CHAPTER IV. Crustacea (_continued_): Cirripedia—Phenomena of Growth and Sex—

CRUSTACEA (CONTINUED): CIRRIPEDIA—PHENOMENA OF GROWTH AND SEX— OSTRACODA

=Order III. Cirripedia.=

The Cirripedes are medium-sized Crustacea, with the body consisting of few segments, and enveloped in a mantle formed as a fold of the external integument, which may be strongly protected by calcified plates. The abdomen is greatly reduced. The larva, after hatching out as a Nauplius, and passing through a Cypris stage, when it resembles an Ostracod, fixes itself to a foreign object by means of the first antennae, and becomes a pupa, which after profound changes gives rise to the adult.

All the Cirripedes, when adult, live either a fixed or parasitic existence, and as so frequently happens with animals of this kind, they have departed widely from the ordinary structure of the class to which they belong. Their anomalous appearance and the mystery surrounding their propagation gave rise, probably, to the old legend that the Barnacles (Lepadidae), which live attached to pieces of floating timber, hatched out into Barnacle geese; and even so late as 1678, in the Royal Society’s Transactions, Sir Robert Moray describes what he takes to be little birds enclosed in Barnacle shells, washed ashore on the coast of Scotland: “The little Bill like that of a Goose, the Eyes marked, the Head, Neck, Breast, Wings, Tail, and Feet formed, the Feathers everywhere perfectly shaped and blackish coloured, and the Feet like those of other Water-fowl, to my best remembrance.” Cuvier in his classification of the animal kingdom included them in the Mollusca; and it was not until 1830 that J. V. Thompson described their larval stages, and showed conclusively that they belonged to the Crustacea. Since the work of this naturalist a number of observers have securely founded our knowledge of the group, but we may especially mention the epoch-making works of Darwin, Hoek, and latterly of Gruvel.

The young Cirripede is hatched out from the maternal mantle-cavity as a free-swimming Nauplius, a larval form common to most of the Entomostraca and to some Malacostraca; the Cirripede Nauplius (Fig. 49) is characterised by the presence of well-developed frontal horns, and usually by the long spiny processes which spring from various parts of the body. As an introduction to the study of the group, it will be well to follow the transformations of this larva in Lepas up to the period when it begins its sessile existence. The liberated Nauplii swim freely near the surface of the sea, and remaining in this condition for several days are dispersed widely from their birthplace; they are then transformed by the process of moulting into the second larval stage, known as the Cypris (Fig. 50), from its resemblance to a bivalve Ostracod. The Cypris larva continues to swim about by means of the six pairs of biramous thoracic legs until it finds a suitable place on which to fix; in the case of Lepas fixation usually takes place on loose floating logs; the Cypris fixes itself by means of the first antennae, at the bases of which a large cement-gland secretes an adhesive substance. The biramous swimming legs are cast off, and six pairs of biramous cirri characteristic of the adult take their place; at this stage the body has the appearance shown in Fig. 51. The region of the head at the base of the antennae now becomes greatly swollen and elongated to form the peduncle or stalk of the adult; the larval bivalve carapace is cast off and on the external surface of the mantle the calcifications begin which will give rise to the exoskeletal plates of the adult. This region is known as the “capitulum” of the adult, as opposed to the “peduncle.” The young Cirripede is now known as a pupa, and from this stage the adult form is reached by a gradual transition.

FIG. 49.—Nauplius larva of Lepas fascicularis, × 12. A{1}, A{2}, 1st and 2nd antennae; B, brain; E, eye; H, fronto-lateral horn; M, mandible; S, stomach. (After Groom.) ]

FIG. 50.—Cypris stage in the development of Lepas australis, × 15. A, Peduncle; A.M, adductor muscle; C, caecum of oesophagus; C.g, cement-glands; Cr, cirri (thoracic appendages); E, compound eye; E^1, simple eye; G, ventral ganglia; I, intestine; M, mouth; M.C, mantle-cavity; O, ovary; S, stomach. (After Hoek.) ]

FIG. 51.—Pupa of Lepas pectinata, × 8. A, Antenna; C, carina; M, adductor muscle; S, scutum; T, tergum. (After Gruvel.) ]

