the terminal group of a series in which the vertebral column was originally homocercal and fin-spines were present, as is the case in most of the Blenniidae and Trachinidae and their near allies. All these families may be assumed to have evolved in several series, often on parallel lines, from some group closely related to the {703}Berycidae; and the resemblance which their terminal forms bear to the Anacanthini is, as pointed out by Regan, probably to be ascribed to convergence, not to any close genetic affinity, as hitherto believed by many authors.

The character of the position of the scapular foramen, either in the scapular bone or between it and the coracoid, which obtains in many genera of this division as well as in most of the Anacanthini, has proved to be unreliable even for the purpose of family definition; it is, however, of assistance in determining the relation of certain obscure, degraded forms placed by some authors with the Anacanthines, by others with the Blenniids.

SYNOPSIS OF THE FAMILIES.

I. Pectoral rays attached to the scapula and to a series of pterygials of which only one or two are in contact with the scapula (see Fig. 427); ventral fins jugular, with 1 spine and 4 or 5 soft rays; anterior dorsal rays usually spinous or not articulated, often forming a detached fin.

A. Epipleurals present.

1. Second suborbital produced inwards to support the eye-ball.

Ventrals close together; scales very small, cycloid, forming oblique bands 1. Trachinidae.

Ventrals widely separated 2. Percophiidae.

2. No subocular shelf.

Ventrals widely separated; two nostrils on each side 3. Leptoscopidae.

Ventrals widely separated; a single nostril on each side 4. Nototheniidae.

Ventrals close together; scales very small, forming oblique bands; head partly covered with bony plates 5. Uranoscopidae.

B. No epipleurals. {704}

Post-temporal forked, articulated to the skull; soft dorsal and anal much elongate 6. Trichodontidae.

Post-temporal closely adnate to the skull; soft dorsal and anal short (with only 7 to 10 rays) 7. Callionymidae.

Post-temporal simple, articulated to the skull; soft dorsal and anal short; a ventral sucker 8. Gobiesocidae.

II. Pectoral rays all attached to the pterygials, of which two or three are in contact with the scapula; ventral fins, if present, jugular or mental, composed of 1 to 4 rays.

A. Ventrals jugular or absent.

Post-temporal distinctly forked; praecaudal vertebrae with transverse processes; some or all of the dorsal rays spinous or not articulated; caudal fin usually distinct 9. Blenniidae.

Post-temporal small and ankylosed to the skull; praecaudal vertebrae without well-developed transverse processes; a very short spinous dorsal; caudal fin distinct 10. Batrachidae.

Post-temporal distinctly forked; praecaudal vertebrae with haemal arches; dorsal rays all spinous; caudal fin distinct 11. Pholididae.

Post-temporal distinctly forked; praecaudal vertebrae with transverse processes; dorsal rays all articulated, or a few of the posterior spinous; no distinct caudal fin 12. Zoarcidae.

Post-temporal forked, ankylosed to the skull; praecaudal vertebrae with transverse processes; no spines; no distinct caudal fin 13. Congrogadidae

B. Ventrals mental (just behind the chin); no spines 14. Ophidiidae.

III. Pectoral rays attached to an undivided cartilaginous plate representing the pterygials; ventral fins jugular, reduced to a filament formed of two adnate rays; fins without spines 15. Podatelidae.

FAM. 1. TRACHINIDAE.—Second suborbital with an internal lamina, supporting the globe of the eye; mouth large, protractile. Ribs and epipleurals nearly equally developed, sessile; posterior praecaudal vertebrae with short parapophyses. Gill-membranes free from isthmus; 6 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae well developed. Scapula and coracoid well developed, a foramen between them; pectoral rays attached to the scapula and to three short and broad pterygials, two of which are in contact with the coracoid. Ventral fins jugular, close together, with 1 spine and 5 soft rays. Body elongate, covered with small cycloid scales forming oblique bands. A short spinous dorsal and a long soft dorsal and anal. Vertebrae 35-43 (10-11 + 25-32). No air-bladder.

This family includes but one genus (Trachinus), the Weevers, with 4 species, occurring on the coasts of Europe, the {705}Mediterranean, and West Africa north of the Equator. A fossil species has been described from the Upper Miocene of Croatia. The two British species, T. draco and T. vipera, are well known for the painful wounds which they are able to inflict through their sharp, grooved dorsal and opercular spines, which convey a very active poisonous fluid secreted by small glands at their base. As these fish like to bury themselves partially in the sands in shallow water, people bathing occasionally tread on them with, as a rule, at least violent pain as a result. The flesh is not bad eating, and great numbers of the larger species (T. draco), are brought to the Paris market.

FAM. 2. PERCOPHIIDAE.—Percophis, with a single species from the coast of Brazil, differs from the Trachinidae in the scapular fenestra being situated entirely in the scapula, in the ventral fins being rather widely separated at the base, and in the quincuncial disposition of the scales. Vertebrae, 57 (22 + 35). Bleekeria and Embolichthys, from the Indian and Japanese seas, with the ventral fins rudimentary or absent, which have been placed in the Ammodytidae, appear to be related to Percophis.

FAM. 3. LEPTOSCOPIDAE.—Differ from the preceding in the absence of a subocular shelf. Scapular fenestra either in the scapula or between the scapula and the coracoid. Mostly Marine Fishes, various in form, from the tropics to the Antarctic circle, some occurring at great depths. About 25 species, referable to 7 genera: Leptoscopus, Parapercis, Neopercis, Pteropsaron, Bembrops, Pleuragramma, Chimarrhichthys. The latter, from New Zealand, is the only freshwater form of the family, and is remarkably adapted for living in alpine torrents. Pleuragramma antarcticum, brought home by the Southern Cross Expedition, comes from 78° 35´ S. lat., the farthest point at which fishes have yet been obtained in the Antarctic region. Macrius amissus, from the Pacific Ocean at a depth of 1000 fathoms, which, judging from a very imperfect description, probably belongs to this family, measures 5 feet, and is the largest known deep-sea Teleostean.

FAM. 4. NOTOTHENIIDAE.—Also closely allied to the Trachinidae. No subocular shelf; a single nostril on each side; ventrals {706}widely separated; pectoral arch usually as in the Trachinidae, but scapular fenestra sometimes in the scapula (Trematomus). Body varying much in shape according to the genera, the form sometimes suggestive of the Cottidae; scales usually ctenoid, sometimes absent; anterior (spinous) dorsal sometimes absent; lateral line often double, or even triple. Mostly from the Southern seas and the Antarctic circle. About 40 species, referable to 19 genera, of which the following are the principal:—Notothenia, Trematomus, Chaenichthys, Champsocephalus, Cryodraco, Acanthaphritis, Eleginops, Bovichthys, Gymnodraco, Gerlachia, Bathydraco, Racovitzaia, Harpagifer, Draconetta.

FAM. 5. URANOSCOPIDAE.—Agree with the Trachinidae in general structure, and in the closely approximated ventrals. Scales very small, in oblique bands, or absent. Pterygials much reduced, fused with the scapula and the coracoid; scapular fenestra in the scapula. Parapophyses strongly developed on the praecaudal vertebrae, with the ribs attached to their upper surface. The head is very large, broad, partly covered with bony plates; cleft of the mouth vertical; eyes on the upper surface of the head. Vertebrae 25 to 30 (12-14 + 13-16). Four genera: Uranoscopus, Anema, Cathetostoma, Ariscopus, with 15 species, from the tropical seas, northwards to the Mediterranean and Japan, southwards to South Australia and New Zealand.

FAM. 6. TRICHONOTIDAE.—Small elongate fishes very nearly related to the Callionymidae, with which they agree in the arrangement of the bones at the base of the pectoral fins and the absence of epipleurals; but post-temporal more distinctly forked and detached from the skull, suborbital arch ossified (without subocular shelf), gill-openings wide, a single long dorsal fin, a long anal fin, and body covered with scales. Vertebrae 48-53. Five marine species, referable to 3 genera: Trichonotus and Taeniolabrus from the Indian Ocean, and Hemerocoetes from New Zealand.

FAM. 7. CALLIONYMIDAE.—Suborbital arch ligamentous; entopterygoid absent; basis cranii simple; mouth rather small, protractile. Vertebrae few (7 + 14), the last two much enlarged; most of the vertebrae with bifid neural processes, simulating a "spina bifida"; first vertebra ribless, second to fourth {707}with sessile ribs and no transverse processes, fifth to seventh with ribs inserted on short transverse processes; no epipleurals. Post-temporal forked, but completely adnate to the skull; scapula separated from the coracoid by a fenestra; pectoral rays attached to the scapula and to three broad pterygials, all three in contact with the coracoid. Ventral fins jugular, widely separated from each other, with 5 soft rays in addition to a short spine. Gill-openings very narrow, generally reduced to a foramen on the upper side of the operculum; 6 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae well developed. Body naked. Two dorsal fins, the first composed of a few flexible spines; second dorsal and anal rather short (7-10 rays).

Small marine fishes, referable to 2 genera; Callionymus, with about 45 species, nearly cosmopolitan, and Vulsus, with a single species from Amboyna and Celebes. In the common British species, the Dragonet (Callionymus lyra), the male acquires very marked secondary characters, the snout becoming more elongate, the second dorsal fin much produced, and the body ornamented with yellow and blue bands. The courtship and pairing have been described by E. W. L. Holt, who observes that this curious fish offers the only instance of a definite sexual intercourse among Teleosteans propagating by pelagic ova. In the Indian C. carebares it is the female that is the more brightly coloured.

FAM. 8. GOBIESOCIDAE.—Suborbital arch absent; entopterygoid absent; basis cranii simple; mouth moderate, protractile. Vertebrae numerous, 27-31 (14-16 + 11-21), the first, if present, rudimentary, the third and following praecaudals with long parapophyses bearing the ribs at their extremity; no epipleurals. Post-temporal simple, articulated to the skull; scapula with a foramen, coracoid much reduced; pectoral rays inserted on the scapula and on four large pterygials, two of which are in contact with the scapula; an adhesive ventral disk, simple or double, supported in front by the clavicles, in the {708}middle and at the sides by the enlarged pelvic bones and fins, and behind by the enlarged lamellar post-clavicles, which are formed of two pieces. Ventral fins jugular, widely separated from each other, formed of 1 short spine and 4 or 5 soft rays. Gill-openings narrow; 5 or 6 branchiostegal rays; gills 3 or 3½; pseudobranchiae well developed. Body naked. Dorsal and anal fins short, composed entirely of soft branched rays.

First placed with the Acanthopterygians by J. Müller, notwithstanding the absence of spinous rays in the vertical fins, and removed from the vicinity of the Cyclopteridae by Günther, raised to the rank of a Sub-order (Xenopteri) near the Anacanthini by Gill, the exact systematic position of this curious type of Fishes has long been a matter of uncertainty. The position of the ventral fins suggests, at first glance, affinity with the Callionymidae, and a comparison of the skeletons of these two types has convinced me that they are really related to each other, although both highly modified in different directions.

The Cling-Fishes are curious small, carnivorous, Marine Fishes, usually found between tide-marks among loose stones and shells, to which they adhere firmly by means of the adhesive ventral disk. They can live a long time out of water. About 50 species are known, from various parts of the world, extending as far north as Scotland and Vancouver Island, and southwards to {709}New Zealand. Three or four species, belonging to the genus Lepadogaster, are known to occur on the British coasts. The principal genera are Gobiesox, Chorisochismus, Sicyases, Cotylis, Lepadogaster, Trachelochismus, Diplocrepis, Crepidogaster, and Leptopterygius.

FAM. 9. BLENNIIDAE.—Suborbitals often forming a more or less distinct subocular shelf; mouth moderate or large, more or less protractile, often bordered to a considerable extent by the maxillaries. Most of the praecaudal vertebrae with strong transverse processes supporting the ribs, which may bear epipleurals. Gill-membranes usually attached to isthmus; 6 or 7 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae usually present. Post-temporal forked; scapula and coracoid more or less developed, sometimes much reduced, the former pierced by a foramen; pectoral rays attached to 4 or 5 hour-glass-shaped pterygials, one or two of which are in contact with the coracoid. Ventral fins jugular, with not more than 4 rays, or absent. Body more or less elongate, sometimes Eel-shaped, naked or with small scales. Dorsal and anal fins elongate, the former constituted entirely of spines, or anteriorly of spines or non-articulated rays, and posteriorly of soft rays. Caudal fin usually distinct, with expanded hypural.

A large family, mostly of small Marine Fishes, the arrangement of which still offers great difficulties. Whether the aberrant genera Cerdale and Ptilichthys deserve to be regarded as the types of distinct families cannot be decided until the skeleton has been examined. The species number about 350, from nearly all the seas, a few inhabiting fresh waters, and are referred to numerous genera, of which the following are the principal:—Gadopsis, Enneanectes, Heterostichus, Acanthoclinus, Clinus, Emmnion, Blennius, Chasmodes, Petroscirtes, Xiphasia, Anarrhichas, Pataecus, Salarias, Ophioblennius, Anoplarchus, Xiphistes, Opisthocentrus, Chaenopsis, Pholedichthys, Lumpenus. Remains of Clinus and Blennius have been described from the Miocene, and the extinct genus Pterygocephalus, from the Upper Eocene, is regarded as allied to Clinus.

The Blenniidae are mostly carnivorous, but a few are herbivorous; some are viviparous (Clinus), others oviparous. Species {710}of Blennius occur in abundance on our coasts, and are among the most familiar tenants of small rock-pools. Their habits have been admirably described by Guitel. The male makes a sort of nest, and defends the brood. Numerous species of the genus Salarias occur in the tropics; these little fish, as their name implies, are remarkable for the long leaps they are able to make. The largest of the Blenniids are the "Wolf-Fishes," often named "Cat-Fishes" (Anarrhichas), of which one species (A. lupus) is common on the British coasts, growing to a length of 5 or 6 feet. "It is impossible," says Brown Goode, "to imagine a more voracious-looking animal than the Sea Cat-Fish, with the massive head and long sinuous, muscular body, its strongly rayed fins, its vice-like jaws, armed with great pavements of teeth, those in front long, strong, pointed, like those of a tiger. It has been known to attack furiously persons wading at low tide among the rock-pools." Its flesh is excellent eating, but generally despised in this country owing to the unprepossessing appearance of the animal.

FAM. 10. BATRACHIDAE.—Suborbital arch absent; basis cranii simple; mouth very large, slightly protractile, bordered to a great extent by the maxillaries. Vertebrae numerous, 29-46 (11-12 + 17-34), without ribs, with sessile epipleurals, simulating ribs; parapophyses rudimentary or absent. Post-temporal small and ankylosed to the skull; scapula and coracoid much reduced, 4 or 5 elongate pterygials, dilated distally, the two lower in contact with the coracoid. Ventral fins jugular, with 1 spine and 2 or 3 branched rays. Gill-openings narrow, the gill-membranes broadly grown to the isthmus; gills 3; pseudobranchiae absent. Head broad and depressed; body naked or with small scales. Spinous dorsal very short, soft dorsal and anal long.

This family is on the whole intermediate between the Blenniidae and the Pediculati. Sluggish, voracious, carnivorous Fishes from the shores of tropical and warm seas, some of them ascending rivers. The species number about 20, referable to {711}5 genera: Batrachus, Opsanus, Thalassophryne, Thalassothia, and Porichthys. The eggs of Batrachus tau are very large, ¼ inch in diameter, and are deposited in a little retreat provided by the parent; the male assumes the care of the brood; the young fasten themselves to rocks by means of an adhesive ventral disk, which soon disappears.

In Thalassophryne, from the coasts of Central America, the opercular spine and the two dorsal spines are perforated, and convey poison from subcutaneous sacs situated at their base. In the American genus Porichthys the head and body bear series of greatly developed mucous pores, some of which simulate the photophores of Scopelus, but are not luminous.

FAM. 11. PHOLIDIDAE.—Suborbitals not forming a subocular shelf; mouth scarcely protractile, with thick lips. Praecaudal vertebrae similar to the caudals, without transverse processes, with hæmal arches; ribs sessile. Gill-membranes free from the isthmus; 4 or 5 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present. Scapular arch as in Blenniidae. Ventral fins jugular and rudimentary, or absent. Body elongate, compressed, with very small scales. Dorsal and anal fins elongate, the former constituted entirely of non-articulated rays or spines. Caudal fin distinct, with expanded hypuraL.

Small shore fishes of the Northern Seas, differing from the Blenniidae in the structure of the praecaudal vertebrae, in spite of the external resemblance which the two known genera, Pholis (Centronotus) and Apodichthys, bears to Anoplarchus and Xiphistes. Species about 10.

{712}A well-known British fish of this family is the little Gunnel or Butter-Fish (Pholis gunnellus), remarkable for the manner in which the female protects her offspring, coiling herself round the eggs, which she rolls up into a ball about the size of a Brazil nut, in holes of the boring Mollusc (Pholas). The male sometimes assists the female.

FAM. 12. ZOARCIDAE.—Suborbitals not forming a subocular shelf; mouth feebly protractile. Praecaudal vertebrae with strong transverse processes bearing ribs and epipleurals. Gill-membranes usually more or less broadly united to isthmus; 5 to 8 branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae present or absent. Scapular arch as in Blenniidae. Ventral fins jugular or absent; if present, with 1 to 4 rays. Body more or less elongate, naked or with very small scales. Dorsal and anal fins elongate, all the rays articulated, or a few of the posterior dorsals spinous. Usually no distinct caudal fin.

These fishes have usually been placed, in part at least, near the Gadids, but they have more in common with the Blenniids, as pointed out by Jordan and Evermann, and may be regarded as degraded forms descended from the latter. The family is widely distributed in all seas, many of the forms being specially adapted to live at great depths. The species known number about 130. Principal genera: Scytalina, Zoarces, Lycodes, Gymnelis, Lycocara, Melanostigma, Derepodichthys, Bathyonus, Porogadus, Bythitis, Neobythitis, Cataetyx, Selachophidium, Acanthonus, Typhlonus, Aphyonus, Tauredophidium, Rhodichthys, Brosmophycis, Brotula, Lucifuga, Lamprogrammus, Diplacanthopoma, Hephthocara.

{713}Some are oviparous, others (Zoarces, Diplacanthopoma, Hephthocara, Lucifuga) viviparous. The eyes are absent, or at least not visible externally in some of the bathybial forms (Typhlonus, Aphyonus, Tauredophidium), as well as in the only known freshwater forms, the Cuban Cave-Fishes Stygicola and Lucifuga, which are evidently allied to the marine Brotula, whilst the blind Cave-Fishes of North America (cf. p. 618) are derived from freshwater types. It is believed that blind fishes are found also in caves of the island of Jamaica, but no specimens have been seen by naturalists. The largest Cuban Cave-Fish is 5 inches long.

FAM. 13. CONGROGADIDAE.—Eel-shaped Fishes without ventrals, allied to the Blenniidae, but with all the rays soft and articulated, the post-temporal small and ankylosed to the skull, and the sub-orbitals produced into laminae supporting the eyeball. Lips much developed; gill-membranes free from isthmus; scales very small.

A single genus, Congrogadus, with three species from the Australian and East Indian coasts. The recently described Japanese genus Hierichthys has been referred to this family.

FAM. 14. OPHIDIIDAE.—Degraded Blenniids, closely related to the Zoarcidae, with pseudobranchiae, with tapering tail without distinct caudal fin, and with the ventral fins each reduced to a pair of filaments or a bifid ray inserted just behind the chin at the extremity of the clavicle, which is produced forwards as a slender rod.

Small marine, carnivorous fishes, from the Atlantic and Southern Pacific coasts as well as from great depths in the Atlantic, Pacific, and Indian Oceans. About 25 species are known. Genera: Ophidium, Lepophidium, Genypterus.

FAM. 15. PODATELIDAE.—Mouth inferior, protractile, toothless or with minute teeth. Praecaudal vertebrae with transverse processes, to which the ribs are attached. Gill-membranes narrowly attached to isthmus; 8 or 9 branchiostegal rays; gills 4; no pseudobranchiae. Supratemporal loosely attached by ligament to the skull; scapula cartilaginous, perforate, bearing the base of the pectoral fin, which is an undivided cartilaginous plate; coracoid small, ossified. Ventral fins jugular, each reduced to a single stout filament made up of two intimately coherent rays. Body short, tail elongate and tapering, {714}compressed; no scales. A short dorsal fin, without spines, situated above the pectorals; a long anal fin, continuous with the caudal.

The genus Podateles (Ateleopus) comprises only two species from the deep sea, one from Japan and one from India.

DIVISION IX.—TAENIOSOMI.

Exceedingly compressed, more or less elongate, often ribbon-like fishes of doubtful affinities, probably related to the earlier Acanthopterygians, the ventral fins, when well developed, comprising as many as 7 to 9 rays. Dorsal fin extending from the head to the end of the tail, its rays simple (separable into lateral halves), the anterior often prolonged; anal fin very short or absent. Pectoral fin with horizontal or nearly horizontal base, the rays supported by the scapula and by three short pterygials, all three, or two at least, of which are related to the coracoid. Ribs small and slender, or absent. Post-temporal simple and solidly attached to the skull. Scales minute or absent.