The body of the adult Lepas is retracted into the mantle, and lies free in the mantle-cavity, but is continuous anteriorly with the tissues of the peduncle, into which the mantle does not extend. The thorax, with its six pairs of legs, can be protruded from the mantle-cavity through the slit-like opening which separates the two valves of the mantle along the ventral middle line; and when the animal is feeding, the thoracic legs are so protruded, and by their concerted waving action they drive the food-particles in the water along the channel between them, until the particles reach the oral cone, where they are masticated by the mandibles and two pairs of maxillae, and so passed into the alimentary canal. When the animal is disturbed it rapidly retracts its limbs, the valves of the mantle are closed by means of a strong adductor muscle in the head, and the animal is protected from all external influences. In the acorn-barnacles (Operculata), which live in great numbers attached to rocks and other objects between tide-marks, the body is constructed on a similar plan, save that there is no stalk, and the body is completely enclosed in a hard calcareous box formed from the mantle, which, when the valves are closed, as they always are during low tide, completely protect the animal inside from desiccation or danger of any kind. Besides the cement-glands situated in the peduncle, we can distinguish the generative organs, consisting of a pair of ovaries and testes, the majority of Cirripedes being hermaphrodite. The testes open at the end of an elongated median penis behind the thoracic limbs, while the ovaries, situated in the peduncle, have paired openings into the mantle-cavity on either side of the head. A pair of maxillary glands or kidneys are present, and the alimentary canal is provided with various digestive glands. Special branchial organs are not present in the Pedunculate Cirripedes, but in the Operculate genera two branchiae are formed from the plications of the internal surface of the mantle. There is no contractile heart, and the circulatory system is poorly developed. The Cirripedes are badly furnished with sensory organs; the remains of a simple Nauplius eye may persist, situated on the upper part of the stomach, but the chief sense-organs are the sensory hairs upon the limbs.

FIG. 52.—=A=, Dwarf male of Scalpellum vulgare, × 27; =B=, diagram of Stalked Barnacle. a, Peduncle; al, alimentary canal; b, brain; c, carina; e, remains of Nauplius eye; gl, cement-gland; m, mantle-cavity; o, its opening; ov, ovary; p, penis; s, scutum; t, testis; tm, tergum, seen in =A= as the shaded body above the reference-line of e and to the right of the carina, on the left of the figure. ]

The recent Cirripedes fall into six clearly defined Sub-orders.

=Sub-Order 1. Pedunculata.=

In this division, sometimes combined with the Operculata as THORACICA, owing to the extremely reduced state of the abdomen, the body is borne on a distinct stalk, and the bivalve arrangement of the mantle is clearly retained. The mantle is protected externally by a number of calcareous plates, the arrangement of which is typical of the various genera. It appears that in the most primitive and geologically oldest Cirripedes, the probable ancestors of the Pedunculate and Operculate sub-orders, the arrangement of the plates was somewhat irregular, and they were far more numerous than in the modern forms, so that passing from these older types to modern times we witness a reduction in the number and a greater precision in the arrangement of the skeletal parts.

FIG. 53.—=A=, Turrilepas wrightianus (Silurian), × 1; =B=, Archaeolepas redtenbacheri (Jurassic), × 1. C, carina; R, rostrum; S, scutum; T, tergum. (After Zittel.) ]

One of the most ancient Cirripedes known is Turrilepas, which occurs in the Silurian deposits of England, but it is also known from earlier deposits, while undoubted Cirripedes have been found in the Cambrian of North America. The body of Turrilepas is enclosed in imbricating plates, as shown in Fig. 53, A.

In Archaeolepas of the Upper Jurassic (Lithographic slates of Bavaria) the arrangement of scutes typical of the Lepadidae is foreshadowed, but the whole of the peduncle is protected by rows of plates (Fig. 53, B), as in Turrilepas.

The above-mentioned genera did not survive into the Cretaceous period, their places being taken by the genera Pollicipes and Scalpellum, which first appeared in the Silurian and persist to the present time, the older and more primitive Pollicipes being represented by about half a dozen living species, while the species of Scalpellum are exceedingly numerous.

=Fam. 1. Polyaspidae.=—This family includes the three genera, Pollicipes, Scalpellum, and Lithotrya.