Deep-sea or pelagic fishes from the Atlantic and Mediterranean and from the Pacific; the life-histories are still very imperfectly known, and great changes of form take place with growth. Only two families.

{715}FAM. 1. TRACHYPTERIDAE.—Mouth very protractile; ventral fins more or less developed, with 6 to 9 rays, or reduced to a single long ray; no anal fin; vent about the middle of the body; caudal rays, if present, divided into two fascicles, the upper sometimes much prolonged and directed upwards.

Two genera. The most generalised is Trachypterus, of which probably only 10 forms are entitled to specific distinction. The best known species is T. arcticus, the Deal-Fish or Northern Ribbon-Fish, which reaches a length of 8 feet or more, and of which a few specimens have been stranded on the coasts of Scotland. Nilsson, who has observed these fishes alive on the Scandinavian coast, says they approach the shore at flood-tide on sandy shelving bottoms, and are often left by the retreating waves; that they move with one side turned obliquely upward, and that they lie on the side like Flat-Fishes on the bottom in 2 or 3 fathoms of water. Regalecus differs in the presence of a single ray to the ventral and the absence of the caudal fin. Some 5 or 6 species may be distinguished. R. glesne, the Oar-fish, or "King of the Herrings," is the best known and the largest species, reaching a length of over 20 feet. About 25 {716}specimens are known to have occurred on the British coasts. Some of the accounts of "Sea-Serpents" are probably based on this fish, which has been observed to swim with undulating motion and with a small portion of the head as well as the crest-like anterior part of the dorsal fin above the water.

The fish named Stylophorus chordatus, which has been referred to this family, is known from a single specimen too imperfectly preserved to afford a clear idea of its affinities.

FAM. 2. LOPHOTIDAE.—Mouth moderately protractile; ventrals very small, if distinct, with 4 or 5 rays; abdominal cavity extending nearly the whole length of the much elongated body, the vent very far back and followed by a short anal fin; caudal fin small, not divided.

A single genus, Lophotes, with 3 or 4 species, from the Mediterranean, the tropical Atlantic, the Cape of Good Hope, Japan, and New Zealand, reaching a length of 6 feet or more. The dorsal fin commences with an extremely long and strong spine on the head, which is much elevated and truncate in front.

SUB-ORDER 11. OPISTHOMI

Air-bladder without open duct. Opercle well developed, hidden under the skin; supraoccipital in contact with the frontals, separating the parietals. Pectoral arch suspended from the vertebral column, far behind the skull; no mesocoracoid. Vertical fins with spines. Ventral fins absent.

This division stands in the same relation to the Acanthopterygii as do the Apodes to the Malacopterygii. The single family is possibly derived from the Blenniidae.

FAM. 1. MASTACEMBELIDAE.—Body more or less Eel-shaped; a series of short spines detached from the very elongate dorsal fin, which is more or less confluent with the likewise very elongate anal fin. A single nostril on each side. Mouth not protractile, bordered by the praemaxillaries, to the upper border of which the maxillaries are attached. Gill-cleft inferior; gills 4; branchiostegal rays 6; no pseudobranchiae. Vertebrae numerous (72-95), the praecaudals with transverse processes bearing the ribs. Scales very small.

Carnivorous fishes, from fresh and brackish waters of Southern Asia and Tropical Africa. 33 species are known, referable to {717}two genera: Mastacembelus and Rhynchobdella. The largest species reach a length of three feet. Little is known of their habits. Of the Indian Rhynchobdella aculeata, Day says it conceals itself in the mud and becomes drowned in water if unable to reach the surface, as it apparently requires to respire air directly.

SUB-ORDER 12. PEDICULATI.

Air-bladder without open duct. Opercle large, hidden under the skin; supraoccipital in contact with the frontals, separating the parietals. Pectoral arch suspended from the skull; no mesocoracoid. No ribs, no epipleurals. Ventral fins jugular. Gill-opening reduced to a foramen situated in or near the axil, more or less posterior to the base of the pectoral. Body naked or covered with spines or bony tubercles.

A small, natural group, connected with the Acanthopterygii Jugulares through the Batrachidae, in which the elongate pterygials of the pectoral fin foreshadow the kind of arm ("pseudobrachium") which is more or less characteristic of these highly aberrant Fishes. As in the Batrachidae, the post-temporal is flat and ankylosed to the cranium, and the suprascapula is much elongated. The pterygials, two or three in number, are separated from the small scapula and coracoid by a broad ligament, the arm-like pectorals being more or less distinctly geniculated and inserted far back behind the cranium. The head is large, the basis cranii simple. The gills are reduced to 2, 2½, or 3. The spinous dorsal, if present, consists of a few rays, which may be modified into tentacles inserted on the head. Vertebrae 17 to 31.

{718}Five families:—

I. Gill-opening in or behind lower axil of pectoral; mouth large, terminal or directed upwards.

Pectoral fin scarcely geniculated; ventrals present 1. Lophiidae.

Pectoral fin scarcely geniculated; ventrals absent 2. Ceratiidae.

Pectoral fin strongly geniculated; ventrals present 3. Antennariidae.

II. Gill-opening behind lower axil of pectoral; mouth large, inferior; ventrals absent 4. Gigantactinidae.

III. Gill-opening above axil of pectoral; mouth rather small, subterminal or inferior; pectoral fin strongly geniculated; ventrals present; spinous dorsal absent or reduced to a small tentacle lodged in a cavity under the snout 5. Malthidae.

FAM. 1. LOPHIIDAE.—Mouth extremely large, terminal, with very strong cardiform teeth. Gill-opening in lower axil of pectoral; pseudobranchiae present. Pectoral fin scarcely geniculated, with two pterygials. Ventral fin with 1 spine and 5 branched rays. Spinous dorsal present. Skin naked.

Twelve species, referable to three genera (Lophius, Chirolophius, and Lophiomus) living on the bottom of the Atlantic, Indian, and Pacific Oceans, at moderate or great depths. Lophius was represented in the Upper Eocene of Monte Bolca.

The Fishing-Frog or Angler (Lophius piscatorius) has a wide distribution, occurring on the coasts of Europe and North America. The first dorsal ray, inserted on the snout, is very long, movable in every direction, and terminates in a dermal flap, which is supposed to be used by the "Angler" as a bait, attracting other fishes, which are soon ingulfed in the enormous gape. It grows to a length of over 5 feet. The ventral rays are very elongate in the young.

{719}FAM. 2. CERATIIDAE.—Mouth extremely large, terminal, with strong cardiform teeth. Gill-opening in lower axil of pectoral; pseudobranchiae absent. Pectoral fin scarcely geniculated, with three pterygials. Ventral fins absent. Spinous dorsal fin usually present, sometimes reduced to a single tentacle on the snout. Skin naked.

The members of this family, about 25 in number, are all inhabitants of great depths (300-2600 fathoms). The colour of the body is usually a deep black, and the first dorsal spine, on the head, may terminate in a luminous bulb with or without filaments. "The Bathybial Sea-devils," writes Günther, "are degraded forms of Lophius; they descend to the greatest depths of the ocean. Their bones are of an extremely light and thin texture, and frequently other parts of their organisation, their integuments, muscles, and intestines are equally loose in texture when the specimens are brought to the surface. In their habits they probably do not differ in any degree from their surface representative, Lophius."

Principal genera: Ceratias, Aceratias, Oneirodes, Himantolophus, Aegaeonichthys, Melanocetus, Liocetus, Linophryne, Caulophryne, Dolopichthys.

{720}FAM. 3. ANTENNARIIDAE.—Mouth large, vertical or very oblique, turned upwards, with cardiform teeth. Gill-opening in or behind lower axil of pectoral; pseudobranchiae absent. Pectoral fin forming an elbow-like angle, with three pterygials. Ventral with 4 or 5 rays. Spinous dorsal present. Skin naked or spinulose.

About 40 species, referable to 5 genera: Pterophryne, Antennarius, Brachionichthys, Saccarius, and Chaunax.

The species of Antennarius live mostly in coral groves, where they lie in wait for prey, well concealed by their protective coloration and the harmonising aspect of their integument and appendages. To this genus also belongs the "Marbled Angler" (A. marmoratus), carried about in mid ocean among the Sargassum weed, to rest on which, from its peculiar arm-like pectoral fins, it is specially fitted; there it makes its wonderful nest of silk-like fibres, probably secreted by the parent as in the Sticklebacks, with large bundles of eggs hanging like grape clusters. The deep-sea Chaunax inflates its abdomen like Tetrodon.

FAM. 4. GIGANTACTINIDAE.—Mouth inferior, snout produced into a long tentacle directed forwards, and bearing a luminous organ. Body covered with small spines. Otherwise as in the Ceratiidae. Gigantactis vanhoeffeni, of Brauer, from the Indian Ocean, at depths of about 1000 fathoms.

FAM. 5. MALTHIDAE.—Mouth rather small, subterminal or inferior, with villiform or cardiform teeth. Gill-opening above pectoral; pseudobranchiae absent. Pectoral fin forming an elbow-like angle, with three pterygials. Ventral with 5 rays. Spinous dorsal absent, or reduced to a more or less developed tentacle lodged in a cavity under the snout. Head and body with bony tubercles or spines.

About 30 species are known, mostly from the deep sea within the tropics (down to 1270 fathoms). Principal genera: Coelophrys, Malthe, Malthopsis, Halieutaea, Halicmethes, Dibranchus.

The "Bat-Fish" (Malthe vespertilio), common in shallow water about the West Indies, is said to assume an almost toad-like attitude on the ground, the head being directed slightly upwards, while the pectorals take on the function of hind legs and the ventrals of fore legs.

{721}SUB-ORDER 13. PLECTOGNATHI.

Air-bladder without open duct. Opercular bones more or less reduced; supraoccipital in contact with the frontals, separating the parietals; maxillary and praemaxillary bones often firmly united. Pectoral arch suspended from the skull. No ribs. Ventral fins thoracic and much reduced if present; the pelvic bones, if present, more or less completely co-ossified. Gill-opening much reduced. Body covered with more or less osseous scales, bony scutes, or spines, or naked.

A highly aberrant group, closely connected with the Acanthopterygii through the Acanthuridae, as pointed out long ago by Dareste. The skeleton is often feebly ossified and the vertebrae much reduced in number, but the jaws, although short, are very strong, usually with large sectorial teeth which may be confluent into a beak; the post-temporal is short and simple, suturally united to the squamosal. These fishes have usually been arranged in three divisions: Sclerodermi, Ostracodermi, and Gymnodontes, but Regan, whose classification is here followed, has shown that the latter include a type (Triodon) which, in spite of its beak-like teeth, is more nearly related to the Sclerodermi, whilst the Ostracodermi have much more in common with the latter than with the Gymnodontes. It therefore appears best to admit only two divisions, the first with 4, the second with 3 families:—

I. SCLERODERMI.—Supraclavicle vertical; pectoral arch of the Perciform type; all the vertebrae with a single neural spine.

A. Body covered with hard or spinous scales; epipleurals present; pelvis present.

Teeth separate; spinous dorsal present; ventrals paired; pelvis immovable 1. Triacanthidae.

A beak; spinous dorsal and ventrals absent; pelvis movable 2. Triodontidae.

Teeth separate; spinous dorsal present; ventrals absent or represented by a single short spine; pelvis movable 3. Balistidae.

B. Body encased in a carapace; no epipleurals; spinous dorsal, pelvis, and ventrals absent 4. Ostraciontidae.

II. GYMNODONTES.—Supraclavicle oblique or nearly horizontal; lower{722} three pterygials enlarged and immovably united to the coraco-scapular cartilage; anterior vertebrae with bifid divergent neural spines; pelvis absent.

Beak with a median suture; interoperculum not connected with suboperculum; caudal fin present; body inflatable. 1. Tetrodontidae.

Beak without median suture; interoperculum attached posteriorly to suboperculum; caudal fin present; body inflatable. 2. Diodontidae.

Beak without median suture; interoperculum attached posteriorly to suboperculum; caudal fin absent, the body non-inflatable, truncate posteriorly, with the dorsal and anal fins confluent. 3. Molidae.

DIVISION I.—SCLERODERMI.

Supraclavicle vertical; pectoral pterygials not enlarged, movably attached by ligament to the scapula and coracoid, three to the former and one to the latter. All the vertebrae with the neural arches forming a single spine. Basis cranii more or less distinctly double; dentary and articular completely co-ossified.

FAM. 1. TRIACANTHIDAE.—Praemaxillaries protractile, free from the maxillaries; teeth in the jaws separate, conical or incisor-like; palatine arch firmly united to the skull. Gills 4. Praecaudal vertebrae with parapophyses; epipleurals present. Spinous dorsal fin with 2 to 6 spines. Ventral fins each represented by a strong spine, with an inner basal knob which locks it when everted, rarely with the addition of 1 or 2 rudimentary soft rays; pelvis present, firmly united to the pectoral arch. Scales small, sometimes spinous or bony. Vertebrae 20.

Marine fishes from the Indian and Western Pacific Oceans. Ten species, referable to three genera: Triacanthus, Triacanthodes, {723}Halimochirurgus. The latter, remarkable for its long, tube-like snout, is the only deep-sea form of this Sub-order; it was recently discovered in the Gulf of Manaar, at a depth of 143 fathoms. Fossil genera are Acanthopleurus, Oligocene, and Spinacanthus, Eocene.

FAM. 2. TRIODONTIDAE.—Praemaxillaries not protractile, firmly united to the maxillaries; teeth coalescent into a beak, the upper jaw divided by a median suture, the lower simple. Praecaudal vertebrae with or without parapophyses; epipleurals present. No spinous dorsal fin. No ventral fins. Abdomen with a dilatable sac, kept expanded by the very long movable pelvis. Body covered with small, spiny, subimbricate, bony laminae. Vertebrae 20.

A single species, the curious Triodon bursarius of the Indian Ocean and Archipelago.

FAM. 3. BALISTIDAE.—Praemaxillaries not protractile, firmly united to the praemaxillaries; teeth incisor-like; palatine movably articulated with ectopterygoid, or entirely free from it. Gills 4. Praecaudal vertebrae with well-developed parapophyses, to which epipleurals are attached. Spinous dorsal fin with 1 to 3 spines. Ventral fins, if present, represented by a single short {724}rough spine at the end of the long, movable pelvis. Body covered with juxtaposed movable scutes or with minute rough scales.

About 100 species are known from the tropical and warm seas, one species (Balistes capriscus) occasionally wandering as far north as the south coast of England. Genera: Balistes, Monacanthus, Paraluteres, Pseudaluteres, Pseudomonacanthus, Aluteres, Psilocephalus. The Oligocene genus Acanthoderma is closely allied to Balistes.

The "File-Fishes" or "Trigger-Fishes" (Balistes), the largest species of which grow to nearly 3 feet, have a powerful dentition, which enables them to break off pieces of corals, on which they feed, and to bore holes in the hard shells of Mollusca in order to extract the soft parts; they are themselves well protected by a mail of hard, rhomboidal scales. The herbivorous Monacanthus is less favoured in this respect, the rough scales being so small as to give the skin a velvety appearance.

Psilocephalus differs from Monacanthus in its very elongate head and body, the very feeble dorsal spine, the presence of a mental barbel, and its more numerous vertebrae (29 or 30 instead of 18 to 21). The flesh of many of these fishes is poisonous. The drumming sounds produced by Balistes have been described by Möbius.

FAM. 4. OSTRACIONTIDAE.—Praemaxillaries not protractile, firmly united to the praemaxillaries; teeth incisor-like; palatine immovable. Gills 4. Praecaudal vertebrae with very feeble parapophyses and no epipleurals. No spinous dorsal fin. Clavicles, coracoids, and post-clavicles much expanded. No ventral fins. Body encased in a carapace formed of large, juxtaposed, mostly hexagonal bony plates. Vertebrae 14 to 16.

The species of "Trunk-Fishes" number about 20, and are referable to 3 genera: Aracana, Ostracion, Lactophrys; all belong to the tropical seas, living near the bottom in shallow water. The genus Ostracion is represented by one species in the Upper Eocene.

The rigid box in which these fishes are encased entails more use of the dorsal and anal fins for progression than is customary {725}among fishes. According to Brown Goode, "the propelling force is exerted by the dorsal and anal fins, which have a half rotary, sculling motion, resembling that of a screw propeller; the caudal fin acts as a rudder, save when it is needed for unusually rapid swimming, when it is used as in other fishes; the chief function of the broad pectorals seems to be that of forming a current of water through the gills, thus aiding respiration, which would otherwise be difficult on account of the narrowness and inflexibility of the branchial apertures. When taken from the water, one of these fishes will live for two or three hours, all the time solemnly fanning its gills, and when restored to its native element seems none the worse for its experience, except that, on account of the air absorbed, it cannot at once sink to the bottom." "No group of tropical fishes," says the same author, "is so thoroughly worked out in the writings of the fathers of natural history as this one. Over 200 years ago every species of trunk-fish now taken from the Atlantic was known to and described by the naturalists, and it is a well-deserved tribute to their discrimination as zoologists to say that none of the many efforts which have since been made to subdivide their species have been at all successful."

DIVISION II.—GYMNODONTES.

Supraclavicle oblique, sometimes nearly horizontal; lower three pectoral pterygials enlarged and immovably united to the coracoscapular cartilage; upper pterygial small, suturally united to the scapula. Anterior vertebrae with bifid divergent neural spines. Basis cranii simple; suture between dentary and articular evident. Pelvis absent.

The spinous dorsal and the ventral fins are constantly absent, the praemaxillaries are united to the maxillaries, and the teeth {726}are coalescent, forming a beak; parapophyses are not developed, and epipleurals are absent.

FAM. 1. TETRODONTIDAE.—Beak with a median suture. Interoperculum a long rod, attached to inner face of praeoperculum, sometimes connected with operculum, never with suboperculum. Gills 3. First 4 or 5 praecaudal vertebrae with bifid neural spine and closed neural arch. Skin naked or with movable spines, rarely with bony plates; belly inflatable. Vertebrae 17 to 29.

The "Puffers" or "Globe-Fishes" comprise about 60 species, referable to 5 genera: Tetrodon, Ephippion, Tropidichthys, Xenopterus, Chonerhinus. They inhabit all the tropical and warm seas, a few species being confined to fresh water. Remains of Tetrodon have been found in Upper Eocene and later formations. They are remarkable for the manner in which they inflate themselves with air. The flesh of most species is poisonous.

FAM. 2. DIODONTIDAE.—Beak without median suture. Interoperculum rod-like, attached posteriorly to the rod-like anterior limb of the suboperculum. Gills 3. All the praecaudal vertebrae with bifid neural spines. Skin with movable spines; belly inflatable. Vertebrae 21 or 22.

Only two genera appear capable of clear definition: Diodon and Lyosphaera; species about 15. Numerous species have been described from the Upper Eocene and later formations.

"Porcupine Fishes" are confined to tropical seas, and have attracted attention from the earliest times, being frequently preserved as "curiosities." Their flesh is regarded as poisonous.

FAM. 3. MOLIDAE.—Beak without median suture. Interoperculum rod-like, attached posteriorly to the rod-like anterior limb of the suboperculum. Gills 4. Anterior praecaudal vertebrae with divergent bifid neural spines and neural canal not roofed in. {727}Body non-inflatable, truncate posteriorly, without caudal peduncle; caudal fin absent, the dorsal and anal fins confluent. Skin rough or tessellated. Vertebrae 17 in Orthagoriscus.

The very young are armed with spines.

The "Sun-Fish" are extraordinary creatures found in the open sea or descending to great depths, and of wide distribution. The number of species is still very uncertain, but two generic forms, Orthagoriscus or Mola and Ranzania, are easily distinguished. Examples of both occur now and then on our coasts. Orthagoriscus mola grows to upwards of 8 feet and to a weight of 1800 pounds. It has been observed to swim slowly about, near the surface, the high dorsal above the water. Its food is said to consist chiefly of jelly-fish and larval fishes; its mode of reproduction and places of breeding are still unknown.

{729}INDEX

Every reference is to the page: words in italics are names of genera or species; figures in italics indicate that the reference relates to systematic position; figures in thick type refer to an illustration; f. = and in following page or pages; n. = note.

Abbott, 193 n. Abcona, 670 Abdominal pores, 401 f. Abramis, 582 Abyssascidia, 73 Acanthaphritis, 706 Acanthias, 455, 298; vertebral column, 198; uterine nutrition, 434; A. vulgaris, 264, 455; pectoral fin, 243 Acanthicus, 595 Acanthistius, 659 Acanthocepola, 662 Acanthoclinus, 709 Acanthocybium, 678 Acanthodei, 440 f., 148 Acanthoderma, 724 Acanthodes, 442; A. wardi, 441 Acanthodidae, 441 Acanthodopsis, 442 Acanthonus, 712 Acanthophthalmus, 582 Acanthopleurus, 723 Acanthopoma, 589 Acanthopterygii, 650 f., 159, 306, 543; diagram showing relationships of groups, 651 Acanthuridae, 668, 357, 651, 652, 654 Acanthurus, 668; A. chirurgus, 357 Acara, 672 Accessory respiratory organs, 292 Acentronura, 634 Acentrophorus, 498 Aceratias, 719 Acerina, 659; A. cernua, 659 Acestra, 595; A. gladius, 595 Acestrorhamphus, 575 Acestrorhynchus, 575 Achilognathus, 582 Achiropsis, 687 Achirus, 687 Acipenser, 492, 149, 262, 264, 273, 274, 276, 282, 348; ribs, 201; lymph follicles, 261; spiral valve, 268; gills, 283, 284; air-bladder, 298, 299; vascular system, 319, 322, 328, 334; spleen, 343; gonoducts of female, 400, 405; distribution, habits, and food, 493; breeding, 494; economic value, 494; A. huso, 494; A. rhynchaeus, pectoral fin, 243; A. ruthenus, 493; scales, 187, 188; brain, 376; micropyles, 411; larva, 494; A. sturio, 493, 494; vertebral column, 200; supra-renals, 346; micropyles, 411 Acipenseridae, 486, 489, 495 Acipenseroidei = Chondrostei, q.v. Acrania, 113 Acrartete, 636 n. Acrochordonichthys, 588 Acrodus, 445 Acrogaster, 656 Acrognathus, 611 Acropoma, 659 Acropomatidae, 659, 654 Acrotus, 644 Actinistia, 477 n. Actinopterygii, 476 Actinotrocha, 28 f., 29 Acysis, 588 Adipose fins, 163 Aegaeonichthys, 719 Aelurichthys, 588 Aethalion, 546 Aetheolepis, 498; scales, 187 Aetheospondyli, 497 Aëtobatis, 465, 466; spines, 177 Agassiz, 558 n., 685 n., 720 n. Ageniosus, 589; A. valenciennesi, 589 Agnathostomata, 145; characters of, 147 Agoniates, 575 Agonidae, 700, 694 Agonopsis, 700 Agonus, 700; {730} A. cataphractus, 700 Agrammus, 696 Agriopus, 695 Aida, 639 Ailia, 588; air-bladder, 302 Ailiichthys, 588 Aipichthys, 666 Air-bladder, as an accessory respiratory organ, 291 f.; Amia, 291; Lepidosteus, 291, 299; Sudis gigas, 291; Erythrinus taeniatus and E. braziliensis, 291; Neoceratodus, 291, 300; Protopterus, 291, 301, 302; Lepidosiren, 291; structure, 297; in different Fishes, 298; "red bodies," 307; "red glands," 307, 308, 309; gases of the air-bladder, 309; functions, 309; in locomotion, 310; as a vocal organ, 358; connexion with the auditory organ, 388, 389, 390 Albula, 548, 541, 549; vestigial conus arteriosus, 329, 548; A. conorhynchus, 548; larva, 548 Albulidae, 547, 544 Alburnus, 582; A. lucidus, 167, 583 Alcock, 695 Alder and Hancock, 38 Alepidosauridae, 614, 606 Alepidosaurus, 614; A. ferox, 614 Alepocephalidae, 569, 544 Alepocephalus, 570; A. rostratus, 570 n. Aleposomus, 570 Alestes, 575 Alimentary canal, of Hemichordata, 11, 25; of Tunicata, 54, 67; of Amphioxus, 120; of Fishes, 252 f.; regions, 252; mesenteries, 258; histology, 259 f.; glands, 270 Allabenchelys, 588 Allis Shad, 564 Allman, 22 n., 705 n. Alopecias vulpes, 451, 452; spiral valve, 265 Aluteres, 724 Amaroucium, 88; A. proliferum, stomach, 88 Ambassinae, 660 Ambassis, 660 Ambloplites, 657 Amblyapistus, 695 Amblyopsidae, 618, 361, 395, 606 Amblyopsis, 618; A. spelaea, 166, 361, 618, 619 Amblyopus, 689 Amblypharyngodon, 582 Amblypterus, 487 Amia, 499, 149, 160, 262, 273, 274, 276, 283, 291, 393, 609 n.; fossil, 501; A. calva, 499, 500; scales, 189; vertebral column, 201, 202, 203; skull, 228; median fins, 235; pectoral fin, 243, 244; spiral valve, 268; pseudobranch, 284; air-bladder, 297, 299, 310; heart, 328; arteries to air-bladder, 337; sensory canal, 385; nephrostomes, 401, 402; segmentation of ova, 409; distribution and habits, 499; spawning, 500; nest, 500, 501; larvae, 501 Amiidae, 499, 299, 497 Amiopsis, 501 Amiurus, 588, 261; liver and pancreas, 273; A. catus, rectal valve, 254; renal portal system, 319; A. nebulosus, 587 n., 592; A. nigrilabris, blind, 394 Ammocoetes, 428, 46, 262, 272, 280, 280 n., 327, 343, 428; metamorphosis, 429; protective value of the skin, 429 Ammocrypta, 659 Ammodytes, 639, 275; A. lanceolatus, 639; A. tobianus, 639 Ammodytidae, 639, 637 Ammopleurops, 687 Ammotretis, 687 Amniota, 145 Ampheristus, 695 Amphioxus, 112 f., 4, 37, 46, 110; general characters, 113; external characters, 114; anatomy, 116, 117; musculature, 117; skeleton, 119; notochord, 119; alimentary canal, 120; branchial bars, 122; endostyle, 123; coelom, 123; vascular system, 124, 125; renal organs, 125, 127; nervous system, 127; sense-organs, 128; gonads, 129; embryology and life-history, 130, 130, 131, 132, 133, 134, 135, 136; compared with Hemichordata, 11 f., 16, 29 f. —see also Branchiostoma Amphipnoidae, 598 Amphipnous, 598; respiratory air-sacs of, 598; A. cuchia, 294, 598 Amphiprion, 672 Amphisile, 633; A. strigata, 633 Amphisilidae, 633, 628 Amphistiidae, 684, 683 Amphistium, 684; A. paradoxum, 684 Amphistylic, 222, 223 Anabantidae, 645, 292, 637; distribution of, 645 Anabas, 645, 355; A. scandens, 645; labyrinthiform organ, 293 Anableps, 616, 419; intromittent organ, 414; development of embryos in ovisacs, 418; A. tetrophthalmus, 617 Anacanthini, 646 f., 543, 702, 703 Anacyrtus, 575 Anadromous, 413 Anallantoidea, 145 Anamniota, 145 Anampses, 673 Anapterus, 611 Anarrhichas, 709; teeth, 251; A. lupus, 710; eggs, 408 Anaspida, 531 f., 149 Anchinia, 96, 100; A. rubra, 100; polymorphism, 100 Anchovy, 564 {731} Ancylodon, 663 Ancylostylus, 549 Andersonia, 588, 589 Andrews, 113 Anema, 706 Angel-Shark, 456 Angler, 718 Anguilla, 601; A. vulgaris (Common Eel), 601; larva, 602; red glands, 307; renal portal circulation, 319, 320 Anguillidae, 600, 163, 405, 649 n. Anodonta, Rhodeus in, 584 Anogmius, 549 Anomalopidae, 660; photophores, 178 Anomalops, 660 Anomalopterus, 570 Anoplarchus, 709 Anoplogaster, 656, 655 Anoplopoma, 697, 693 Anoplopterus, 588 Anostominae, 576 Anostomus (Characinidae), 576; (Mugilidae), 640 Ansorge, 560 Antennariidae, 720, 718 Antennarius, 720; nest, 414; A. hispidus, scales, 191; A. marmoratus, 720; scales, 191 Anthias, 659, 660 Antiarchi, 532 f., 149 Antigonia, 667 Antimora, 648 Anurella, 78; development of, 78; A. roscovita, larva, 78 Apateodus, 611 Apeltes, 630 Aphanopus, 679 Aphareus, 660 Aphia, 689; A. pellucida, 690 Aphiocharax, 575 Aphoristia, 687 Aphredoderus, 656, 655; A. sayanus, 656 Aphyonus, 712, 713 Apistus, 695, 692 Aplidium, 88; A. zostericola, stomach, 88 Apodes, 599 f., 306 Apodichthys, 711 Apogon, 660 Apolectus, 677 Apostasis, 668 Appendicularia, 68, 37; A. sicula, 66 Appendicularians, 64 Appendiculariida, 65, 66 Appendiculariidae, 68, 38; nervous system, 53 Appendix digitiformis, 276 Aprion, 660 Aprionodon, 448 Apua, 582 Aracana, 724 Arapaima, 557; A. gigas, 556, 558 Arch-centra, 196 Archaeomaenidae, 545, 544 Archaeomenes, 545 Archaeoteuthis, 669 Archaeus, 677 Archenteron, 20, 56, 130, 132 Archeobatis, 446 Archinephric duct, 397, 398 Arctoscopus, 663 Argenteum, 168 Argentina, 565, 569 Arges, 595 Arginae, 595 Argyriosus vomer, 363 Argyropelecus, 571 Ariscopus, 706 Aristotle, 36 Arius, 588, 587, 589; A. australis, 593; deposition of eggs, 415; A. commersonii, 593 Armed Bullhead, 700 Arnoglossus, 687 Arripis, 663 Arthrodira, 535 f., 149 Arterial system, 329 f. Artificial pearls, 167 Ascelichthys, 698 Ascidia, 72, 73; structure, 39 f., 43, 45; test, 40, 41; body-wall, 42; mantle, 42; branchial cavity, 43, 48; atrial or peribranchial cavity, 43; other cavities of body, 44; tentacles, 44; endostyle, 46; branchial sac, 47; heart and circulation, 49, 51; blood, 49; neural gland, 52; dorsal tubercle, 52; nervous system, 53; sense-organs, 53; alimentary canal, 54; renal organ, 54; reproductive organs, 55; embryology and life-history, 55, 57, 60; metamorphosis, 61; A. challengeri, dorsal tubercle, 79; A. mentula, structure, 39 f., 40; test, 42; endostyle, 46; pharynx, 47; nervous system, 52; larva, 78; A. meridionalis, dorsal tubercle, 79; A. pyriformis, dorsal tubercle, 79; A. translucida, dorsal tubercle, 79; A. virginea, 39 f. Ascidiacea, 70 f., 64 Ascidiae Compositae, 80 f., 64; structure, 81 Ascidiae Luciae, 90 f., 86 Ascidiae Simplices, 71 f. Ascidians, 35, 64, 70 Ascidiella aspersa, 77 Ascidiidae, 72, 64, 110 Ascidiinae, 72 Ascidiozooids, 35, 84 Ascopera gigantea, branchial sac, 77; dorsal tubercle, 79 Asineops, 656 Aspidophora, 5 n. Aspidophoroides, 700 {732} Aspidorhynchidae, 502 Aspidorhynchus, 502; A. acutirostris, 502 Aspius, 582 Aspredinidae, 596, 575 Aspredo, 596, 416 Aspro, 659 Asprotilapia, 672 Asteracanthus, 445 Asterolepidae, 534 Asterolepis, 534 Asterospondylic, 198 Astroblepus, 595 Astronesthes, 571, 570, 572; A. niger, photophores, 178 Asymmetron, 137, 129; distribution, 138; A. bassanum, 137; A. caudatum, 137, 138; A. cingalense, 137; A. cultellum, 137; A. hectori, 137; A. lucayanum, 137, 138; A. maldivense, 137 Ateleaspidae, 528 Ateleaspis, 528, 530; A. tessellata, 528 Ateleopus, 714 Atheresthes, 687, 685 Atherina, 639, 640 Atherinella, 639 Atherinichthys, 641 Atherinidae, 639, 637 Atherinops, 639 Atherinopsis, 639 Atopochilus, 588 Atrial cavity, 43, 44, 59, 63, 67 Atriopore, 113, 115, 117 Atrium, of Amphioxus, 113, 118, 121, 135 Atypichthys, 666 Atyposoma, 666 Auchenaspis, 529 Auchenipterus, 589; A. nodosus, elastic-spring-apparatus, 358 Auchenoglanis, 588 Auditory organs, 387 f., 388; connexion with the air-bladder, 389, 390 Audouin, 37 Aulacocephalus, 659 Auliscops, 632 Auliscus, 631 Aulolepis, 560 Aulopus, 611 Aulopyge, 582 Aulorhamphus, 668 Aulorhynchidae, 631, 628 Aulorhynchus, 631, 632 Aulostoma, 632; A. coloratum, 632 Aulostomatidae, 632, 628 Aulostomatomorpha, 570 Autostylic, 223 Auxis, 678 Azurina, 672

Badis, 658 Bagarius, 588 Bagrichthys, 588 Bagrinae, 588 Bagroides, 588 Bagropsis, 588 Bagrus, 588 Balanoglossus, 3, 5 f., 24, 25, 28, 30 f., 123; embryo, 8; history of name, 17 Balanoglossus, 17, 5, 6, 13, 16; B. aurantiacus, 7, 11, 15; B. biminiensis, 18, 21 n.; B. clavigerus, 6, 17; B. gigas, 5 Balistes, 724, 264, 354; coloration, 174; scales, 190; sound production, 357; B. aculeatus, 357; B. capriscus, 724; B. vetula, 361 Balistidae, 723, 652, 721, 163 Ballowitz, 591 n. Bancroft, 85 Band-Fish, 662 Barbels, 154 Barbus, 582, 584; B. mosal, 584; B. tropidolepis, 583; lower pharyngeals, 583; B. viviparus, 584 Barilius, 582 Barracudas, 642 Barramunda, 558 Barrois, 39 Basking Shark, 453 Bat-Fish, 720 Bateson, 5, 6, 11, 14, 20, 30 n. Bathyagonus, 700 Bathyclupea, 657, 656 Bathydraco, 706 Bathygadus, 647 Bathylaco, 571 Bathylagus, 566, 565, 569 Bathylychnus, 571 Bathymaster, 661 Bathymyzon, 426 Bathyoncus, 74 Bathyonus, 712 Bathypterois, 611, 613; B. dubius, 162, 612; B. longipes, 613 Bathysaurus, 611 Bathythrissa, 548 Bathytroctes, 570 Batoidei, 457 f., 148 Batrachidae, 710, 651, 704, 361; photophores, 179 Batrachus, 711; B. tau, 711, 361 Baudelot, 584 n. Bdellostoma, 423; external characters, 151; skull, 220, 221; pancreas, 273; gill-sacs, 282, 423; vascular system, 315 n.; distribution, 423; habits, 423; eggs, 424; B. stouti, 423; embryo, 425 Bdellostomatidae, 423 Belodontichthys, 588 Belone, 638, 411; B. annulata, 638 Belonesox, 616 Belonoglanis, 588, 589 Belonorhynchidae, 485, 488 Belonostomus, 502 {733} Bembras, 700 Bembrops, 705 Beneden, Van, 37, 39 Benham, 113 Bentenia, 682 Benthodesmus, 679 Benthophilus, 689 Benthosaurus, 611 Berycidae, 655, 651, 652, 653, 703, 303, 361, 389 Beryx, 656; B. affinis and B. mülleri, coloration, 165; B. splendens, 655 Betta, 669; B. pugnax, 669 Bib, 649 Bibronia, 685 Birkenia, 531, 149; B. elegans, 532 Birkeniidae, 531 Bitterling, 584, 416 Black Bass, 657 Black-Fish, 610, 643 Blastocoele, 44, 49, 52, 130 Blastogenetic acceleration, 84 Blastopore, 20, 130 Blastosphere, 20, 130 Blastozooid, 84, 93 Blastula, 56 Bleak, 583, 167 Bleekeria, 705 Blenniidae, 709, 651, 702, 703, 704, 163, 271, 302, 418 Blennius, 709, 710 Blepsias, 698 Blind Fishes, 394, 713; of Mammoth Cave, 619 Blochius, 680 Blood, of Hemichordata, 15; of Tunicata, 49; of Amphioxus, 124; of Fishes, 341 Blood-glands, 342 Blue Shark, 448 Boar-Fish, 666 Boas, 548 n. Bola, 582 Boleophthalmus, 689 Boleosoma, 659 Boltenia, 75; B. pachydermatina, B. tuberculata, dorsal tubercle, 79 Bolteninae, 75 Bombay-duck, 613 Bonnet Shark, 450 Bonnier and Pérez, 95 Borgert, 99 Bothriolepis, 534; B. canadensis, 534 Botia, 582 Botryllidae, 88, 81, 82, 84, 110; vessels, 42; neural gland, 52 n.; reproductive organs, 56; ascidiozooid, 82 Botrylloides, 88 Botryllus, 88, 36, 90; stigmata, 59; diagram of budding, 90; B. schlosseri, 80; B. violaceus, 89 Bottard, 669, 696 n. Boulenger, 459 n., 477 n., 481 n., 551 n., 584 n., 586 n., 593 n., 683 n. Boulengerella, 575 Bourne, 19 Boveri, 113, 126 Bovichthys, 706 Bow-Fin, 499 = Amia, q.v. Box, 664, 665, 264, 276; B. vulgaris, rectal caecum, 254 Brachionichthys, 720 Brachiopoda, 35 Brachychalcinus, 575 Brachymystax, 565 Brama, 682; B. longipinnis, 682 n.; B. raii, 682 n. Bramidae, 682, 676 Branchial bars, of Amphioxus, 120, 122 Branchial cavity, of Tunicata, 43, 101 Branchial clefts, 155, 277 f. Branchial sac, 43, 44, 45, 53; of Appendicularians, 67; of Thaliacea, 96, 104, 105 Branchial septa, of Hemichordata, 12; of Amphioxus, 120 Branchiostoma, 137, 112, 138; distribution, 138; B. belcheri, 137, 138; B. californiense, 137; B. capense, 137; B. caribbaeum, 137; B. elongatum, 137, 138; B. indicum, 137; B. lanceolatum, 112, 114 n., 115, 137, 138; transverse section, 118, 121; nephridium, 126; nervous system, 128, 129; gonads, 129; B. nakagawae, 137, 138; B. pelagicum, 129 n., 137, 138 —see also Amphioxus Branchiostomatidae, 137, 112 Brauer, 571 Breathing sounds, 357 Breathing-valves, 288 Bregmaceros, 648 Breitensteinia, 588 Brevoortia, 563 Bridge, 555, 558 n. Bridge and Haddon, 574 n., 587 n., 590 n. Brill, 687 Brooks, 39, 101 n., 105 Brosmius, 648 Brosmophycis, 712 Brotula, 712, 713 Brychaetus, 557 Brycon, 575; B. fulcatus, mouth, 577 Bryconaethiops, 575 Bryconodon, 575 Bryconops, 575 Bucklandium, 589 Budding, in Pterobranchia, 24, 27; in Tunicata, 71, 80, 81 f., 90, 97, 103 Budgett, 482, 483 n., 514, 552, 558 Bull-heads, 698 Bullhead Sharks, 444 Bunocephalichthys, 596 Bunocephalus, 596 Burbot, 649 {734} Bursa Entiana, 254 Bury, 31 n. Butirinus, 548 Butter-Fish, 712, 415 Bythitis, 712