FIG. 54.—Pollicipes mitella, × 1. (After Darwin.) ]

Pollicipes is not only very ancient geologically (being found from the Ordovician upward), but it preserves the primitive characteristic of numerous skeletal plates, the peduncle being frequently covered with small calcareous pieces, which graduate into the larger more regularly placed scutes on the capitulum (Fig. 54). The species of this genus, many of which are among the largest Cirripedes, are widely distributed in the temperate and tropical seas, living for the most part attached to rocks and often in deep water. P. cornucopia occurs off the English and Scottish coasts.

The members of the genus Scalpellum, which is represented by exceedingly numerous species in the Cretaceous period, also possess a large number of plates on the capitulum, and often on the peduncle as well, but never so many as in Pollicipes. Although the arrangement of the plates varies much in the different species, we may describe a fairly typical case, that of the common Scalpellum vulgare (Fig. 55, B).

The valves of the capitulum are held together by the median dorsal piece called the “carina”; the other unpaired skeletal piece is the “rostrum,” in front, just below the place where the valves gape to allow the protrusion of the limbs. The paired pieces receive the names “scutum,” “tergum,” and “laterals,” and the peduncle is covered with rows of small plates.

The genus Scalpellum is a very large one, and is widely distributed, though at the time at which Darwin wrote only six species were known. The reason for this is to be found in the fact that the great majority of the species live at great depths, so that they remained unknown until the expeditions of the Challenger and other deep-sea expeditions brought them to light. They may affix themselves, generally in considerable numbers together, on branching organisms, such as Corals, Polyzoa, and Hydroids, but often also on empty shells, rocks, and other foreign bodies. The body is colourless or of a pale flesh colour, but a colony of these animals, expanded and drooping in various attitudes from a piece of coral, gives the appearance of some graceful exotic flower.

FIG. 55.—=A=, Complemental male of Scalpellum peronii, × 20; =B=, hermaphrodite individual of S. vulgare, × 2. a, Complemental males, in situ; b, rostrum. (=A=, after Gruvel; =B=, after Darwin.) ]

Perhaps the most interesting feature of the genus is the remarkable variation in the sexual constitution of some of the species. The great majority of the Pedunculata and all the Operculata are hermaphrodites, which habitually cross-fertilise one another, and this they are well fitted to do, since they all live gregariously and are provided with a long exsertile penis for transferring the spermatozoa from one to the other. In Pollicipes, however, the individuals of which often live solitarily, it appears that self-fertilisation may occur. In Scalpellum three different kinds of sexual constitution may occur: (1) According to Hoek in S. balanoides, taken by the Challenger, the individuals are ordinary cross-fertilising hermaphrodites. (2) In the great majority of species, including the common S. vulgare, as originally described by Darwin, and since confirmed by Hoek and Gruvel, the individuals are hermaphrodite, but there are present affixed to the adult hermaphrodites, just inside the opening of the valves in a pocket of the mantle, a varying number of exceedingly minute males, called by Darwin “complemental males.” These tiny organisms are really little more than bags of spermatozoa, but they possess to varying degrees the ordinary organs of the adult in a reduced condition. The male of S. peronii (Fig. 55, A) retains the shape and skeletal plates of the ordinary form, and differs chiefly in its reduced size; but the more common condition is exhibited by the male of S. vulgare (Fig. 52, A), where the scutes are reduced to vestiges round the mantle-opening, and almost the whole of the body is occupied by the greatly developed generative organs. (3) In a few species, e.g. S. velutinum and S. ornatum, the individuals are purely dioecious, being either females of the ordinary structure resembling the hermaphrodites of the other Lepadidae, or dwarfed males resembling closely the complemental males described above for S. vulgare.

FIG. 56.—Lithotrya dorsalis, x 1. B, Basal calcareous cup; C, carina; R, rostrum; S, scutum; T, tergum. (After Darwin.) ]

The nature and derivation of these various conditions will be discussed when the parallel cases found in Ibla and among the Rhizocephala have been described.

The remaining genus of the Polyaspidae, also characterised by the presence of numerous skeletal plates on the capitulum, is Lithotrya (Fig. 56), which bores into rocks and shells, and is an inhabitant of the warm and tropical seas.

The peduncle of the full-grown animal is completely imbedded in the rock or shell to which it is attached, and at the basal end of the peduncle is situated a cup composed of large irregular calcified pieces. This cup is, however, not formed until the animal has ceased to burrow. The excavation of the substratum is effected by means of a number of small rasping plates which cover the peduncle, the whole being set in motion by the peduncular muscles.