Cachius, 582 Cadophore, 97 Caecum cloacae, 276 Caenotropis, 576 Caesio, 664, 264 Caesioperca, 659 Calamichthys, 484, 158, 202; distribution, 484; C. calabaricus, 484 Calamostoma, 634 Callanthias, 659 Callichrous, 588 Callichthyinae, 588 Callichthys, 588, 587, 153, 302; intestinal respiration, 292; C. littoralis, 592; C. paleatus, 414, 592 Calliodon, 674 Callionymidae, 706, 704 Callionymus, 707; C. carebares, 707; C. lyra, 707, 420 n. Callomystax gagata, stridulating mechanism, 356 Callophysus, 588 Callopristodus, 446 Callopterus, 204 Callorhynchus, 471; vertebral column, 199; frontal clasper, 223; branchial blood-vessels, 334; distribution, 471; egg-case, 471; C. antarcticus, 470 Calotomus, 674 Campanula Halleri, 393, 394 Campostoma, 582 Candiru, 593 Canobius, 487 Cantharus, 664 Capelin, 568 Capoëta, 582 Caprodon, 659 Caproidae, 666, 653 Capros, 666; C. aper, 666, 357 Carangidae, 677, 652, 676, 158, 303 Carangodes, 677 Carangopsis, 677 Caranx, 677, 161, 284; with Medusae, 643; C. hippos, 363; C. rhonchus, 363; C. trachurus, 677, 307 Carapus, 579 Carbonnier, 592 Carcharias (Carcharinus), 448; succession of teeth, 250; C. glaucus, 448; C. nicaraguensis, 448 Carchariidae, 448, 449 Carcharinus—see Carcharias Carcharodon rondeletii, 451, 452 Careproctus, 698 Cariba, 578 Carp, 583, 584; Leather-, 584 Carpiodes, 581 Caryo-enteric, 102, 108 Castle, 56 n. Cat-Fishes, 710, 587 Cataetyx, 712 Cathetostoma, 706 Catla, 582; C. buchanani, 584 Catlocarpio, 582 Catopra, 658 Catoprion, 576 Catopteridae, 488, 486 Catosteomi, 626 f., 306 Catostominae, 581 Catostomus, 581 Caturus, 499; C. furcatus, 499; vertebrae, 203 Caudal fin, 156, 159, 237 Caullery, 85 Caulolatilus, 661 Caulolepis, 656 Caulophryne, 719 Cave-Fishes, 394 f., 618 f., 713 Centrarchidae, 657, 653 Centrarchus, 657 Centrina, 455; C. salviani, 455 Centriscidae, 633, 628, 357 Centriscus, 633, 154; C. scolopax, 633; scales, 189, 190; stridulation, 357 Centrogenys, 659 Centrolepis, 487 Centrolophus, 643; C. britannicus, 643; C. niger, 643 Centromochlus, 588, 361 Centronotus, 711 Centrophorus, 455 Centropogon, 695 Centropominae, 660 Centropomus, 660 Centropristes, 659 Centroscyllium, 455 Cephalacanthus, 702 Cephalaspidae, 528 Cephalaspis, 529, 149; C. lyelli, 529; C. magnifica, 529; C. murchisoni, 528 Cephalochordata, 112 f., 4, 38 Cephalodiscus, 21 f., 5, 28, 29, 31, 32; distribution, 23; budding, 24; structure, 24 f.; males, 26; C. dodecalophus, 22, 23, 24, 25 Cepola, 662; C. rubescens, 662 Cepolidae, 661, 653 Ceratias, 719; phosphorescent organ, 174, 178; C. bispinosus, 174 Ceratichthys, 582 Ceratiidae, 719, 718 Ceratodontidae, 507 Ceratodus, 508, 519 Ceratoptera, 465; C. vampyrus, 466 Ceratotrichia, 234 Cerdale, 709 Cestracion = Heterodontus, q. v. Cetengraulis, 563 {735} Cetomimidae, 614, 606 Cetomimus, 614 Cetopsis, 588, 587 Cetorhinidae, 453 Cetorhinus, see Selache Chaca, 588; C. lophioides, 589 Chaenichthys, 706 Chaenobryttus, 657 Chaenopsis, 709 Chaerops, 673 Chaetobranchus, 672 Chaetodon, 668 Chaetodontidae, 667, 654, 361; coloration, 166 Chaetostomus, 595; C. cirrhosus, sexual differences, 594; C. gigas, 595 Chalceus, 575; C. angulatus, mouth, 577 Chalcinus, 575 Chamisso, 36 Champsocephalus, 706 Champsodon, 641; C. vorax, 642 Chaninae, 563 Channa, 645 Channalabes, 588, 589 Channomuraena, 605; C. vittata, 605 Chanoides, 563 Chanos, 563, 563; C. salmoneus, gill-helix, 294 Characidium, 576 Characinidae, 575, 574, 294; distribution, 576; teeth, 250; sound production, 361; Weberian ossicles, 389, 573 Characodon, 616 Charax, 665 Charitosomus, 572 Charr, 567 Charybdia, 685 Chascanopsetta, 687 Chasmodes, 709 Chatoessus, 563, 563 Chauliodontinae, 571 Chauliodus, 571, 570, 572 Chaunax, 720 Cheiracanthus, 442 Cheirodus, 488; C. granulosus, 488 Cheirolepis, 485, 487 Chela, 582 Chelaethiops, 582 Chelidoperca, 659 Chelmo, 668 Chelyosoma, 73; C. macleayanum, 72 Chiaje, Delle, 17 Chiasmodon, 641, 642 Chiasmodontidae, 641, 637 Chilinus, 673 Chilio, 673 Chilobranchus, 598 Chilodactylus, 664 Chilodipterinae, 660 Chilodipterus, 660 Chiloglanis, 588 Chilorhinus, 601 Chiloscyllium, 446, 447; pectoral girdle, 239; pectoral fin, 239; pelvic girdle, 240; pelvic fin, 240; brain, 374 Chimaera, 469, 382; vertebral column, 199; skull, 223; frontal clasper, 223, 469; gills, 282, 283; lateral sensory organs, 386; auditory organ, 387, 388; distribution, 469; egg-case, 470; segmentation of egg, 474; fossil, 474; C. affinis, 469; C. colliei, 469, 473; C. monstrosa, 469; vertebral column, 199; skull, 223; pectoral fin, 243; spiral valve, 267 Chimaeridae, 468, 474 Chimaeropsis, 468 Chimarrhichthys, 705 Chirocentridae, 561, 544 Chirocentrites, 561 Chirocentrodon, 563 Chirocentrus, 562; vestigial spiral valve, 269; C. dorab, skull and pectoral arch, 562 Chirodon, 575 Chirolophius, 718; C. naresii, 718 Chironemus, 664 Chirostoma, 639 Chirothricidae, 615, 606 Chirothrix, 615; C. libanicus, 615 Chlamydoselachidae, 443 Chlamydoselachus, 443, 279; rectal gland, 276; branchial clefts, 277; efferent branchial vessels, 332 n.; C. anguineus, 443, 444; C. lawleyi, 443 Chlamydothorax, 17 n. Chlorophthalmus, 611; C. gracilis, 613 Chloroscombrus, 677 Chologaster, 618; C. agassizii, 618; C. cornutus, 618; C. papilliferus, 618 Chondrostachys, 85 Chondrostei, 485 f., 149 Chondrosteidae, 489, 486 Chondrosteus, 489; C. acipenseroides, 489, 490 Chondrostoma, 582 Chonerhinus, 726 Chorda-centra, 196 Chordata, characters of, 3 f., 35, 38 Choridactylus, 695, 692 Chorimycterus, 576 Chorinemus, 677 Chorisochismus, 709 Chorizocormus, 89; C. reticulatus, 90 Choroid gland, 393, 394 Chromatophores, 166 Chromides, 671 Chrysichthys, 588 Chrysophrys auratus, 420 Cichla, 672, 671 Cichlidae, 670, 654; distribution, 672; eggs carried in the mouth, 416 Cichlops, 661 Ciliation, of the alimentary canal, 262 {736} Cimolichthys, 609 Ciona, 72; development, 56 n.; C. intestinalis, 73, 77 Cirrhilabrus, 673 Cirrhites, 660 Cirrhitichthys, 660 Cirrhitinae, 660; coloration, 166 Cirri, of Amphioxus, 128 Citharichthys dinoceros, 685 Citharidium, 576 Citharininae, 576 Citharinus, 576, 578; C. geoffroyi, 579 Citharus, 687 Cladistia, 481 f., 477 Cladocyclus, 561 Cladodus, 438; C. neilsoni, 438 Cladoselache, 438, 148, 153, 160, 162, 197; pectoral and pelvic girdles, 239; pectoral and pelvic fins, 242, 245; characters, 436 f.; C. fyleri, 437 Cladoselachidae, 438 Clariallabes, 588 Clarias, 588, 153, 302; accessory respiratory organs, 293, 294; C. anguillaris, 590; C. lazera, 590 Clariinae, 588 Clarke, 608 Clarotes, 588 Claspers, 162, 246, 414, 432, 469, 470, 471 Clavelina, 71, 83; nervous system, 53; stigmata, 59; hibernation, 88; C. lepadiformis, 71, 72 Clavelinidae, 71, 81, 83, 109, 110 Climatius, 441 Climbing Perch, 645, 292 Cling-Fishes, 708 Clinus, 709 Cloaca, of Tunicata, 44, 45, 81, 82, 92; of Fishes, 156, 256 Cloacal aperture, of Tunicata, 80 Cloacal cavity, of Hemimyaria, 101 Club-shaped gland, 134 Clupea, 563, 564; C. alosa, 564; C. finta, 564; C. harengus, 564, 307; = Herring (q.v.); C. pilchardus, 564; C. sprattus, 564 Clupeichthys, 563 Clupeidae, 562, 544; gill-helix, 294; connexion of air-bladder with auditory organ, 303, 389; eggs, 412 Clupeinae, 563 Cnidoglanis, 588 Coal-Fish, 649 Cobitidinae, 582, 585 Cobitis, 582, 271; C. taenia, 358 Cobitopsis, 639; C. acuta, 639 Coccodus, 498 Coccolepis, 487 Coccosteus, 535, 149, 536; C. decipiens, 535, 536 Cochliodon, 595 Cochliodontidae, 445; teeth, 251 Cochliodus, 445 Cochlognathus, 582 Cod, 648, 648; = Gadus morrhua, q.v. Coelacanthidae, 480, 481 Coelacanthus, 481, 160 Coelocormidae, 86 Coelocormus huxleyi, 86; reproductive organs, 56 Coelolepidae, 524, 530 Coelom (body-cavity), in Hemichordata, 8, 21, 24 f.; in Tunicata, 44; in Amphioxus, 123, 132 f.; in Fishes, 397 Coelonotus, 634, 635 Coelophrys, 720 Coffer-Fishes, 152, 361 Coilia, 563, 563 Cole, 25 n. Cole and Johnstone, 687 n. Colella, 85, 83; C. pedunculata, 85; C. quoyi, 80 Collar-pore, 9, 25, 27 Collett, 714 n. Coloconger, 601 Colocopus, 668 Colour, of Fishes, 164 f.; brilliancy and variety, 164; cause of, 166; changes, 169; protective value, 171; aggressive and alluring, 173; warning, 174 Columbia, 621; C. transmontana, 621 Comephoridae, 696, 692, 694, 418 Comephorus, 697, 692 n. Compound Ascidians, 35, 36 f., 70, 71, 80 f., 110; eggs, 56 Conchopoma, 507 Conger, 601; C. vulgaris, skull and pectoral arch, 600 Congrogadidae, 713, 704 Congrogadus, 713 Congromuraena, 601 Conocara, 570 Conorhynchus, 588 Conte and Vaney, 27 n. Cope, 477 n., 542, 543, 573, 619, 626 Copidoglanis, 588 Copodus, 446 Coprolites, 268 Coregonus, 565, 568, 311; C. clupeoides, 568; C. lavaretus, 568; C. oxyrhynchus, 568; C. pollan, 568; C. vandesius, 568 Corella, 73; stigmata, 48; C. japonica, branchial sac, 73; C. parallelogramma, 73 Corellinae, 73 Coreoperca, 659 Coridodax, 674 Coris, 673 Corvina, 663; C. lobata, air-bladder, 304 Corydoras, 588; C. paleatus, 592 Corynascidia, 73; C. suhmi, 72; branchial sac, 73 Corynopoma, 575 {737} Coryphaena, 681 Coryphaenidae, 681, 676 Coryphaenoides, 647 Cossyphus, 673 Costa, 112 Coste, 630 n. Cottidae, 697, 692, 694 Cottocomephorus, 697, 692 n. Cottunculus, 698 Cottus, 698; C. bubalis, 698; C. gobio, 698; C. scorpius, 698, 357 Cotylis, 709 Cranial nerves, 378, 380 Craniata, 4, 38; characters of, 141; classification, 145 Cranoglanis, 588 Creagrutus, 575 Crenicichla, 672 Crenidens, 665 Crenuchus, 575 Crepidogaster, 709 Crista acustica, 60 Cromeria, 573 Cromeriidae, 573, 545 Cromileptes, 659 Crossognathidae, 565 Crossognathus, 565 Crossopholis, 492 Crossopterygii, 476 f., 149; distribution, 483 Crossorhinus, 447 Cryodraco, 706 Cryphiolepis, 487 Cryptopterus, 588 Cryptotomus, 674 Crystallaria, 659 Crystallogobius, 689 Ctenochaetus, 668 Ctenodentex, 665 Ctenodontidae, 505 Ctenodus, 506 Ctenolabrus, 673 Ctenolates, 659 Ctenothrissa, 560; C. vexillifer, 559 Ctenothrissidae, 559, 544 Cubiceps, 643 Cuchia, 598, 294 Cuénot, 584 n. Culeolus, 75; spicules, 87; C. murrayi, 42; C. moseleyi, dorsal tubercle, 79; C. wyville-thomsoni, 75 Culter, 582 Cunningham, 565 n., 602 n., 687 n. Curimatus, 576 Cutaneous sense-organs, of Fishes, 383 f., 385, 386 Cuvier, 36 Cyathaspis, 527 Cyathozooid, 91, 93, 94 Cybium, 678 Cyclobatis, 462 Cyclomyaria, 95, 101 Cyclopterichthys, 698 Cyclopteridae, 698, 692, 694, 302 Cyclopterus, 698, 321, 408, 415; sucker, 162; C. lumpus, 699; rectal valve, 254 Cyclosalpa, 108; C. pinnata, 102, 108 Cyclospondylic, 198 Cyclostomata, 421 f., 145 f.; characters, 146; external features, 150; skeleton, 197 f.; teeth, alimentary canal and digestive glands, 247 f.; respiratory organs, 279 f.; vascular system, 315 f.; blood-glands, 343; nervous system, 367 f.; organs of special sense, 383 f.; kidneys and reproductive organs, 399 f. Cyclothone, 571 Cymatogaster, 670 Cynoglossus, 687; C. lingua, 686; C. semilaevis, 393 Cynolebias, 616 Cynthia, 75; branchial sac, 74; C. cerebriformis, dorsal tubercle, 79; C. formosa, 76; dorsal tubercle, 79; C. papietensis, dorsal tubercle, 79 Cynthiidae, 74, 64, 110; branchial sac, 48; neural gland, 52 n.; reproductive organs, 55; eggs, 56; dorsal tubercle, 79 Cynthiinae, 75; tentacles, 75 Cyphosidae, 657, 653 Cyphosus, 657 Cyprinidae, 581, 575, 271; ventral suckers, 162; absence of jaw-teeth, 251; air-bladder, 302; breathing sounds, 358; connexion of auditory organ with air-bladder, 389 Cyprininae, 582, 583 Cyprinion, 582 Cyprinodon, 616 Cyprinodontidae, 616, 606, 163, 275, 414, 418; distribution, 617 Cyprinus, 582, 155; pharyngeal teeth, 252; C. auratus, eyes, 155; coloration, 171; C. carassius, 585; C. carpio, 583, 307, 358 Cystoarian, 403 Cystodytes, 85; spicules, 87 Cyttoides, 683 Cyttopsis, 683 Cyttus, 683

Dab, 687; Long Rough, 687; Smear, 687 Dactylopogon, 611 Dactylopteridae, 701, 694, 361 Dactylopterus, 701, 693; D. volitans, 161, 355, 357, 361, 399, 701; pectoral arch, 693 Dallia, 610; D. pectoralis, 610, 611 Dalliidae, 610, 606 Danio, 582 Dapedius, 498, 541 n.; D. politus, 498 Dapedoglossus, 557; D. testis, 556 Dareste, 721 {738} Darters, 659 Dascyllus, 672 Dasyatis sabina, 464 Dasyscopelus, 611 Datnioides, 658 Dawydoff, 16 n. Day, 569 n., 593 n. Deal-Fish, 715 Delhez, 551 Deltodus, 445 Deltoptychius, 445 Demersal, 408 Dendrodoa, 76 Dentex, 664 Dentition, 247 f.; dentinal tissues, 249; fixation of teeth, 249; succession and replacement, 250; shape, 251; sexual differences, 251; pharyngeal teeth, 251; Cyclostomata, 247; Petromyzon, 248; Fishes, 248 f.; Scyllium, 249; Carcharias, 250 Dercetidae, 623, 622 Dercetis, 623 Derepodichthys, 712 Derichthys, 601 Dermal denticles, 183 f.; structure and development, 183, 184, 185 Diagramma, 664 Dibranchus, 720 Dicerobatis, 465 Dicrotus, 679 Didemnidae, 86, 82, 110 Didemnum, 87 Didymaspis, 530 Dinichthys, 537, 149 Dinolestes, 659 Dinoperca, 659 Diodon, 726, 163, 354, 361; scales, 191; teeth, 251; D. geometricus, 726; D. hystrix, 364 Diodontidae, 726, 722 Diphycercal, 159 Diplacanthidae, 441 Diplacanthopoma, 712, 713 Diplacanthus, 441 Diplesium, 659 Diplocrepis, 709 Diplomystes, 588, 587 Diplomystus, 563 Diplophos, 571 Diplophysa, 582 Diplopterus, 477 Diplosoma, 87; section, 87 Diplosomatidae, 87, 110 Diplosomoides lacazii, larva, 78 Diplospondylic, 198 Diplurus, 481, 161 Dipneusti—see Dipnoi Dipnoi (Dipneusti), 505 f., 149; distribution, 512 Dipterodon, 665 Dipterus, 506, 519; D. valenciennesi, 506 Diptychus, 582 Disartete, 636 n. Discocephali, 691 f., 651, 652 Discognathus, 582 Discopyge, 464 Distaplia, 85; D. magnilarva, larva, 78 Distichodontinae, 576 Distichodus, 576; D. niloticus, mouth, 577 Distoma, 85 Distomatidae, 85, 81, 82, 83, 86, 110; reproductive organs, 56; ascidiozooid, 82; transverse section, 86 Ditrema, 670; D. temminckii, 670 Dog-Fishes, 446 Dolchinia, 96, 100; D. mirabilis, 100 Dolichoglossus, 17, 5, 13; D. kowalevskii, 6, 7, 9, 20; D. otagoensis, 11 Dolichorhynchus, 137; D. indicus, 137 Doliolidae, 96, 39, 110 Doliolum, 96, 37; structure, 96; life-history, 97, 98; occurrence, 99; budding, 97; polymorphism, 98; D. nationalis, 99; D. tritonis, 96, 99 Dollo, 518 Dolopichthys, 719 Dolphins, 681 Doradinae, 588 Dorado, 578 Doras, 588, 590, 592, 363; intestinal respiration, 292; stridulation, 357; elastic-spring-apparatus, 359; D. maculatus, air-bladder, 303; nature of sounds, 362 Dorsal lamina, 45, 47, 52, 101, 105 Dorsal pore, 18, 19, 31 Dorsal tubercle, 52, 79 Doryichthys, 634, 635 Doumea, 588 Doydixodon, 664 Draconetta, 706 Dragonet, 707 Drasche, 38 Drepanaspidae, 525, 530 Drepanaspis gemündenensis, 526, 530, 525, 526 Drepane, 668 Drepanidae, 668, 654 Drum, 663, 304, 362 Ductor, 677 Dufosse, 355 Dussumieria, 563, 563 Dybowski, 697 n. Dysichthys, 596 Dysomma, 603 Dysommatopsis, 603

Eagle Rays, 465 Echeneididae, 691 Echeneis, 691; cephalic sucker, 161 Echidna, 605 {739} Echidnocephalus, 624 Echinorhinus, 455; E. spinosus, 456 Echiostoma, 571 Ecteinascidia, 71 Edaphodon, 474 Edinger, 271 Eels, 599 f., 601 f., 163, 405 Eel-Fares, 602 Egertonia, 674 Eggs, of Hemichordata, 20, 26; of Tunicata, 55, 73, 84, 97, 106; of Amphioxus, 130; of Fishes, 408 f., 423, 424, 428, 432, 433, 435, 456, 470, 471, 494, 504, 510; alecithal and telolecithal, 410; holoblastic and meroblastic segmentation, 409, 410; micropyles, 411, 412; deposition, 411; attachment, 411, 424, 433, 434; demersal and pelagic, 411, 412; spawning, 412; relative fecundity, 412; fertilisation, 413; sexual relations, 413; sexual congress, 414, 427, 428, 432; time of hatching, 417, 433, 494, 500, 504; influence of temperature, 417; of Aspredo, 596; of Batrachus tau, 711 Ehrenbaum, 685 n. Eigenmann, 602 n., 619 n., 670 n. Eigenmannia, 579 Elacate, 678 Elaeoblast, 107, 107 Elasmobranchii, 431 f., 148 Elasmodus, 474 Elassoma, 657 Electric, Cat-Fish, 591; Eel, 580, 365 f.; Rays, 462; Mormyridae, 550; organs, 365 f. Eleginops, 706 Eleotris, 689; E. marmorata, 689 Ellipesurus, 465 Elonichthys, 487 Elopidae, 546, 544 Elopopsis, 547 Elops, 547; E. lacerta, 547; E. saurus, 547 Elvers, 602 Embiotoca, 670 Embiotocidae, 670, 654, 418, 419 Embolichthys, 705 Embryology (development), of Hemichordata, 18; of Tunicata, 55; of Amphioxus, 130; of Fishes, 417 Embryonic nutrition; food-yolk, 409, 417; ovarian secretion, 419; oviducal or uterine secretion, 434, 435; placentas, 98, 101, 107, 434 Emery, 622, 623, 685 n., 714 n. Emmnion, 709 Empetrichthys, 616 Encheliophis, 625; E. vermicularis, 625 Enchelycore, 605 Enchodontidae, 608, 606 Enchodus, 609 End-buds, 383 Endostyle, 46, 53, 58, 67; of Amphioxus, 122 Engraulinae, 563 Engraulis, 563; E. encrasicholus, 564 Enneanectes, 709 Entero-epicardiac budding, 82 Enteropneusta, 5 f., 30 n.; distribution, 5, 6; coloration, 7; habitat, 6, 7; divisions of body, 7; burrowing, 7; body-cavities, 8; body-wall, 9; nervous system, 9; alimentary canal, 11; vascular system, 15; excretion, 15; reproductive organs, 16; regeneration, 16; development, 18 f. Entersphenus, 426 Eocottus, 698 Eothynnus, 678 Epapterus, 589 Ephippion, 726 Ephippus, 668; E. faber, 668 Epibulus, 673 Epicardiac budding, 81 Epicardium, 44, 60, 71, 83 Epigonus, 659 Epinephelus, 659, 660 Epinnula, 679 Epipharyngeal groove, 123 Eques, 663 Equula, 663 Eremophilus, 589 Eretmodus, 672 Ereunias, 698 Erythrininae, 575 Erythrinus, 575, 578; air-bladder, 305, 306; E. taeniatus, E. braziliensis, air-bladder as a respiratory organ, 291 Esocidae, 609, 606, 275; distribution, 610 Esox, 609; teeth, 250, 262; E. lucius, 609, 307; skeleton, 609; E. nobilior, 609; E. lepidotus, 610 Essence orientale, 585 Esunculus, 548 Etelis, 660 Etheostoma, 659 Etroplus, 672 Etrumeus, 563 Euanemus, 359 Eucalia, 630 Euchilichthys, 588, 587 Eucirrhichthys, 582 Eugnathichthys, 576 Eugnathidae, 498, 497 Eugnathus, 499 Eugyra, 77, 78; E. glutinans, 79; E. kerguelenensis, branchial sac, 77 Eukeraspis pustulifera, 529 Eumeda, 588 Euoxymetopon, 679 Euphanerops, 532 Eupleurogrammus, 679 Eurycormus, 499; {740} E. speciosus, vertebrae, 203 Eurynotus, 487, 488; E. crenatus, 487 Eurypharynx, 604 Eurypholis, 609 Eusthenopteron, 478, 202, 246; E. foordi, 479 Euthacanthus, 441 Euthynotus, 204 Eutropiichthys, 587 Eutropius, 588 Exocoetoides, 615 Exocoetus, 638, 161, 173, 353, 411 Exoglossum, 582 Exostoma, 588, 586, 587 External characters, Cyclostomata, 150 f.; Fishes, 152 f. Eycleshymer, 592 n. Eye, 155, 393, 394, 395; degeneration in deep-sea and cave Fishes, 394; telescopic, 395 Eyelids, 395