FIG. 57.—Conchoderma virgata, × 1. C, Carina; S, scutum; T, tergum. (After Darwin.) ]

=Fam. 2. Pentaspidae.=—In this family are placed a number of genera, and among them the common Lepas, the species of which possess typically five skeletal plates, viz., a carina and a pair of scuta and of terga, the peduncle being naked. These forms are a later development of Cirripede evolution, and did not come into existence till Tertiary times. Some of them, e.g. Oxynaspis, live at considerable depths attached to corals, etc., but large numbers float on the surface of the sea, fixed often on logs and wreckage of various kinds. Dichelaspis is found attached to the shells of large Crustacea.

Conchoderma is an interesting genus, the species of which live affixed to various floating objects, the keels of ships, etc.; the mantle is often brilliantly coloured, as in C. virgata, and the skeletal plates are reduced to the merest vestiges, leaving the greater part of the body fleshy.

FIG. 58.—Ibla cumingii, ♀, × 1. S, Scutum; T, tergum. (After Darwin.) ]

FIG. 59.—Ibla cumingii, dwarf male, × 32. A, Antennae; B, part of male imbedded in the female, to which the torn membrane M belongs; E, eye; Th, thoracic appendages or cirri. (After Darwin.) ]

=Fam. 3. Tetraspidae.=—This family includes the single genus Ibla (Fig. 58), which possesses only four skeletal plates, a pair of terga and of scuta, coloured blue, while the peduncle is covered with brown spines. There are only two very similar species known, I. cumingii, which is found attached to the peduncle of Pollicipes mitella, and I. quadrivalvis, living on masses of the Siphonophore Galeolaria decumbens. These two species are quite different in the partition of the sexes. In I. cumingii the large individuals of normal structure are females, inside the mantle-cavities of which are attached dwarf males of the form shown in Fig. 59.

These organisms have the peduncle buried completely in the substance of the female’s mantle, inside which they live; they exhibit a degenerate structure, but still retain two pairs of cirri. The large individuals of I. quadrivalvis, on the other hand, are hermaphrodites, but they harbour within their mantles minute complemental males similar to those of I. cumingii, though they are rather larger.

FIG. 60.—Diagram of the shell of an Operculate Cirripede. a “Ala,” or overlapped portion of a “compartment”; B, basis; C, carina; C.L, carino-lateral; L, lateral; R, rostrum; r, “radius,” or overlapping portion of a compartment; R.L, rostro-lateral. (After Darwin.) ]

=Fam. 4. Anaspidae.=—This includes the remaining pedunculate genera, characterised by the fleshy nature of the mantle and peduncle, which are both entirely devoid of calcifications. The species of Alepas live upon Echinoderms and various other animals; Chaetolepas upon Sertularia, and Gymnolepas upon Medusae. Anelasma squalicola is an interesting form, living parasitically upon the Elasmobranch fishes, Selache maxima and Spinax niger in the North Sea. The peduncle is deeply buried in the flesh of the host, so that only a portion of the dark blue capitulum protrudes to the surface. From the whole surface of the peduncle a system of branching processes is given off, which ramify far into the tissues of the fish, and communicate inside the peduncle with the lacunar tissue, which is packed round all the organs of the Cirripede. There can be small doubt that the Anelasma derives its nutriment parasitically through this root-system, since the cirri are mere fleshy lobes unadapted to securing food, and the alimentary canal is always empty. This animal has a suggestive bearing on the Rhizocephala, which, as will be shown, derive their nutriment from a system of roots penetrating the host and growing out from what corresponds morphologically to the peduncle.

=Sub-Order 2. Operculata.=

FIG. 61.—Balanus tintinnabulum, with the right half of the shell and of the operculum removed, seen from the right side. A, Antennae, the size of which is exaggerated; A.M, adductor muscle; B, basis; C, carina; Cr, cirri or thoracic appendages; D, oviduct; G, ovary; L, lateral compartment; Lb, labrum or upper lip; M, M, depressor muscles of scutum and tergum; M.C, mantle-cavity; O, orifice of excretory organ; O.M, opercular membrane; R, rostrum; S, scutum; St, region of stomach; T, tergum. (After Darwin.) ]

The “acorn-barnacles” appear later in geological time than the earlier stalked forms. Verruca and Chthamalus are found in the Chalk, and survive down to the present day, but Balanus does not occur until middle Tertiary times. Representatives of the last-named genus are familiar to every one, as the hard sharp objects which cover rocks and piles near high-water mark on every sea-coast. If we examine the hard skeleton of one of these animals, we find that, unlike the Pedunculata, they possess no stalk, the capitulum being fused on to the surface of attachment by a broad basal disc. Typically, there may be considered to be eight skeletal pieces forming the outer ring which invests the soft parts of the animal, an unpaired rostrum and carina, and laterally a pair of rostro-lateral, lateral, and carino-lateral “compartments,” as shown in Figs. 60, 63.