Facciola, 685 n. Fatio, 569 n. Felchen, 568 Fierasfer, 625, 622, 399; F. acus, 156, 626 Fierasferidae, 625, 623 File-Fishes, 724 Filippi, 690 n. Fins, of Fishes, 156; median, 156; paired, 157; pectoral, 157; lobate, 157; pelvic, 158; caudal, 159; modified to form suckers, 161; degeneration and atrophy, 162; endoskeletal elements, 234, 235, 236, 237, 239, 240, 241, 242, 243, 244, 245, 246; exoskeletal elements, 158, 234, 236 Fin-rays, of Amphioxus, 120; of Fishes, 158, 234, 236 Fishes, 141 f.; systematic position and classification, 141; external features, 152; coloration, 164; poison glands, 176; phosphorescent organs, 178; skin and scales, 182; skeleton, 193; dentition, alimentary canal, and digestive glands, 247; respiratory organs, 277; air-bladder, 297; vascular system, lymphatics, and blood-glands, 313; muscular system, locomotion, sound-producing organs, and electric organs, 349; nervous system and organs of special sense, 367; kidneys and reproductive organs, breeding, 397; systematic, 431 f. Fishing-Frog, 718 Fistularia, 632, 154; F. tabaccaria, 632 Fistulariidae, 632, 628 Flat-Fishes, 685 Flounder, 687; coloration, 167, 170 Flute-mouths, 154 Flying-Fish, 638, 161, 173, 353, 411; fresh-water, 559 Flying Gurnards, 701, 355, 361 Fol, 38 Forbesella, 75; F. tessellata, 76; dorsal tubercle, 79 Forskål, 36 Fowler, 23 Frilled Sharks, 443 Fritillaria, 69, 70; F. furcata, 70; F. megachile, 66 Fritsch, 551, 591 n. Frost-Fish, 679 Fundulus, 616

Gadidae, 647, 702, 158, 303, 307, 361, 389, 412; photophores, 179 Gadomus, 647 Gadopsis, 709 Gadow, 193 n. Gadus, 648, 168; G. aeglefinus, 649, 308, 361; G. luscus, 649; G. merlangus, 649; coloration, 168; pyloric caeca, 275; G. morrhua, 648, 648; vertebrae, 205; skull, 230; pseudobranch, 284; air-bladder, 303; red gland, 307, 308; vascular system, 320, 323, 336, 337; spleen, 343; sounds, 361; G. pollachius, 649; G. virens, 649; sensory canals, 386; tail, 646 Gagata, 588 Galaxias, 607, 608; G. attenuatus, 607, 608; G. brevipinnis, 607 Galaxiidae, 607, 606, 163, 405; distribution, 607 Galeichthys, 588, 587, 593 Galeocerdo, 448; G. arcticus, 448; G. tigrinus, 448 Galeoides, 641 Galeus, 449, 298; G. canis, 449 Gambusia, 616; development of embryos in ovisacs, 418 Ganodus, 474 Ganoidei, 149 Gar-Fish, 411 (Belone) Gar-Pike, 638 (Belone); 503 (Lepidosteus)—see also Lepidosteus Garman, 616 n., 619, 698 n. Gastric glands, 270 Gastrochisma, 678 Gastromyzon, 582; G. borneensis, 586; sucker, 162 Gastropelecus, 575 Gastrosteidae, 629, 627; distribution, 631 Gastrosteus, 630, 163, 169, 271, 288; G. aculeatus, 357, 630; pectoral arch of, 630; G. pungitius, 630; G. spinosus, 354; G. texanus, 631 n. Gastrostomus, 604 Gastrotoceus, 634, 635 Gastrula, 20, 56, 130, 131 Gavialiceps, 603 {741} Gazza, 663 Gegenbaur, 5 n., 37, 570 n. Gempylus, 679 Genidens, 588 Genital pores, 403 Genital wings, of Enteropneusta, 16, 10 Genyomyrus, 551, 549 Genypterus, 713 Geoffroy, 677 n. Geophagus, 672 Geotria, 426 Gephyroberyx, 656 Gephyrocercal, 161 Gephyroglanis, 588 Gerbe, 673 n. Gerlachia, 706 Gerres, 663 Gerridae, 663, 654 Giard, 38, 39 Gigantactinidae, 720, 718 Gigantactis vanhoeffeni, 720 Gilbert, 548 Gilbertia, 659 Gilchrist, 571 Gill, of Salpa, 105 Gill, T., 542, 570 n., 599 n., 629 Gill-helix, 294 Gill-slits, 4; of Enteropneusta, 11, 12, 20, 30; of Pterobranchia, 25, 27; of Tunicata, 47, 67, 105; of Amphioxus, 120; of Fishes, 155, 277 f. Gillaroo Trout, 260 Gills—see Respiratory organs Ginglymostoma, 447 Girardinus, 616; sexual congress, 414 Girella, 664 Glandiceps, 17, 5, 6, 13, 14, 15; G. abyssicola, 6; G. hacksi, 6; G. talaboti, 5 Glandicipitidae, 17 Glandula pterygopodia, 182 Glanidium, 588 Glaniopsis, 582 Glaucosoma, 660 Globe-Fishes, 726, 152, 163 Glomerulus, of Enteropneusta, 15; of Fishes, 398, 399 Glomus, 398 Glossobalanus, 17, 5, 13; G. minutus, 10, 11; G. ruficollis, 17 n.; G. sarniensis, 5, 10 Glossodus, 446 Glyphidodon, 672 Glyptocephalus, 687; G. cynoglossus, 687; G. microcephalus, 687 Glyptolepis, 480 Glyptopomus, 477 Glyptosternum, 588 Gnathacanthus, 695 Gnathonemus, 551, 549; G. curvirostris, 550; G. numenius, 550 Gnathostomata, 145; characters of, 147 Gobiesocidae, 707, 704 Gobiesox, 709 Gobiidae, 689 Gobiiformes, 688 f., 651, 652 Gobio, 582 Gobiodon, 689 Gobiosoma, 689 Gobius, 689, 690; G. minutus, care of eggs, 415; G. ruthensparri, 689 Gold-Fish, 585, 155, 171 Gomphosus, 673 Gonads (reproductive organs), of Hemichordata, 16, 25; of Tunicata, 55, 67, 76, 93, 97, 105; of Amphioxus, 129; of Fishes, 402 f., 400, 403 Gonoducts, of Fishes, 404 f.; Elasmobranchs, 400, 404; Teleostei, 400, 404, 406; Holocephali, 405; Dipnoi, 405, 406, 407; Crossopterygii, 405; Chondrostei, 405; Holostei, 405 Gonorhynchidae, 572, 545 Gonorhynchus greyi, 572, 572 Gonostoma, 571 Gonostomatinae, 571 Gonozooid, 99, 100 Goode, Brown, 680 n., 710, 725 Goode and Bean, 614 n. Goodrich, 29, 113, 126 Goodsiria, 89; G. placenta, 89 Gourami, 669 Grammicolepis, 683 Grammistes, 660 Grammistinae, 660 Grassi, 21 n. Grassi and Calandruccio, 602 Grayling, 568 Greene, 711 n. Greenland Shark, 455 Gressin, 705 n. Grey Mullets, 640 Grobben, 39 Groove of Hatschek, 128 Guanin, 167 Guaninkalk, 167 n. Guitel, 690 n., 709 n., 710 Gunnel, 712 Günther, 543, 557, 569 n., 621 n. 632, 674, 711 n., 719 Gurnards, 701—see also Trigla Gwyniad, 568 Gymnallabes, 588 Gymnapistus, 695 Gymnarchinae, 551 Gymnarchus, 551; larval gills, 290, 418, 419; abdominal pores, 401; size of eggs, 408; G. niloticus, 552; embryo, 419 Gymnelis, 712 Gymnoarian, 403 Gymnocypris, 582 Gymnodontes, 725 f., 721, 272, 361 Gymnodraco, 706 {742} Gymnotidae, 579, 574, 163, 256, 302, 389 Gymnotus, 579, 581, 649 n.; electric organs, 365, 366; G. electricus, 580 Gyrinochilus, 582 Gyrodus, 498 Gyrolepis, 487 Gyroptychius, 479 Gyrosteus, 489

Haddock, 649, 308, 361 Haddon, 289 Haemulon, 664 Hag-Fishes, 421 Hake, 649, 308 Haldeman, 19 n. Halec, 609 Half-Beak, 154, 414 Halibut, 687 Halicmethes, 720 Halieutaea, 720 Halilophus mirabilis, 21 Halimochirurgus, 723 Haloporphyrus, 648 Halosaurichthys, 624 Halosauridae, 623; photophores, 178 Halosauropsis, 624; H. macrochir, 621 n., 624 Halosaurus, 624 Hammer-head Sharks, 449 Hancock, 53, 591 n., 592 Haplistia, 477 n. Haplochilus, 616 Haplochiton, 608 Haplochitonidae, 608, 606 Haplodactylidae, 664, 650, 654 Haplodactylus, 664 Haplomi, 605 f., 651, 306 Haplonotus, 663 Harmout, 590 Harpagifer, 706 Harpodon, 611; H. nehereus, 613; H. squamosus, 613 Harrimania, 17, 5; H. kupfferi, 20 n. Harrimaniidae, 17 Harriotta, 471, 154, 223; distribution, 473; H. raleighana, 472; young, 473 Hartmeyer, 38 Hasselt, van, 36 Hatching, of eggs, 417 Hatschek, 113; grove of, 128 Haus, of Appendicularians, 66 Head, shape and relative size, 153 Heart, of Tunicata, 49, 67; of Fishes, 327, 328 Heart-vesicle, of Hemichordata, 15 Heincke, 565 n. Helgia, 582 Heliastes, 672 Helicophagus, 588 Heller, 38 Helodus, 445 Helogenes, 589 Helostoma, 669 Hemerocoetes, 706 Hemibranch, 278 Hemibranchii, 627, 629 Hemichordata, 4 f., 3, 38; affinities, 30 f. Hemichromis, 671 Hemiexocoetus, 638 Hemimyaria, 101 f., 95, 96 Hemiodontinae, 576 Hemiodus, 576 Hemipimelodus, 588 Hemirhamphus, 638; beak, 154; intromittent organ, 414 Hemirhynchus, 680 Hemisilurus, 588 Hemithyrsites, 679 Hemitripterus, 698 Hemprich and Ehrenberg, 558 n. Heniochus, 668 Henoplosus, 666 Hensel, 593 n. Hephthocara, 712, 713 Heptanchus, 443; branchial clefts, 277; H. cinereus, 443; skull, 222 Heptapterus, 588 Herdman, 38, 108 n., 109 n. Hermaphrodite Fishes, 420 Hermaphroditism, 420 Hermosilla, 657 Heros, 672 Herring, 564, 389; coloration, 173; gonads, 403; eggs, 412; influence of temperature on time of hatching, 417 Heterobranchus, 588, 590; accessory gill, 293 Heterochaerops, 673 Heteroconger, 601 Heterodont, 251 Heterodontidae, 444; teeth, 251; persistent nephrostomes, 401, 445 Heterodontus, 444; skull, 223; teeth, 251; spiral valve, 267; H. philippi, 445 Heteromi, 621 f., 306 Heteropleuron, 137, 138; H. bassanum, 137; H. cingalense, 137; H. cultellum, 137; H. hectori, 137; H. maldivense, 137 Heterostichus, 709 Heterostraci, 524 f., 149 Heterothrissa, 563 Heterotis, 557, 555; larval gills, 290, 418; size of eggs, 408; H. ehrenbergii, gill-helix, 294; H. niloticus, 556, 558 Hexagrammidae, 696, 694 Hexagrammus, 696 Hexanchus, 443; branchial clefts, 277; H. griseus, 443 Hickson, 289, 690 n. Hierichthys, 713 {743} Himantolophus, 719; H. reinhardti, 719 Hincks, 23 n. Hippocampus, 634, 635, 154, 156, 283; prehensile tail, 163; H. brevirostris, 361, 362; H. guttulatus, 635 Hippoglossina, 687 Hippoglossoides, 687; H. limandoides, 687 Hippoglossus, 687; H. vulgaris, 687 Histiocephalus, 695 Histiophoridae, 679, 676 Histiophorus, 680 Histiopterus, 660 Histiothrissa, 563 Hjort, 39, 83 Holacanthus, 668, 361 Holargyreus, 648 Holaspis, 527 Holmwood, 692 n. Holoblastic, 97, 409, 410 Holobranch, 278 Holocentrum, 656, 655; H. sogho, 361 Holocephali, 466 f., 148 Holoptychidae, 479, 202 Holoptychius, 480; H. leptopterus, 236; H. flemingi, 479 Holosomata, 88 f., 81, 82, 110 Holostei, 495 f., 149, 497 Holothurians, Fierasfer in, 626; Syngnathus and Doryichthys in, 635 Holoxenus, 695; H. cutaneus, coloration, 165 Holt, 690 n., 707 Homaloptera, 582 Homalopterinae, 582, 585 Homocercal, 160, 237 Homosoma, 643 Homosteus, 536 Hoplichthyidae, 699, 694 Hoplichthys, 700 Hoplognathidae, 662, 653 Hoplognathus, 662 Hoplopagrus, 660 Hoplopteryx, 656; H. lewesiensis, 656 Horse-Mackerel, 677, 158 Houting, 568 Humboldt, 581, 595 Hundsfisch, 610 Huot, 627 Huxley, 30 n., 37, 112 Hybodus, 445; vertebral column, 197 Hydrocyon, 575; H. goliath, 578 Hydrocyoninae, 575 Hydrolagus colliei, 469 Hygrogonus, 672 Hymenocephalus, 647 Hyodon, 553; H. alosoides, skull and pectoral arch, 553 Hyodontidae, 552, 544, 389, 405 Hyostylic, 222 Hyperlophus, 563 Hyperopisus, 549; H. bebe, 552 Hyperpharyngeal groove, 123 Hypnos, 464; electric organs, 365; H. subnigrum, 258 Hypobranchial groove, 46 Hypobythiinae, 72 Hypobythius, 72; H. calycodes, 72, 72; H. moseleyi, 72 Hypomesus, 566 Hypopharyngeal groove, 122, 123 Hypophthalmichthys, 582; H. molitrix, 584 Hypophthalminae, 589 Hypophthalmus, 589 Hypophysial canal, 58 Hypophysis, 129, 370 Hypoprion, 448 Hypoptopoma, 595 Hypoptychus, 639 Hypostoma—see Plecostomus Hypostomides, 628 Hypsocormus, 501; H. insignis, 502 Hyrtl, 558 n., 599 n. Hysterocarpus, 670

Iasis, 108; I. cordiformis-zonaria, 108 Icelus, 698 Ichthyoborinae, 576 Ichthyoborus, 576 Ichthyocampus, 634 Ichthyococcus, 571 Ichthyodectes, 561 Ichthyodorulites, 159, 435, 446, 467 Ichthyomyzon, 426 Ichthyopsida, 145 Ichthyotomi, 438 f., 148 Icichthys, 644 Icosteidae, 644, 637 Icosteus, 644; I. enigmaticus, 644 Iguanodectes, 575 Ihering, 593 n. Ikeda, 29 Ilyophis, 601 Infundibular organ, 127 Inter-renal bodies, 346, 346 Intromittent organs, 414 Ipnops, 611, 613; I. murrayi, 612; photophore, 179 Iridocytes, 166 Ischnacanthus, 441 Ischyodus, 474 Isichthys, 551 Iso, 639 Isospondyli, 543, 620 Istieus, 549 Isurichthys, 678

Jacoby, 601, 602 n. Janessa, 446 Jenynsia, 616 Jobert, 593 n. John Dory, 683, 308, 361, 363 Jones, 720 n. {744} Jordan, 542 Jordan and Evermann, 569 n., 620 Jordan and Goss, 687 n. Jordania, 698 Joturus, 640 Joues cuirassées, 692 Jugulares, 702 f., 651, 652, 306 Julin, 38, 39, 52 Julis, 673, 674 Jullien, 23 Jumping-Fish, 690

Kalymmocytes, 40, 56, 93, 107 Kelb-el-Bahr, 578 Kidneys, 397 f.; development, 397; in different Fishes, 399 Kilch, 311 King-Fish, 628 King of the Herrings, 715 Kner and Steindachner, 626 Kneria, 616; K. angolensis, 616; K. spekii, 616 n. Kneriidae, 615, 606 Korotneff, 39 Kowalevskia, 68 Kowalevskiidae, 68 Kowalevsky, 7 n., 37, 39, 94 n., 113 Krohn, 37 Krohnius, 647 Krukenberg, 271 n. Kuhl, 36 Kuhlia, 657 Kupffer, 38 Kurtidae, 687 Kurtiformes, 687 f., 651, 652 Kurtus, 687; K. indicus, skeleton of, 688 Kyle, 687 n.

Labeo, 582; L. falcifer, 583 Labidesthes, 639 Labrichthys, 673 Labridae 673, 654, 271, 275; coloration, 164, 166 Labrodon, 674 Labrus, 673, 674; pharyngeal teeth, 252, 288; L. labrax, 275; L. maculatus, pharyngeal bones, 673; L. rupestris, 674 Labyrinthiform organs, 292, 293 Lacaze-Duthiers, 38 Lactariidae, 663, 654 Lactarius delicatulus, 663 Lactophrys, 724 Laemargus, 455; L. borealis, 455, 456; intestinal caeca, 274; eggs, 435 Laïs, 588 Lamarck, 36 Lamna, 451; L. cornubica, 451 Lamnidae, 450 Lampetra, 426; L. wilderi, 427 Lampreys, 425 f. Lamprididae, 628, 627 Lampris, 628, 629; L. luna, 628; coloration, 164 Lamprogrammus, 712 Lamprologus, 671 Lanarkia, 524, 525; L. spinosa, 524 Lancelet, 112 Lang, 31 n. Langerhans, 112 Lankester, 5 n., 23, 112, 113 Larimus, 663 Larvacea, 64 f., 64, 110; structure, 65; habits, 65; tail, 66; classification, 68; occurrence, 69 Larval Fishes—Teleosts, 417 f., 418, 419; Cyclostomata, 428; Crossopterygii, 290, 483, 484; Chondrostei, 494; Holostei, 501, 504; Dipnoi, 514, 515, 517, 518 Larval gills, 290 f.; Elasmobranchii, 289, 290; Polypterus, 290, 483, 484; Heterotis, 290; Gymnarchus, 290, 418, 419; Misgurnus, 290; Salmon, 290; Protopterus, 291, 515, 515; Lepidosiren, 291, 517, 518 Larval organs, 418; adhesive or cement organs, 418, 494, 501, 504, 514, 515, 517, 518; external gills, 289, 290, 419; cutaneous gills, 290, 483, 484, 517, 518; defensive spines, 418 Lasanius, 532; L. problematicus, 532 Lateolabrax, 659 Lateral line sensory organs, 163, 384, 385, 386, 387 Lates, 660 Latilus, 661 Latrididae, 663, 654 Latris, 663; L. hecateia, nocturnal colour-changes, 170 Launce, 639 Lavaret, 568 Leather Carp, 584 Lebiasina, 575, 578 Lefevre, 83 Lefua, 582 Lemon Sole, 687 Lentipes, 689 Lepadogaster, 709, 191 Lepidocephalichthys, 582 Lepidocottus, 698 Lepidomeda, 582 Lepidopus caudatus, 679 Lepidorhombus, 687; L. megastoma, 687 Lepidosiren, 511, 149, 153, 261, 401; pectoral fins, 157; skull, 231, 233; gills, 286; larval gills, 291, 517, 518; lung, 291, 301; vascular system, 527; thyroid, 344; growling sounds, 363; relations with other Dipnoi, 518, 519; L. paradoxa, 516; distribution, habits, and food, 516; nocturnal colour change, 517; hibernation, 517; {745} nest, 517; filaments on pelvic limb of male, 517; larva, 517, 518 Lepidosirenidae, 511 Lepidosteidae, 502 Lepidosteoidei, 495 f. Lepidosteus, 503, 149, 153, 160, 258, 262, 273, 274, 283, 284, 291; scales, 185, 186, 188; vertebral column, 201, 202, 204; skull, 229; alimentary canal, 257; spiral valve, 268; pyloric caeca, 274; air-bladder, 297, 299, 310; spiracular pseudobranch, 334, 335; branchial circulation, 334 n., 336; brain, 376; gonads, 400, 402, 405, 407; segmentation of the egg, 409; distribution, 503; habits, 503; breeding, 503; larvae, 504; fossil, 504; L. osseus, 503, 504; L. platyrhynchus, pectoral fin, 243; L. platystomus, 503; L. viridis, 503 Lepidothynnus, 678 Lepidotrichia, 234 Lepidotrigla, 701 Lepidotus, 498; L. minor, 497 Lepomis, 657 Lepophidium, 713 Leporinus, 576 Leptagoniates, 575 Leptecodon, 623 Leptichthys, 563 Leptobarbus, 582 Leptocephalid, of Albula, 548 Leptocephalus, 600, 21 n.; L. brevirostris, 602 Leptochilichthys, 570 Leptoclinum, 87; section, 86; spicules, 87; L. neglectum, 80 Leptoderma, 570 Leptodoras, 588 Leptolepididae, 546, 544 Leptolepis, 546; L. dubius, 546 Leptopterygius, 709 Leptoscopidae, 705, 703 Leptoscopus, 705 Leptosomus, 611 Leptotrachelus, 623 Lethrinus, 665 Leucaspius, 582 Leuciscus, 582, 288 Leuckart, 37 Leucosomus, 582 Levinsen, 23 Leydig, 584 n. Lichia, 677 Ling, 649 Linophryne, 719 Liocassis, 588 Liocetus, 719 Lionurus, 647 Liopropoma, 659 Liparis, 698, 699, 411 Liparops, 698 Lipogenyidae, 624, 622, 623 Lipogenys, 624; L. gillii, 624 Liposarcus, 595 Lirus, 643; L. medusophagus, 643; L. perciformis, 643 Liuranus, 601 Liver, 271; of Amphioxus, 121, 123 Loaches, 585, 261, 290, 292; Pond Loach, 585. Lobotes, 658 Lobotidae, 658, 653 Locomotion of Fishes, 349 f., 351 Lohmann, 66 Longchamps, de Selys, 29 Lophiidae, 718 Lophiomus, 718 Lophius, 718, 153; L. piscatorius, 718, 355; lure, 161, 173; teeth, 250 Lophobranchii, 628, 543, 629 Lopholatilus, 661; L. chamaeleonticeps, 661 Lophotes, 716 Lophotidae, 716 Lorenzini's ampullae, 384 Loricaria, 595, 292 Loricariidae, 594, 575, 256, 292 Loricariinae, 595 Lota, 648; L. vulgaris, 649 Lotella, 648 Löwig and Kölliker, 37 Lucifuga, 712, 713, 395; L. subterranea, 361 Luciobrama, 582 Luciocephalus, 669 Luciogobius, 689 Lucioperca, 659 Luciosoma, 582 Lump-Sucker, 699, 162, 321, 408, 415 Lumpenus, 709 Lunel, 635, 682 n. Lutjaninae, 660 Lutjanus, 660 Lütken, 614 n., 682 n., 714 n. Lutodeira, 256 Luvaridae, 681, 676 Luvarus imperialis, 681 Lycocara, 712 Lycodes, 712, 702 Lycodontis, 605 Lyconus, 647 Lycoptera, 546 Lymph-hearts, 342 Lymphatic system, 342 Lymphoid tissue, 348 Lyomeri, 622 Lyosphaera, 726

MacBride, 16, 31 n., 113, 133 n. M‘Intosh and Masterman, 565 n. Mackerel, 678, 168, 275, 302 Macquaria, 659 Macrius, 705; M. amissus, 705 Macrodon, 575; {746} M. trahira, mouth, 577 Macrones, 588 Macropharynx, 604 Macrophthalmia chilensis, 426 Macropodus viridi-auratus, 669 Macropoma, 481, 202, 268 Macrosemiidae, 498, 497 Macrosemius, 498 Macrostomias, 571 Macruridae, 647, 702 Macruronus, 647 Macrurus, 647; M. carminatus, 647 Maena, 664 Mahaseer, 584 Maigre (= Meagre), 361, 362, 663 Malacanthus, 661 Malacichthys, 659 Malacocephalus, 647 Malacopterus, 673 Malacopterygii, 543 f., 159, 306 Malacosarcus, 619 Malacosteus, 571; M. indicus, 570; photophoreo, 178 Mallotus, 566; M. villosus, 568, 569 Malm, 685 n. Malopterurinae, 588 Malopterurus, 588, 593, 359; electric organs, 365, 366; M. electricus, 591, 362 Malthe, 720; scales, 190, 191; M. vespertilio, 720 Malthidae, 720, 718 Malthopsis, 720 Mantle, 42 Marcusenius, 551 Marsupial pouches, 416 Mastacembelidae, 716 Mastacembelus, 717; M. maculatus, 717 Masterman, 23 n., 25, 27, 28, 29, 31 n. Maurolicus, 571, 570 Mazza, 714 n. M‘Biriki, 583 Meagre (= Maigre, q.v.) Meda, 582 Medialuna, 657 Median fins, 156, 234 f. Medusae, with Caranx, 643 Meek, 714 n. Megalichthys, 477, 478; spiral valve, 268 Megalocercus, 68; M. abyssorum, 66, 68 Megalops, 547; M. atlanticus, 547; M. cyprinoides, 547 Megalurus, 501 Megrim, 687 Melambaphes, 664 Melamphaes, 656, 620, 655 Melanocetus, 719 Melanostigma, 712 Melanostoma, 659 Melanotaenia, 639 Membrana nictitans, 395 Membranellae, 19 Menaspis, 445 Mene, 677 Merluccius, 648, 648 n.; M. vulgaris, 649; red gland, 308 Meroblastic, 93, 409, 410 Merosomata, 85, 81, 110 Mertens, Von, 66 Mesacanthus, 442 Mesiteia, 447 Mesoborus, 576 Mesocoracoid arch, 543, 573 Mesodon, 498 Mesonephros, 397, 398, 400 Mesoprion gembra, radialia of the dorsal fin, 235 Mesorchium, 402 Mesovarium, 402 Mesturus, 498 Metcalf, 38, 52, 109 Metschnikoff, 19, 31 n., 39 Metynnis, 576 Meyen, 677 n. Micracanthus, 669 Micralestes, 575 Microbrachius, 534 Microcoelia, 611 Microcosmus draschii, dorsal tubercle, 79 Microdon, 498 Micropogon undulatus, air-bladder and its muscles, 360, 361 Micropterus, 657 Micropyles, 411, 412 Microspathodon, 672 Microstoma, 566, 565 Miller's Thumb, 698 Milne-Edwards, 37 Minnow, 288 Minot, 30 n. Minous, 695, 692; M. inermis, 695 Misgurnus, 582, 290; intestinal respiration, 261, 292; oviducts, 405; M. fossilis, 358, 585 Mistichthys luzonensis, 689 Möbius, 630 n., 673, 724 Mochocus, 588 Mola, 727 Molgula, 77; M. citrina, 79; M. oculata, 77, 79; M. pyriformis, branchial sac, 77; dorsal tubercle, 79 Molgulidae, 77, 64, 110; nervous system, 53; reproductive organs, 55 Molidae, 726, 722 Mollienesia, 616 Molluscoidea, 35 Molva, 648; M. vulgaris, 649 Monacanthus, 724; coloration, 165; warning colours, 174; scales, 190; sounds, 354, 357; M. sp., 723; M. pardalis, 361; M. scopas, scale, 191 Monascidiae, 35 {747} Monocentridae, 656, 653 Monocentris, 656 Monocirrus, 658 Monogamy, 413 Monopterus, 597; M. javanensis, head, skull, pectoral arch, vertebrae, 598 Moon-eyes, 553 Moon-fish, 579 Morchellium, 88; M. argus, stomach, 88 Mordacia, 426 Morgan, 11 n., 20, 21 n., 31 n., 32 Moringua, 601 Mormyridae, 549, 544, 154; larval gills, 290; abdominal pores, 401 Mormyrinae, 551 Mormyrops, 551, 549 Mormyrs, 551 Mormyrus, 551; M. caballus, 549; electric organs, 365, 366, 401 Morone, 659, 660 Moseley, 95, 613, 702 n. Motella, 649, 288 Mouth, position, size, and shape; of Cyclostomata, 150; of Fishes, 153 f. Moxostoma, 581 Mud-Fish, 610 Mugil, 640, 264; M. capito, 640 Mugilidae, 640, 637, 256; gizzard, 260 Müller, H., 37 Müller, J., 19, 31, 112, 129 Müller, O. F., 36 Müllerian duct, 398, 400, 404 Mullets, Grey, 640; Red, 665 Mullidae, 665, 654 Mulloides, 665 Mullus, 665; M. barbatus, 666; M. surmuletus, 666; pectoral arch, 666 Muraena, 605; M. helena, 600, 605 Muraenesox, 601 Muraenichthys, 601 Muraenidae, 604, 163, 259 Muraenolepididae, 649 Muraenolepis, 649 Murray, 613 Muscular system, 350 Mustelus, 448, 298; placenta, 434, 449; M. antarcticus, radialia of the dorsal fin, 234; vascular system, 316, 317, 330, 331, 334; placenta, 435; M. laevis, 449; M. vulgaris, 449 Myletes, 576 Myleus, 576 Myliobatidae, 465; teeth, 251 Myliobatis, 465, 466; M. aquila, 465, 466 Mylostoma, 537 Myocoele, 397, 398 Myocomma, 117 Myomyrus, 551 Myotome, 117, 133 Myriacanthidae, 468 Myriacanthus, 468; M. granulatus, 468 Myripristis, 656 Myroconger, 605 Myrophis, 601 Myrus, 601 Myxine, 422, 147; external characters, 151; thread-cells, 182; vertebral column, 197; teeth, 248; liver, 273; pancreas, 273; gill-sacs, 281; blood corpuscles, 341; brain, 372; auditory organ, 387, 388; pituitary involution, 391; degenerate eyes, 394, 395; pronephros, 399, 400; genital pore, 403; distribution, 422; habits, 422; hermaphroditism, 423; M. glutinosa, 151, 422, 423 Myxinidae, 422 Myxinoides, 421 f. Myxus, 640

Nandidae, 658, 653 Nandus, 658 Nannaethiops, 576 Nannobrachium, 611 Nannocampus, 634, 635 Nannocharax, 576 Nanoglanis, 588 Nanognathus, 576 Nanostomus, 576 Nansenia, 566, 565 Narcetes, 570 Narcine, 464 Naseus, 668 Naucrates, 677; N. ductor, 677 Nealotus, 679 Neatherina, 639 Nebris, 663 Nedystoma, 588 Needle-Fish, 634 Nemachilus, 582 Nematabramis, 582 Nematistius, 677 Nematogenys, 588, 587 Nematonotus, 611 Nemichthyidae, 603 Nemichthys, 603 Nemopteryx, 648 Neobola, 582 Neoborus, 576 Neobythitis, 712 Neoceratodus, 508, 259, 291; skull, 231; pectoral fin, 244; gills, 285, 286; lung, 291, 299, 300; vascular system, 323, 324, 329, 338; grunting sounds, 363; brain, 377; abdominal pores, 401; Müllerian ducts in the male, 407; distribution and habits, 508, 512; spawning, 510; eggs, 510; young, 510, 511; relations with other Dipnoi, 518, 519; N. forsteri, 508, 509 Neochanna, 608 Neolebias, 576 {748} Neopempheris, 657 Neopercis, 705 Neoscopelus, 611 Neosilurus, 588 Nephridia, of Amphioxus, 125 Nephrostomes, 398; persistent, 401 Nephrotome, 397, 398 Nerophis, 634, 635 Nerve eminences, 383 Nervous system, of Chordata, 4; of Hemichordata, 9, 25, 30; of Tunicata, 53, 58, 66; of Amphioxus, 127 f., 131; of Fishes, 367 f. Nesiarchus, 679 Nesting habits of Sticklebacks, 630 Nests of Fishes, 414 f., 427, 500, 501, 514, 515, 517; of Arius, 593; of Doras and Callichthys, 592; of Gobius, 690; of Gymnarchus, 552; of Heterotis, 558; of Spinachia, 631 Nettastoma, 601 Nettenchelys, 601 Nettophichthys, 601 Neural gland, 52 Neurenteric canal, 57, 131, 133 Neuromeres, 195 n. Neuropore, 10, 58, 59 Nilhechte, 550 Nilsson, 715 Niphon, 659 Nishikawa, 685 n. Noll, 584 n. Nomeus, 643; N. gronovii, 643 Norman, 23 Notacanthidae, 624 Notacanthus, 625; N. bonapartii, 625 Notagogus, 498 Notidanidae, 442, 279; persistent nephrostomes, 401 Notidanus, 443, 345; spiral valve, 267; N. cinereus, branchial clefts, 277; skull, 222; N. griseus, branchial clefts, 277 Notochord, 3; of Enteropneusta, 14; of Cephalodiscus, 24, 25; of Rhabdopleura, 27; of Actinotrocha, 28; of Tunicata, 57, 60, 61; of Amphioxus, 119, 118, 119, 132; of Fishes, 193 f. Notoglanidium, 588 Notoglanis, 588 Notogoneus, 572 Notopteridae, 554, 544, 303, 389, 405 Notopterus, 555, 153, 305, 306; N. afer, skeleton, 554; N. chitala, 555 Notothenia, 706 Nototheniidae, 705, 704 Noturus, 588, 590 Novacula, 673 Nucleus, of Thaliacea, 101, 105 Nuria, 582 Nurse, 97

Oar-Fish, 715, 163 Oblata, 664 Ocelli, 53 Octacnemidae, 108, 101 Octacnemus, 109, 101; O. bythius, 109; O. patagoniensis, 109 Odax, 674 Odaxothrissa, 563 Odontaspis, 451 Odonteus, 673 Odontonectes, 660 Odontostomus, 611, 613 Oenoscopus, 545 Oesophageo-cutaneous duct, 281 Oikoplasts, 65 Oikopleura, 68, 67, 69; transverse section, 67; longitudinal section, 68; O. cophocerca, 66; O. dioica, 70; O. flabellum, transverse section, 69 Olfactory organs, 155, 390 f., 391, 392 Olfactory pit, 129 Oligopleuridae, 545, 544 Oligopleurus, 545 Oligorus, 659 Oligosarcus, 575 Oligotrema, 78, 111 n. Olt, 584 n. Olyra, 588 Omble Chevalier, 567 Omiodon, 611 Omosudis, 611 Oncorhynchus, 566 Oneirodes, 719; O. eschrichtii, 174 Onus, 648; coloration of larvae, 175 Oozooid, 84, 91, 93 Opah, 628, 164 Operculum, 25, 155, 278, 282, 283 Ophichthys, 601 Ophidiidae, 713, 651, 702, 704, 163, 361 Ophidium, 713, 361; air-bladder, 302 Ophioblennius, 709 Ophiocephalidae, 644, 637, 163, 293, 361; distribution, 645 Ophiocephalus, 645; labyrinthiform organ, 293; O. marulius, 361; O. gachua, 361 Ophiodon, 696; O. elongatus, skull, 692 Opisthocentrus, 709 Opisthognathus, 661 Opisthomi, 716 f., 651, 306 Opisthomyzon, 691 Opisthonema, 563 Opisthoproctus soleatus, telescopic eyes, 395 Opisthopteryx, 611 Opostomias, 571; O. micripnus, photophores, 178, 179, 181 Opsanus, 711 Opsariichthys, 582 Oral hood, 116, 119, 136 Oreinus, 582 Oreosoma, 683 {749} Orestias, 616 Orodus, 445 Orthagoriscus, 727, 153, 354; spinal cord, 367; O. mola, 602, 727, 348, 357 Ortho-enteric, 102, 108 Osmeroides, 547 Osmerus, 566, 565; O. eperlanus, 568; gonoducts, 405, 411 Osphromenidae, 669, 654; labyrinthiform organ, 293 Osphromenus, 669, 293; O. olfax, 669 Osphyolax, 634 Ostariophysi, 573 f., 306, 389 Osteochilus, 582 Osteogeniosus, 588, 593 Osteoglossidae, 555, 544, 294, 405; larval gills, 290; distribution, 557 Osteoglossum, 557; O. bicirrhosum, 556 Osteolepida, 477 f. Osteolepidae, 477 Osteolepis, 477; O. macrolepidota, 477 Osteostraci, 527 f., 149 Ostracion, 724, 152, 361; colours, 165, 174; scales, 191; O. ornatus, 165; O. quadricornis, 725; O. trigonus, 362 Ostraciontidae, 724, 721 Ostracodermi, 522 f., 149, 721 Otocinclus, 595 Otolithus, 663; air-bladder, 304; O. regalis, 361 Oviducal gland (shell-gland), 400, 407 Ovipositor, 408 Oxuderces, 690 Oxydoras, 588, 359 Oxygnathus, 487 Oxylebius, 696 Oxymetopon, 689