The skeletal ring is roofed over by a pair of terga at the carinal end and a pair of scuta at the rostral end; these four plates make up the operculum by which the animal can shut itself completely up in its shell, or between the valves of which it can protrude its limbs for obtaining food.

FIG. 62.—Diagrammatic section of the growing shell of Balanus porcatus. C, Canals; Ct, cuticle; H, hypodermis (= epidermis); H′, part of shell secreted by the hypodermis; Hl, hypodermal lamina; M, part of shell secreted by the mantle. (After Gruvel.) ]

The relation of the animal to its shell is shown in Fig. 61. The shell in the Operculata is not merely secreted as a dead structure on the external surface of the epidermis, but represents a living calciferous tissue interpenetrated by living laminae (Fig. 62, Hl) derived partly from the external hypodermis and partly from the lining of the mantle. The hard parts of the shell usually also contain spaces and canals (C).

FIG. 63.—Diagrams of shells of Operculata. =A=, Catophragmus (Octomeridae); =B=, Balanus, Coronula, etc. (Hexameridae); =C=, Tetraclita (Tetrameridae). C, carina; C.L, carino-lateral; L, lateral; R, rostrum; R.L, rostro-lateral. ]

The various forms of Acorn-barnacle may be classified according to the number of pieces that go to make up the skeleton; thus starting with the typical number eight (Fig. 63, A), we find that in various degrees a fusion between neighbouring pieces has taken place in the different families.

=Fam. 1. Verrucidae.=—The ancient genus Verruca, which is still widely distributed in all seas, and is found fixed upon foreign objects on the sea-bottom at various depths, is interesting on account of the asymmetry of its shell, which bears a different aspect according to which side one regards it from. This asymmetry is brought about by the skeletal pieces (carina, rostrum, and paired terga and scuta) shifting their positions after fixation has taken place.

=Fam. 2. Octomeridae.=—In this family the eight plates composing the shell are separate and unfused (Fig. 63, A). The majority of the species come from the Southern hemisphere, e.g. the members of the genera Catophragmus and Octomeris, but Pachylasma giganteum occurs in deep water in the Mediterranean, where it has been found fixed upon Millepore corals.

=Fam. 3. Hexameridae.=—This family includes by far the greater number of the Acorn-barnacles, in which only six plates are present, the laterals having fused with the carino-laterals (Fig. 63, B). The very large genus Balanus belongs here, the common B. tintinnabulum of our coasts being found all over the world, and occurring under a number of inconstant varietal forms. Especial interest attaches to certain other genera, from their habit of living parasitically on soft-bodied animals, whose flesh they penetrate.

Coronula diadema and Tubicinella trachealis live embedded in the skin of whales, the shell of the first-named being of a highly complicated structure with hollow triangular compartments into which the mantle is drawn out.

Xenobalanus globicipitis lives attached to various Cetacea, and is remarkable for the rudimentary condition of its skeleton, the six plates of which form a mere disc of attachment from which the greatly elongated naked body rises, resembling one of the naked Stalked Barnacles.

=Fam. 4 Tetrameridae.=—In this family only four skeletal plates are present (Fig. 63, C). This family is chiefly confined to tropical seas or those of the Southern hemisphere. The chief genera are Tetraclita and Pyrgoma, found in British seas.

=Sub-Order 3. Acrothoracica.=

Gruvel includes in this sub-order four genera (Alcippe, Cryptophialus, Kochlorine, and Lithoglyptes) the species of which live in cavities excavated in the shells of molluscs or in the hard parts of corals.