Pachycormidae, 501 Pachycormus, 501 Pachylebias, 616 Pachymetopon, 665 Pachyrhizodontidae, 569 Pachyrhizodus, 569 Pachystomias, 571; P. microdon, photophores, 178, 179, 180 Pachyula, 588 Paddle-Fish, 491; = Polyodon, q.v. Pagellus, 665; P. centrodontus, 346 Pagrus, 665; P. auratus, 665 Palaeolycus, 609 Palaeomylus, 468 Palaeoniscidae, 486, 485; range in time, 487 Palaeoniscus, 486, 487; P. macropomus, 486 Palaeorhynchidae, 680 Palaeorhynchus, 680 Palaeoscyllium, 447 Palaeospinax, 445; vertebral column, 197 Palaeospondylidae, 521 f., 149 Palaeospondylus gunni, 521, 522 Palimphyes, 678 Pallas, 112 Pancreas, 273 Pangasius, 588, 305, 359; P. micronema, 359 n. Pantodon buchholzi, 558, 559 Pantodontidae, 558, 544 Papillae, adhering, 61 Paracentroscyllium, 455 Paradiplomystes, 588 Paragoniates, 575 Parailia, 588 Paralepis, 611 Paralichthys, 687 Paraliparis, 698 Paraluteres, 724 Paranthias, 659 Parapegasus, 636 Parapelecus, 582 Parapercis, 705 Paraphago, 576 Paraphractura, 588, 589 Parapriacanthus, 657 Parapsettus, 668 Parascopelus, 611 Parasitism, of Rhodeus, 584, 416; of Stegophilus, 594 Paratilapia, 671 Paratrachichthys, 656, 655 Paratrygon, 465 Parental care, 415, 416, 500, 501 Parexus, 441 Parietal budding, 82 Parietal eye, 395 f., 396 Pariolius, 588 Parker, T. J., 714 n. Parker, W. N., 705 n. Parodon, 576 Paropsis, 677 Parrot-Wrasses, 674 Pataecus, 709 Pearl Oysters, Fierasfer in, 625 Pectoral fins, 157, 242, 243, 244 Pectoral girdle, 239, 240 Pediculati, 717 f., 651, 306 Pegasidae, 635, 626, 628 Pegasus, 636 Pegea, 108; P. scutigera-confoederata, 108 Pelagic ova, 408 Pelargorhynchus, 623 Pelecus, 582 Pellegrin, 671 n., 724 n. Pellona, 563, 563 Pellonula, 563 Pelonaia, 74; branchial sac, 74; P. corrugata, 76 Pelor, 695 Peloria, 685 Pelvic fins, 158, 245 f., 246 Pelvic girdle, 239 f., 240, 241 {750} Pempheridae, 656, 653 Pempheris, 657; P. muelleri, 657 Penetopteryx, 634, 635 Pentaceropsis, 660 Pentaceros, 660 Pentacerotinae, 660 Pentanemus, 641; P. quinquarius, 162 Pentapus, 664 Pentaroge, 695 Peprilus, 643 Perca, 659, 262; alimentary canal, 275; veins, 323; thyroid, 343; auditory organ, 388; P. fluviatilis, 659, 321, 323 Percalates, 659 Percarina, 659 Percesoces, 636 f. Perch, 659; deposition of eggs, 411; = Perca, q.v. Percichthys, 659, 660 Percidae, 658, 654, 412; coloration, 166 Perciformes, 652 f., 651 Percilia, 659 Percina, 659 Percophiidae, 705, 703 Percophis, 705; P. brasilianus, pectoral arch of, 703 Percopsidae, 620, 606 Percopsis, 621 Peribranchial budding, 82 Peribranchial cavity, 43, 44, 59, 63; of Thaliacea, 95, 101 Pericardium, of Hemichordata, 15; of Tunicata, 44, 49; of Fishes, 327 Perihaemal spaces, 9 Periophthalmus, 689, 690, 355; eyes, 155; pectoral fins, 161; tail as a respiratory organ, 289; P. koelreuteri, 690 n. Peripharyngeal bands, 45, 46, 52, 53 Peripharyngeal spaces, 9 Peristedion, 701 Péron, 36 Perophora, 72, 84; P. listeri, 72 Petalodontidae, 446 Petalodus, 446 Petalopteryx, 498 Petersen, 690 n. Petersius, 575 Petrocephalus, 551 Petromyzon, 426, 147, 258, 367, 382; external characters, 150; skull, 217; teeth, 247, 248; spiral valve, 264; liver, 272; pancreas, 273; gill-sacs, 279, 280; heart, 327; arteries, 329, 330; blood corpuscles, 341; thyroid, 343, 344; supra-renal bodies, 346; brain, 371, 372, 392; spinal nerves, 378; auditory organ, 387; olfactory organ, 391, 392; naso-pituitary involution, 391; pituitary caecum, 392; parietal eye, 395, 396; kidneys, gonads, and genital pores, 399, 400, 403; distribution, 426; P. fluviatilis, 426; P. marinus, 426; P. planeri, ova, 428; P. wilderi, spawning, 427; larva, Ammocoetes stage, 428 Petromyzontes, 425 f. Petromyzontidae, 426 Petroscirtes, 709 Phago, 576 Phallusia, 72 Phaneropleuron, 506, 519; P. andersoni, 506 Phanerosteon, 487 Pharyngeal teeth, 251 Pharyngodictyon, 87; P. mirabile, 80 Pharyngognathi, 543 Pharyngopneusta, 30 n. Phisalix, 705 n. Phlyctaenaspis, 536 Pholedichthys, 709 Pholididae, 711, 704 Pholidophoridae, 545, 541 n., 544 Pholidophorus, 545 Pholis, 711, 712; P. gunnellus, 712, 415 Phoronidea, 27 f., 5 Phoronis, 27 f., 5; regeneration, 16, 30; P. buskii, 28 Phorozooid, 99, 100 Phosphorescent organs, 178 Photichthys, 571, 570 Photoblepharon, 660 Photonectes, 571 Photophores, 178, 612, 613, 624, 711 Photostomias, 571 Phractolaemidae, 560, 544 Phractolaemus ansorgii, 560, 560 Phractura, 588 Phthinobranchii, 629 Phycis, 648 Phyllodoce, nephridium, 127 Phyllodus, 674 Phyllopteryx, 634; P. eques, 635 Phylogephyra, 565 Physailia, 588 Physalia, with Nomeus, 643 Physiculus, 648 Physoclisti, 306, 307, 311 Physopyxis, 588 Physostomi, 543, 306, 311 Piabucina, 575; P. argentina, mouth, 577 Pike, 609, 250, 307 Pike-Perch, 659 Pilchard, 564, 389 Pilot-Fish, 677 Pimelepterus, 657 Pimelodina, 588 Pimelodus, 588, 589, 361 Pimephales, 582 Pipe-Fish, 634, 154 Piramutana, 588 {751} Piranha, 578 Pirate Perch, 656 Piratinga, 361 Pirinampus, 588 Pit-organs, 383 Pituitary body, 129 Placenta, of Doliolum, 98; of Hemimyaria, 101, 107; of Fishes, 434 Plagiostomi, 442 f., 148 Plagusia, 687 Plagyodus, 614 Plaice, 687 Platax, 668 Platycephalidae, 699, 694, 650 Platycephalus, 699, 693 Platychaerops, 673 Platycormus, 643 Platyglossus, 673 Platylaemus, 674 Platypoecilus, 616 Platyrhinoidis, 460 Platysomatichthys, 687, 685 Platysomidae, 487, 485 Platysomus, 488 Platystoma, 594; air-bladder and its extrinsic muscles, 360, 361, 362; P. coruscans, 594 Platytroctes, 570 Plecodus, 672 Plecoglossus, 566, 569 Plecostomus, 595, 256, 292; (Hypostoma) scales, 190; P. commersonii, 192 Plectognathi, 721 f., 543, 651, 205, 231, 275, 306, 354, 418 Plectromus, 656 Plectropoma, 659; P. richardsoni, coloration, 165 Plesiops, 659 Plethodus, 549 Pleuracanthidae, 440 Pleuracanthus, 440, 159, 235, 236, 438; vertebral column, 197; pectoral and pelvic girdles, 239; pectoral fin, 242; pelvic fins, 245; P. ducheni, 439 Pleuragramma, 705; P. antarcticum, 705 Pleurogrammus, 696 Pleuronectes, 687; P. flesus, 687; coloration elements, 167, 170; P. limanda, 687; P. platessa, 686, 687 Pleuronectidae, 684, 683, 152, 264, 275, 284, 302, 412; protective coloration, 172 Pleuropholis, 545 Pleuroplax, 445 Pleuropterygii, 436, 148 Plication, of branchial sac, 48 Plotosus, 588, 587, 408 Podateles, 714; P. indicus, pectoral arch of, 714 Podatelidae, 713, 704 Poecilia, 616 Poecilodus, 445 Poecilopsetta, 687 Poey, 713 n. Pogge, 700 Pogonias, 663; P. chromis, 663, 361; air-bladder, 304, 305; drumming sounds, 362 Poison, of Trachinus, 705 Poison glands, 176 Pollack, 649 Pollan, 568 Polyacanthonotus, 625 Polyacanthus, 669, 293; P. opercularis, 669 Polyandry, 413 Polycarpa, 74, 76; reproductive organs, 55; P. aurata, 76; dorsal tubercle, 79; P. comata, 76; P. glomerata, larva, 78; P. pedata, 76; P. tinctor, 76; dorsal tubercle, 79 Polycarps, 76 Polycaulus, 695 Polycentropsis, 658 Polycentrus, 658 Polyclinidae, 87, 81, 83, 110; ascidiozooid, 82 Polyclinum, 88; P. molle, stomach, 88 Polycyclus renieri, larva, 78 Polygamy, 413 Polyipnus, 571 Polymixia, 656 Polynemidae, 640, 637 Polynemus, 641; P. quadrifilis, 641; shoulder-girdle and pelvis of, 640; P. vereker, coloration, 165 Polyodon, 491, 154, 284, 405; vestigial dermal denticles, 188; ribs, 201; skull, 225; pelvic fin, 245; teeth, 249; pyloric caeca, 274, 276; gills, 282; spiracle, 283; gill-rakers, 288; P. folium, 491; habits and distribution, 491; breeding, 492 Polyodontidae, 491, 486, 489 Polyphyodont, 250 Polyprion, 659 Polypteridae, 481; distribution, 483 Polypterus, 482, 149, 323; scales, 185, 187; vertebral column, 202; ribs, 206; skull, 226, 227, 229; median fins, 235; pelvic girdle, 241; pectoral fin, 243; tongue, 252; spiral valve, 268; pyloric caecum, 274; spiracle, 283; pseudobranch, 284; larval gills, 290; air-bladder, 298; arteries to air-bladder, 337; sounds, 362; habits and food, 482; breeding, 483; P. bichir, 482; P. congicus, 290; P. lapradei, 483; larvae, 483, 484; P. senegalus, 482, 483 Polyrhizodus, 446 Polystyelidae, 89, 81, 110 Polyzoa, 35 {752} Pomacanthus, 668 Pomacentridae, 672, 654; coloration, 166 Pomacentrus, 672 Pomatominae, 660 Pomatomus, 660 Pomoxys, 657 Pope, 659 Porbeagle Sharks, 450 Porcupine Fishes, 726, 163 Porichthys, 711; photophores, 179; P. porosissimus, 711 Porogadus, 712 Portheus, 561 Post-abdomen, 83 Pouchet, 7 n. Pout, 649 Powell, 714 n. Prebranchial zone, 45 Premnas, 672 Prenadillas, 595 Priacanthinae, 660 Priacanthus, 660 Prince, 630 n. Prionodon, 448 Prionolepis, 609 Prionotus, 701, 361 Prionurus, 668 Priscacara, 672 Pristidae, 459, 458 Pristigaster, 563 Pristiophoridae, 457, 458 Pristiophorus, 457 Pristipoma, 664, 361 Pristipomatidae, 664, 654 Pristis, 460, 272; rostral denticles, 184; P. antiquorum, 459 Pristiurus, 446, 447 Proantigonia, 667 Proboscis-gland, 15 Proboscis-pore, 9, 24, 27 Proboscis-skeleton, 14 Procatopus, 616 Prochanos, 563 Prochilodus, 576 Prolates, 660 Prolebias, 616 Proles gregaria, 102 Proles solitaria, 102 Promethichthys, 679 Promyliobatis, 466 Pronephros, 397, 400 Pronotacanthus, 622 Propagation, of Eel, 601 Propoma, 659 Propristis, 460 Protaulopsis, 638, 632 n. Prothymallus, 565 Protocampus, 634 Protocercal, 159 Protopterus, 511, 149, 153, 259, 261, 262, 272, 273; nostrils, 155; pectoral fins, 157; cutaneous glands, 182; skull, 231, 232; pelvic girdle, 241; alimentary canal, 257; spiral valve, 268; pancreas, 274; gills, 286; larval gills, 291, 515; lung, 291, 300, 301, 302; vascular system, 326, 327, 329, 339, 340; thyroid, 343; thymus, 345; sounds, 362; brain, 376, 377; auditory organ, 387; abdominal pores, 401; testis, 406; Müllerian ducts in male, 407; distribution, 511, 512; habits, 512; summer sleep, 513; cocoon, 513; nest, 514, 515; larva, 515; relations with other Dipnoi, 518, 519; P. aethiopicus, 512; P. annectens, 509, 512; P. dolloi, 512 Protosphyraena, 502 Protospondyli, 497 Protostigmata, 59 Protosyngnathidae, 631, 628 Protosyngnathus sumatrensis, 632 Prototroctes, 608 Prototunicata, 110 Psammodontidae, 446 Psammodus, 446 Psammoperca, 660 Psammosteidae, 526, 530 Psammosteus, 527 Psenes, 643 Psenopsis, 643 Psephodus, 445 Psephurus, 492; P. gladius, 492 Psetta (= Rhombus), 687, 685; P. laevis, 687; P. maxima, 686, 687 Psettodes, 687, 684, 685; P. erumei, 686 Psettus, 666; P. sebae, 666, 667 Pseudaluteres, 724 Pseudecheneis, 588, 589 Pseudetroplus, 672 Pseudeutropius, 588 Pseudobagrus, 588 Pseudoberyx, 563 Pseudoblennius, 698 Pseudobrachium, 717 Pseudochromididae, 661, 653 Pseudochromis, 661 Pseudocorynopoma, 575 Pseudogobio, 582 Pseudomonacanthus, 724 Pseudomugil, 639 Pseudoplesiops, 659 Pseudopriacanthus, 660 Pseudoscarus, 674 Pseudoscopelus, 641, 642 Pseudosphaerodon, 674 Pseudosyngnathus, 634 Pseudotriakis microdon, 447 Pseudoxiphophorus, 616 Psilocephalus, 724 Psilorhynchus, 582 Psychrolutes, 698 Pteraclis, 682 Pteraspidae, 527, 530 {753} Pteraspis, 527, 530, 149; P. rostrata, 527 Pterichthys, 534, 149; P. milleri, 533 Pterobranchia, 21 f., 5, 31 Pterois, 695 Pterophryne, 720 Pterophyllum, 672 Pteroplatea, 464; P. micrura, intra-uterine nutrition, 435 Pteropsarion, 582 Pteropsaron, 705 Pterothrissus, 548 Pterycombus, 682 Pterygocephalus, 709 Pterygoplichthys, 595 Ptilichthys, 709 Ptychobarbus, 582 Ptychodera, 17, 5, 13, 16, 25 n.; P. bahamensis, 5, 10; P. flava, 10, 11 Ptychoderidae, 17 Ptyctodontidae, 468 Ptyctodus, 468 Puffers, 726 Punnett, 17 n., 21 n. Putnam, 619 n., 625 Pycnodontidae, 498, 497 Pycnodus, 498 Pycnosterinx, 656 Pygochord, 15 Pyloric caeca, 274, 275 Pyrosoma, 91, 91, 36, 37, 70, 86, 108; structure, 91 f., 92; development and life-history, 93, 94; occurrence, 94; P. aherniosum, 94; P. atlanticum, 94; P. elegans, 94; P. excelsior, 95; P. giganteum, 94; P. minatum, 94; P. spinosum, 94 Pyrosomatidae, 91, 81, 110 Pyrrhulina, 575 Pythonichthys, 605

Quinnat Salmon, 566, 569

Raad, 591 Raconda, 563 Racovitzaia, 706 Räderorgan, 129 Raffaele, 614 n. Rag-Fishes, 644 Raia, 461; dermal spines, 184; pectoral fin, 243; teeth, 251; spiral valve, 265, 266; inter-renal bodies, 347; electric organs, 365; distribution, 461; coloration, 461; sexual dimorphism, 462; R. abyssicola, 462; R. alba, 462; R. batis, 462; R. circularis, 462; R. clavata, 462; R. fullonica, 462; R. macrorhynchus, 462; R. maculata, 267, 462; R. mamillidens, 462; R. microcellata, 462; R. murrayi, 461; R. oxyrhynchus, 462; R. radiata, 462 Raiidae, 461 Ramsay, 635 Raniceps, 648 Ranzania, 727 Rasbora, 582 Rathbunella, 661 Rays = Raia, q.v. Rectal gland, 276 Red-currant Squirter, 74 Red Mullets, 665 Regalecus, 715; R. banksii, prehensile tail, 163; R. glesne, 715 Regan, 542, 595 n., 643 n., 646, 702, 721 Reinhardt, 579, 594 Remora, 691; cephalic sucker, 161; R. brachyptera, 691 Renal organ, of Ascidians, 54; of Fishes—see Kidneys Reproductive organs, of Hemichordata, 16, 25; of Tunicata, 55, 67, 76, 93, 97, 105; of Amphioxus, 129; of Fishes, 402 f., 400, 403 Respiration, mechanism of, 288 Respiratory organs—Elasmobranchii, 276, 278; Cyclostomata, 279, 280, 281; Holocephali, 282, 283; Teleostomi, 282, 283; Dipnoi, 285, 286; mechanism of respiration, 288; larval gills, 289, 290; air-bladder as a respiratory organ, 291; accessory organs of respiration, 292, 293, 294, 295 Retropinna, 566, 565 Retzius, 113 Rhabdocynthia, 75; spicules, 87 Rhabdopleura, 21 f., 5, 26 f.; R. compacta, 23; R. grimaldii, 23; R. manubialis, 23; R. normani, 22, 23, 26 Rhachicentridae, 677, 676 Rhachicentrum, 678 Rhamphichthys, 579 Rhamphocottidae, 697, 694 Rhamphocottus richardsonii, 697 Rhamphognathus, 640 Rhamphosternarchus, 579; R. curvirostris, 580; R. tamandua, 580 Rhamphosus, 633 Rhina, 457; R. squatina, 456, 457 Rhineaster, 589 Rhinelepis, 595 Rhinellus, 611 Rhinichthys, 582 Rhinidae, 456, 458; persistent nephrostomes, 401 Rhinobatidae, 460, 458 Rhinobatus, 460; R. granulatus, 460 Rhinodon, 454; R. typicus, gill-rakers, 287 Rhinodontidae, 454 Rhinodoras, 359 Rhinogobio, 582 Rhinoptera, 465, 466 Rhipidistia, 477 n. {754} Rhizodontidae, 478, 202 Rhizodopsis, 479, 228; R. sauroides, skull, 478 Rhizodus, 478, 202 Rhodeus, 582; R. amarus, 584; oviducal tubes, 408; embryos in gill-cavities of Unio, 416 Rhodichthys, 712 Rhodosoma, 73; R. callense, 72 Rhombatractus, 639 Rhomboidichthys, 687 Rhombosolea, 687 Rhombus, 687, 264; R. aculeatus, 262; R. maximus, 264, 275 —see also Psetta Rhyacichthys, 689 Rhynchobatus, 460 Rhynchobdella, 717 Rhynchodus, 468 Rhypticus, 660 Ribbon-Fish, 715, 152, 160 Ribs, 205 Ridewood, 542 n. Rita, 588; R. crucigera, 303 Ritter, 8 n., 39, 83, 690 n. Rivulus, 616 Rocklings, 649, 288 Rohde, 113 Rohteichthys, 582 Rolph, 113 Rondeletia, 614 Rosette-organ, 97 Roule, 38 Rowntree, 579 n., 621 n. Rudder-Fish, 643 Ruvettus, 679 Ryder, 711 n.

Saccarius, 720 Sacchi, 696 n. Saccobranchus, 588, 590; air-sacs, 295; S. fossilis, 295; S. singio, 295 Saccodon, 576 Saccopharyngidae, 603 Saccopharynx, 604; S. ampullaceus, 604 Sachs, 581 n. Sagemehl, 542, 573, 575, 579 n., 586 n., 649 n. Sagenodus, 507 Sail-Fishes, 679 Salanx, 566 Salarias, 709, 710 Salensky, 39, 411 Salminus, 575; S. orbignianus, 578 Salmo, 565, 566 f., 567, 290; pyloric caeca, 275; spiracles in the embryo, 283; pseudobranch, 284; S. alpinus, 567; S. fario, 567; caudal portion of the vertebral column, 237; pectoral girdle, 240; dissection to show the internal organs, 255; brain, 375; section of the eye, 394; S. fontinalis, 567; S. irideus, 567; S. salar (Salmon), 567, 323; skull, 211 f., 212, 213, 214; kidneys, gonoducts, and abdominal pores in the female, 405, 406; deposition of eggs, 415; length of hatching, 417; S. salvelinus, pectoral fin, 243; S. stomachicus, gizzard, 260; S. trutta, 567 Salmon, 566 f.—see also Salmo salar. Salmonidae, 565, 544, 269; distribution, 566; liver, 272; abdominal pores, 402; oviducts, 405; eggs, 411 Salmopercae, 620 Salpa, 101, 36, 37; arrangement of zooids, 102, 103; structure, 104, 105; alternation of generations, 105; development and life-history, 106; classification, 108; S. democratica-mucronata, young, 107; S. hexagona, 106; S. pinnata, endostyle and stolon, 103; S. runcinata-fusiformis, 102, 108 —see also Cyclosalpa, Iasis, Pegea, and Thalia Salpians, 64 Salpidae, 101, 39, 110 Salvelinus, 567 Sand-Eel, 639, 275 Sander, 659 Sarcodaces, 575 Sarda, 678; S. orientalis, caudal fin of, 675 Sardine, 564 Sardinioides, 611 Sardinius, 611 Sargus, 665 Sars, G. O., 21, 23 Sars, M., 21 Saurenchelys, 601 Saurida, 611 Saurie, 638 Saurocephalus, 561 Saurodon, 561 Saurodontidae, 561, 544 Saurogobio, 582 Sauropsida, 145 Saurus, 611 Saury Pike, 411 Savi's vesicles, 384 Savigny, 36 Saw-Fishes, 457, 459 Scabbard-Fish, 679 Scales, 185 f.; Acipenser ruthenus, 187; Antennarius hispidus, 191; Balistes capriscus, 191; Centriscus scolopax, 190; Hypostoma commersonii, 192; Lepidosteus, 188; L. osseus, 186; Monacanthus scopas, 191; Salmo fario, 190; as an index of age, 191 Scapanorhynchus, 453 Scaphirhynchus, 495; S. platyrhynchus, 495 Scaridae, 674, 651, 654 Scarus, 674; {755} teeth, 251; S. strongylocephalus, pharyngeal bones, 674 Scatophagus, 668 Scaumenacia, 506, 519 Schilbe, 588 Schilbichthys, 588 Schimkewitsch, 21 n. Schizocardium, 17, 5, 6, 13, 14; S. brasiliense, 6, 13, 14 Schizothorax, 582 Schlosser and Ellis, 36 Schmidt, 705 n. Schmidt, C., 37 Schnapper, 665 Schultz, 29, 30 Sciades, 588 Sciaena, 663; S. aquila, 663, 362, 361 Sciaenidae, 663, 653, 361, 363; branching of the air-bladder, 303 Scissor, 575 Sclerocottus, 698 Sclerodermi, 722 f., 721, 357 Sclerognathus, 581 Scleropages, 555; S. leichardti, 556, 558 Scleroparei, 692 f., 651, 652 Sclerorhynchus, 460 Scoliodon, 448 Scolopsis, 664 Scomber, 678, 272; S. brachyurus, 357; S. colias, 302; S. pneumatophorus, 302; S. scombrus, 678, 275, 302; coloration, 168 Scombramphodon, 678 Scombresoces, 636 n. Scombresocidae, 637, 636 Scombresox, 638, 411 Scombridae, 678, 676, 158, 303, 412 Scombriformes, 675 f., 651, 652 Scombrinus, 678 Scombroclupea, 563 Scombrocottus, 697 Scombrocypris, 582 Scombrops, 660 Scopelarchus, 611 Scopelengys, 611 Scopelidae, 611, 606, 302, 396; photophores, 178 Scopelogadus, 656 Scopeloides, 611 Scopelosaurus, 611 Scopelus, 611, 613, 620; S. benoitii, photophores, 178, 179, 181; S. crocodilus, 612 Scorpaena, 695, 693; S. grandicornis, 695 Scorpaenichthys, 698; S. marmoratus, pectoral arch of, 693 Scorpaenidae, 694, 692, 153, 418 Scorpaenoides, 695 Scorpididae, 666, 651, 652, 653 Scorpis, 666 Scyllaemus, 565 Scylliidae, 446, 401, 402; fossil, 447 Scylliogaleus, 449 Scylliorhinus—see Scyllium profundorum Scyllium, 446, 198, 271, 446; dermal denticles, 184; teeth in embryo, 249; internal organs, 253; spleen, 343; fossil, 447; S. canescens, 447; S. canicula, 446; vertebral column, 193, 194, 195; skull, 207 f., 208; spiral valve, 267; liver, 273; pancreas, 273; lateral veins, 318 n.; brain, 373; abdominal pores, 401; oviparous, 433; attachment of eggs, 434; supra- and inter-renal bodies, 346, 433; S. catulus, 446; S. (Scylliorhinus) profundorum, 447 Scymnus, 455, 456; vertebral column, 198 Scyphophori, 543 Scytalina, 712 Sea-Breams, 664 Sea-Horse, 634, 154, 163, 361, 362 Sea-Snails, 699, 411 Sebastes, 695, 692, 693; S. norvegicus, 419; S. percoides, pectoral arch, 693 Sectator, 657 Seeliger, 39 Selache (Cetorhinus), 453, 264; fossil, 454; S. aurata, gill-rakers, 287; S. maxima, 453; gill-rakers as a plankton-filter, 287; distribution and food, 453, 454 Selachii, 442, 148 Selachophidium, 712 Selene, 677 Selenichthyes, 627, 629 Semionotidae, 497 Semionotus, 498 Semiophorus, 677 Semiplotus, 582 Semon, 510, 593 Semper, 625 Sense-organs, of Ascidians, 53; of Amphioxus, 128; of Fishes, 383 Seriola, 677 Seriolella, 643 Seriolichthys, 677 Serranidae, 659, 651, 652, 653, 361, 389 Serraninae, 659 Serranus, 659, 660; hermaphroditism, 420; S. cabrilla, 660, 420; S. hepatus, 660, 420; S. scriba, 660, 420 Serrasalmo, 576; S. niger, 578; S. rhombeus, mouth, 577 Serrasalmoninae, 576 Serrivomer, 603 Setarches, 695 Sexual dimorphism, 419 f., 432, 462, 469, 471, 483, 500 Shad, 564 Shape of the body, in Fishes, 152 Sheep's-Head, 665 Shipley, 20 n. Sicyases, 709; S. sanguineus, 708; sucker, 162 Sicydium, 689 {756} Siel-Smelt, 569 Sik, 568 Sillaginidae, 662, 653 Sillago, 662 Silondia, 588 Siluranodon, 588 Silurichthys, 588 Siluridae, 586, 575, 163, 251, 275, 283; barbels, 154; intestinal respiration, 292; accessory respiratory organs, 293, 294; air-bladder, 302, 303, 305; sound-production, 356-361, 363; connexion of the air-bladder and auditory organ, 389, 390; nests, 416; protection of young, 593 Silurinae, 588 Silurodon, 588 Silurus, 588, 342; S. glanis, 592, 593, 358 Silvestri, 594 Simenchelys, 601; S. parasiticus, 603 Simple Ascidians, 35, 36 f., 39 f., 70, 71, 110 Siniperca, 659 Siphons, 43, 53 Siphonognathus, 674 Siphonostoma, 634; S. typhle, protective coloration, 172 Sirenoidei, 149 Sisor, 588, 589 Skates = Raia, q.v. Skeleton, of Enteropneusta, 14; of Amphioxus, 119; of Fishes, 193 f. Skin, of Cyclostomata, 182; of Fishes, 182 Skull, composition of, 206; development, 206; Scyllium canicula, 207, 208; Salmo salar, 211, 212, 213, 214; Cyclostomata, 216 f.; Petromyzon marinus, 216 f., 217; Bdellostoma, 220 f., 220, 221; Elasmobranchii, 222 f.; Notidanus cinereus, 222; Chimaera monstrosa, 223; Chondrostei, 224 f.; Sturgeon, 224; Polyodon, 225; Crossopterygii, 226 f.; Polypterus, 227; Holostei, 228 f.; Teleostei, 229 f.; Dipnoi, 231 f.; Protopterus, 231 f., 232; Lepidosiren, 231 f., 233 Sluiter, 38 Smelt, 568, 411 Smitt, 569 n., 714 n. Social Ascidians, 71 Sole, 687 Solea, 687, 685; S. vulgaris, 686, 687 Solenocytes, 29, 126, 127 Solenognathus, 634 Solenorhynchus, 634 Solenostomidae, 633, 626, 627, 628; marsupial pouches, 416 Solenostomus, 633 Soleotalpa, 687 Sollas, I., 522 Sollas, W. J., 522 Sörensen, 355, 590 n. Sorubim, 588, 361 Sosia, 588 Sound-producing organs, 355 f. Spalt-papillen, 384 Spaniodon, 563, 564 n. Sparidae, 664, 654, 303, 389; teeth, 251 Sparisoma, 674 Sparnodus, 665 Sparus, 665; S. unicolor, 665 Spathiurus, 545 Spathodactylus, 561 Spengel, 5, 7, 8, 13 n., 16, 17, 18 n., 19 Spengelia, 17, 5, 6 Spermatozoa, 413 Sphaerodon, 665 Sphenacanthus, 445 Sphenocephalus, 656 Sphyraena, 642 Sphyraenidae, 642, 637 Sphyraenodus, 678 Sphyrna, 449, 153; liver, 272; fossil, 450; distribution, 449; S. malleus, 266; spiral valve, 266, 267; S. tiburo, 450; S. tudes, 449; S. zygaena, 450 Sphyrnidae, 449 Spinacanthus, 723 Spinachia, 630; S. vulgaris, 631 Spinacidae, 454; photophores, 179; fossil, 456 Spinal cord, 367 Spinal nerves, 378 Spinax, 455 Spinivomer, 603 Spiny Dog-Fishes, 455 Spiracle, 279, 283 Spiracular pseudobranch, 279, 284; in Teleostomi, 284; blood supply, 335 Spiral valve, 264 f. Spix, 558 n. Spleen, 342 Spoon-Bill, 491; = Polyodon, q.v. Sprat, 564 Squaliobarbus, 582 Squaloraia, 468 Squaloraiidae, 468 Star-gazers, 155 Starks, 627 n. Steatogenys, 579 Steenstrup, 36, 685 n. Stegophilus, 589; S. insidiosus, 594 Stegostoma, 446; S. tigrinum, 446 Steindachneria, 647 Steinegeria, 682 Stenodus, 565 Stephanoberycidae, 619, 606 Stephanoberyx, 619; S. gillii, 620; S. monae, 620 Stereobalanus, 17, 5; S. canadensis, 16, 17 Stereolepis, 659, 660 Sterlet, 493 Sternarchus, 579; {757} S. albifrons, 580; S. macrostoma, 580 Sternoptychinae, 571 Sternoptyx, 571; S. diaphana, 571, 178 Sternopygus, 579 Stichonodon, 575 Sticklebacks, 630, 163, 169, 288, 354, 357 Stigmata, 47; of Appendicularians, 67; of Cyclomyaria, 95 Stigmatophorus, 634, 635 Stolon, 71, 93, 95, 97, 103 Stolonial budding, 81 Stomatorhinus microps, 551 Stomias, 571, 572 Stomiatidae, 570, 545; photophores, 178 Stomiatinae, 571 Stomodaeum, 59, 120, 136 Storms, 691 n. Stratodus, 623 Strepsodus, 479 Stridulation, 356 Stromateidae, 643, 637 Stromateoides, 643 Stromateus, 643 Sturgeon, 154, 334, 493; = Acipenser, q.v. Styela, 74, 76; branchial sac, 74; S. bythia, 76; S. clava, 76; S. etheridgii, dorsal tubercle, 79; S. squamosa, 76; S. whiteleggii, dorsal tubercle, 79 Styelinae, 74; tentacles, 75 Styelopsis, 74; branchial sac, 74; S. grossularia, 74, 76 Stygicola, 713 Stygogenes, 595 Stylophorus, 716 Sucker-Fish (Sucking-Fish), 691, 161 Sudis, 611; S. gigas, air-bladder as a respiratory organ, 291 Sun-Fish, 657 (Lepomis); 727 (Orthagoriscus)—see also Orthagoriscus Supra-pericardial organs, 344 Supra-renal bodies, 346, 346 Surf-Fishes, 670 Swinnerton, 573, 627, 636 n. Sword-Fish, 681, 154 Symbranchidae, 597, 283, 294 Symbranchii, 597 f. Symbranchus, 597, 598, 156 Symphurus, 687 Symphysodon, 672 Synagris, 664 Synagrops, 659 Synancia, 695; S. verrucosa, poison-glands, 177 Synaphobranchidae, 603 Synaphobranchus, 603 Synapticula, 13, 12, 122 Synaptura, 687 Synascidiae, 80, 35 Synchirus, 698 Synechodus, 445 Syngnathidae, 634, 627, 628, 163, 205, 251, 354, 361, 416; colours, 166; protective coloration, 172 Syngnathus, 634, 627, 635; mouth, 154; S. acus, 262, 271; S. intestinalis, 635; S. pelagicus, 634 Synodontis, 588, 590, 359; S. batensoda, 591; S. decorus, 591; S. membranaceus, 591 Syntegmodus, 549

Tadpole, Ascidian, 59, 62 Taeniolabrus, 706 Taeniosomi, 714 f., 651, 652 Tamiobatidae, 462 Tamiobatis vetustus, 462 Taractes, 682 Tarpon, 547 Tarrasiidae, 477 n. Tarrasius problematicus, 477 n. Tattersall, 137 n. Tauredophidium, 712, 713 Tauroglossus, 17 n. Tautoga, 673 Taylor, 599 n. Tectospondylic, 198 Teleostei, 504, 541 f.; classification, 542, 543, 149 Teleostomi, 475 f., 149 Telepholis, 615 Telescope Fish, 585 Telescops, 659 Tellia, 616 Telmatherina, 639 Telolecithal, 409, 410 Tench, 583, 171, 260, 320, 321 Tentacles, of Ascidians, 44; of Amphioxus, 116 Test, 40, 63; of Appendicularians, 65 Testa-cells, 56 Tethyum, 36 Tetragonopterus, 575 Tetragonuridae, 642, 637 Tetragonurus cuvieri, 642 Tetranematichthys, 589 Tetrapturus, 680; T. belone, 680 Tetraroge, 695 Tetrodon, 726, 152, 354, 361; warning colours, 163; scales, 191; teeth, 251; T. honckenii, 362; T. mbu, pectoral arch of, 722 Tetrodontidae, 726, 722 Tetronarce, 464 Teuthis, 669 Teuthididae, 668, 654 Thalassophryne, 711; T. reticulata, poison glands, 177 Thalassothia, 711 Thaleichthys, 566 Thalia, 108; T. democratica-mucronata, 108 Thaliacea, 95 f., 38, 64, 81, 110 Thaumatostomias, 571 {758} Thaumaturus, 565 Thelodus, 524; T. pagei, 524; T. tulensis, 525 Therapon, 660, 361 Thilo, 682 n. Thoracopterus, 545 Thoracostei, 627, 629 Thread-cells, in Myxinoids, 182 Threpterius, 664 Thresher Shark, 451, 452 Thrissopater, 562 Thrissopatrinae, 562 Thrissops, 546 Thryptodus, 549 Thunnus, 678, 275; T. thynnus, 678 Thursius, 477 Thylacium, 89 Thymallus, 565; T. vexillifer = vulgaris, 568 Thymus, 344, 345 Thyrina, 639 Thyroid gland, 46, 343, 344 Thyrsites, 679 Thyrsitocephalus, 679 Thyrsoidea, 605; T. macrura, 605; T. meleagris, skull, 604 Tilapia, 671; T. dardennii, 671; T. dolloi, 152 Tile-Fish, 661 Tinca, 582; T. vulgaris, 583, 171, 260, 320; renal portal system, 320, 321 Titanichthys, 537 Todaro, 39 Tongue, 252 Tongue-bars, 12, 12, 13, 120, 135 Tope, 449 Tornaria, 18, 18 f., 28, 31; T. agassizii, 21; T. grenacheri, 19; T. krohni, 19 Torpedinidae, 462 Torpedo, 463; electric organs, 365, 366; T. hebetans, 463; T. marmorata, 463; T. narce, 463; T. ocellata, 463 Toxotes, 658; T. jaculator, 658 Toxotidae, 658, 653 Trachelochismus, 709 Trachelyopterus, 589, 361 Trachichthys, 656, 655 Trachinidae, 704, 703 Trachinops, 659 Trachinus, 704; warning colours, 174; poison-glands and spines, 176; T. draco, 705, 176; pectoral arch of, 703; T. vipera, 705, 176 Trachyglanis, 588 Trachynotus, 677 Trachypoma, 659 Trachypteridae, 715, 152 Trachypterus, 715; T. arcticus, 715; T. iris, 715; T. taenia, 160 Trachyrhynchus, 647 Traquair, 436, 523, 530, 537, 687 n. Traustedt, 38, 101 n. Tremataspidae, 530 Tremataspis, 530 Trematomus, 706 Tremblador, 580 Triacanthidae, 722, 721 Triacanthodes, 722 Triacanthus, 722; scales, 190; sound-production, 357; T. biaculeatus, 361; T. brevirostris, 361; pectoral arch of, 722 Trichiurichthys, 679 Trichiuridae, 679, 676 Trichiurus, 679 Trichodon, 663 Trichodontidae, 663, 654, 704 Trichogaster, 669, 293 Trichomycterinae, 589 Trichomycterus, 589 Trichonotidae, 706 Trichonotus, 706 Trigger-Fishes, 724, 174, 357 Trigla (Gurnards), 701, 693; liver, 272; air-bladder, 305; sound-production, 361, 363; T. cuculus, 701, 363; T. gurnardus, 701, 363; rectal valve, 254; T. hirundo, 701, 363; T. lineata, 701; T. lyra, 701, 363; T. obscura, 701 Triglidae, 700, 694; coloration, 164; sounds, 361, 363 Triglops, 698 Triglopsis, 697 Trigonodon, 665 Triodon, 723, 721; T. bursarius, 723 Triodontidae, 723, 721 Triplophos, 571 Tripterophycis, 648 Trissolepis, 487 Tristichopterus, 478; T. alatus, 479 Tristychius, 445 Troglichthys rosae, 619 Tropheus, 671 Trophozooid, 98, 100 Tropidichthys, 726 Troschel, 682 n. Trout, 566 f., 567; Brook-, 567; Brown, 567; Gillaroo, 260; Sea-, 567 Trumpet-Fish, 154 Trunk-Fishes, 724 Trygon, 464; spines, 177; uterine nutrition, 434; T. pastinaca, 464; T. sabina, 464 Trygonidae, 464, 465 Trygonorhina, 460, 461 Trypauchen, 689 Trypauchenichthys, 689 Tunic, 63 Tunicata, 63, 35, 36 f., 39, 4, 30 n.; affinities, 62; external form, 64 Tunicine, 37 Tunny, 678, 678 Turbot, 687 Turner, 593 n. Twait Shad, 564 {759} Typhlichthys, 618; T. rosae, 619; T. subterraneus, 619 Typhlogobius, 689, 394; T. californiensis, 690 Typhlonus, 712, 713; T. nasus, 712 Typhlosole, 54

Uljanin, 39 Ulocentra, 659 Umbra, 609; U. crameri, 610; U. limi, 610 Umbrina, 663; U. cirrhosa, 361 Undina, 481, 161; U. gulo, 480 Unio, Rhodeus in, 584 Upeneichthys, 665 Upeneoides, 665 Upeneus, 665 Uranoscopidae, 706, 703 Uranoscopus, 706; eyes, 155 Urenchelys, 601 Urocampus, 634 Urochord, 66 Urochordata, 4, 63, 38 Uroconger, 601 Urogymnus, 464 Urolepis, 487 Urolophus, 464 Uronemidae, 507 Uronemus, 507, 519 Urosphen, 632 Ussoff, 38

Vaillant, 596 n., 621 n., 720 n. Vallentin, R., 608 Vandellia, 589; V. cirrhosa, entering urethra, 593 Vascular system, 313 f.; general, 313; Cyclostomata, 315, 327, 329, 330; Elasmobranchii, 316, 318, 327, 328, 330, 331, 333, 334; Teleostomi, 319, 320, 321, 322, 328, 333, 335, 336, 337; Dipnoi, 323, 324, 326, 329, 338, 340; blood, 341; blood-glands, 342 Velum, 120 Vendace, 568 Venous system, 315 f. Vermiform process, of Enteropneusta, 14, 17 Vertebral column, 193 f.; Scyllium canicula, 193, 194, 195; Cyclostomata, 197; Elasmobranchs, 197, 198; Chimaera monstrosa, 199; Dipnoi, 199; Chondrostei, 200; Acipenser sturio, 200; Crossopterygii, Holostei, and Teleostei, 201 f.; Amia, 201, 203; Lepidosteus, 201, 204; Caturus furcatus, 203; Eurycormus speciosus, 203; Gadus morrhua, 205 Vertebrates, characters of, 3 f. Vessels, of Ascidians, 41—see also Vascular system Vidalia, 546 Vipan, 592 Vireosa, 689 Visceral arches, 207 Viviparous Fishes, 418 f., 433, 435 Vogt, 37 Vomeropsis, 677 Vulsus, 707

Wagner, 595 n. Waite, 643 Walking-Fish, 690 Wallago, 588 Warington, 630 n. Watase, 585 n. Weber, E. H., 573 Weber, Max, 661 Weberian ossicles, 389, 573 f. Weevers, 704, 174 Weiss, 568 (= Coregonus) Weiss, F. E., 126 Wels, 593 Wertheimeria, 588 Wetterfisch, 585 Whiff, 687 Whip-tailed Rays, 464 Whitebait, 564, 608 White-Fish, 568, 569 Whiting, 649, 168, 275 Whymper, 595 n. Willey, 5, 7, 9, 11, 13 n., 15 n., 16, 19 n., 58 n., 113, 633 Willeyia, 17 n. Williamson, 602 n. Wilson, 113 Witch, 687 Wodnika, 445 Wolf-Fishes, 710, 251, 408 Woodward, A. Smith, 458, 477 n., 480 n., 497, 541 n., 542, 543, 622, 624, 626, 680 Wrasses, 673, 164, 166 Wright, Ramsay, 587 Wyman, 593 n., 596 n.

Xanthochroism, 171 Xenichthys, 659 Xenocharax, 576, 578 Xenocypris, 582 Xenocys, 659 Xenodermichthys, 570 Xenomystus, 555 Xenopholis, 498 Xenopteri, 708 Xenopterus, 726 Xenotilapia, 671 Xiphasia, 709 Xiphias, 681; beak, 154; X. gladius, 681 Xiphiidae, 681, 676 Xiphiorhynchus, 680 Xiphistes, 709 Xiphostoma, 576 {760} Xiphostominae, 576

Yarrell, 112

Zalocys, 677 Zanclus, 668 Zaniolepis, 696 Zapteryx, 460 Zeidae, 683, 361 Zenion, 683 Zeorhombi, 682 f., 651, 652 Zeugopterus, 687 Zeus, 683; Z. faber, 683, 361, 363; red gland, 308 Zoarces, 712, 713; Z. viviparus, 419 Zoarcidae, 712, 704, 361, 395 Zograf, 697 n. Zygaena, 449

END OF VOL. VII

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THE CAMBRIDGE NATURAL HISTORY

Edited by S. F. HARMER, Sc.D., F.R.S., Fellow of King's College, Cambridge, Superintendent of the University Museum of Zoology; and A. E. SHIPLEY, M.A., F.R.S., Fellow of Christ's College, Cambridge, University Lecturer on the Morphology of Invertebrates.

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