FIG. 64.—Alcippe lampas. =A=, ♀, × about 10, seen from the right side, with part of the right half of the animal removed; =B=, dwarf male, × about 30. A.M, adductor muscle; An, antenna; C, 1st pair of cirri; Cr, posterior thoracic appendages; E, eye; G, testis; M.C, mantle-cavity; O, ovary; P, penis; T, penultimate thoracic segment; V. vesicula seminalis. (After Darwin.) ]

Darwin discovered and described Cryptophialus minutus, and placed it in a sub-order Abdominalia, believing that it was distinguished from all the foregoing Cirripedes by the presence of a well-developed abdomen. Since the discovery of other allied genera, it has been decided that the abdomen is equally reduced in these forms, and that the terminal appendages do not belong to this region, but to the thorax.

The sexes are separate. The body of the female (Fig. 64, A) is enclosed in a chitinous mantle, armed with teeth by which the excavation is effected, and is attached to the cavity in the host by means of a horny disc. Upon this disc the dwarf males (B) are found.

Alcippe lampas inhabits holes on the inner surface of dead Fusus and Buccinum shells; Cryptophialus minutus the shells of Concholepas peruviana; C. striatus the plates of Chiton; Kochlorine hamata the shells of Haliotis; and Lithoglyptes varians shells and corals from the Indian Ocean.

=Sub-Order 4. Ascothoracica.=

These are small hermaphrodite animals completely enveloped in a soft mantle, which live attached to and partly buried in various organisms, such as the branching Black Corals (Gerardia). They retain the thoracic appendages in a modified state, and the body is segmented into a number of somites, the last of which probably represents an abdomen.

Laura gerardiae, described by Lacaze Duthiers, is parasitic on the stem of the “Black Coral,” Gerardia (vol. i. p. 406); it has the shape of a broad bean, the body being entirely enclosed in a soft mantle, with the orifice in the position corresponding to the hilum of a bean. The body lying in the mantle is composed of eleven segments, and is curved into an =S=-shape. Its internal anatomy is entirely on the plan of an ordinary Cirripede.

Petrarca bathyactidis, G. H. Fowler, was found in the mesenteric chambers of the coral Bathyactis, dredged by the Challenger from 4000 metres. The body is nearly spherical, and the mantle-opening forms a long slit on the ventral surface. The mantle is soft, but is furnished on the ventral surface with short spines.

The antennae, which form the organs of fixation, remain very much in the state characteristic of the Cypris larvae of other Cirripedes, being furnished with two terminal hooks by which attachment is effected. The thoracic appendages, of which there are the normal number six, are reduced flabellate structures, and the abdomen forms an indefinitely segmented lobe of considerable size.

The animal appears to be in an arrested state of development, and so retains some of the characteristics of the Cypris larvae, but it is very doubtful how far these characters can be considered primitive.

Other forms are Dendrogaster astericola on Echinoderms, and Synagoga mira on the “Black Coral,” Parantipathes larix, at Naples.

=Sub-Order 5. Apoda.=

FIG. 65.—Proteolepas bivincta, × 26. A, Antennae; a, b, 1st and 2nd abdominal segments; O, ovary; P, penis; T, telson; 1–8, thoracic segments. (After Darwin.) ]

Darwin described a small hermaphrodite parasite in the mantle chamber of Alepas cornuta from Saint Vincent, West Indies, which he named Proteolepas bivincta.

The body (Fig. 65) is distinctly segmented into eleven somites, the last three of which are supposed to belong to the abdomen; there are no appendages except the antennae by which fixation is effected. The mouth-parts are of normal constitution.

This animal has not been found again since Darwin’s discovery, but Hansen describes a number of peculiar Nauplius larvae taken in the plankton of various regions, which he argues probably belong to members of this group. A wide field of work is offered in attempting to find the adults into which various larvae grow.

=Sub-Order 6. Rhizocephala.=

These remarkable animals are Cirripedes which have taken to living parasitically on various kinds of Crustacea; the majority infest species of Decapoda, e.g. Peltogaster on Hermit-Crabs, Sacculina on a number of Brachyura, Sylon on Shrimps, Lernaeodiscus on Galathea; but one genus, Duplorbis, has been found in the marsupium of the Isopod Calathura brachiata from Greenland. Most of the species are solitary, but a few, e.g. Peltogaster sulcatus, are social. In the adult state the body consists of two portions: a soft bag-like structure, external to the host, carrying the reproductive, nervous, and muscular organs, and attached to some part of the host’s abdomen by means of a chitinous ring; and a system of branching roots inside the host’s body, which spring from the ring of attachment and supply the external body with nourishment.

FIG. 66.—Nearly median longitudinal section (diagrammatic) of Peltogaster. gn, Brain; m, mantle; mc, mantle-cavity; mes, mesentery; op, mantle-opening; ov, ovary; ovd, oviduct; ring, ring of attachment; t, testis; vd, vas deferens. ]

FIG. 67.—Diagrammatic median longitudinal section through a normal Cirripede, gn, Brain; op, mantle-opening; ovd, oviduct; vd, vas deferens. ]

The structure of the external bag-like portion is very simple, and varies only in details, chiefly of symmetry, in the different genera. In Peltogaster, which preserves the simplest symmetrical arrangement of the organs, a diagrammatic section through the long axis of the body (Fig. 66) shows that it consists of a muscular mantle (m) surrounding a visceral mass, and enclosing a mantle-cavity (mc) or brood-pouch, which stretches everywhere between mantle and visceral mass, except along the surface by which the parasite is attached to its host, where a mesentery (mes) is formed. The ring of attachment is situated in the middle of this mesentery; the mantle-cavity, which is completely lined externally and internally with chitin, opens anteriorly by means of a circular aperture (op) guarded by a sphincter muscle. The visceral mass is composed chiefly of the two ovaries (ov), which open on either side of the mesentery by means of a pair of oviducts (ovd); the paired testes (t) are small tubes lying posteriorly in the mesentery, and the nervous ganglion (gn) lies in the mesentery between oviducts and mantle-opening. A comparison with the condition of a normal Cirripede (Fig. 67) shows us that the mesenterial surface of the parasite by which it is fixed corresponds to the dorsal surface of an ordinary Pedunculate Cirripede, and that the ring of attachment corresponds with the stalk or peduncle of a Lepas. The root-system passes out through the ring of attachment into the body of the host, and ramifies round the organs of the crab; the roots are covered externally with a thin chitinous investment, and consist of an epithelium and an internal mass of branching cells continuous with the lacunar tissue in the visceral mass.

FIG. 68.—Development of Sacculina neglecta. =A=, Nauplius stage, × about 70; =B=, Cypris stage, × about 70. A{1}, A{2}, 1st and 2nd antennae of Nauplius; Ab, abdomen; Ant, antenna of Cypris; E, undifferentiated cells; F, frontal horn; G, glands of Cypris; H, tendon of Cypris; M, mandible; T, tentacles. ]

The developmental history of the Rhizocephala is one of the most remarkable that embryology has hitherto revealed. It has been most accurately followed in the case of Sacculina. The young are hatched out in great numbers from the maternal mantle-cavity as small Nauplii (Fig. 68, A) of a typical Cirripede nature, but without any alimentary canal. They swim near the surface of the sea, and become transformed into Cypris larvae of a typical character (Fig. 68, B). The Cypris larva, after a certain period of free existence, seeks out a crab and fixes itself by means of the hooks on its antennae to a hair on any part of the crab’s body. Various races of Sacculina are known which infest about fifty different species of crabs in various seas; the best known are S. carcini parasitic on Carcinus maenas at Plymouth and Roscoff, and S. neglecta on Inachus mauritanicus at Naples. The antenna, by which the Cypris is fixed, penetrates the base of the hair; the appendages are thrown away, and a small mass of undifferentiated cells is passed down the antenna into the body-cavity of the crab. Arrived in the body-cavity it appears that this small mass of cells is carried about in the blood-stream until it reaches the spaces round the intestine in the thorax. Here it becomes applied to the intestine, usually at its upper part, immediately beneath the stomach of the crab (Fig. 69), and from this point it proceeds to throw out roots in all directions, and as it grows to extend its main bulk, called the central tumour (c.t), towards the lower part of the intestine. As the posterior border of the central tumour grows down towards the hind-gut, the future organs of the adult Sacculina become differentiated in its substance; the mantle-cavity being excavated and surrounding the rudiment of the visceral mass, while as the central tumour grows downwards it leaves behind it an ever extending system of roots. When the central tumour in process of differentiation has reached the unpaired diverticulum of the crab’s intestine, at the junction between thorax and abdomen, all the adult organs are laid down in miniature, and the whole structure is surrounded by an additional sac formed by invagination known as the perivisceral space (Fig. 70). The young “Sacculina interna” remains in this position for some time, and being applied to the ventral abdominal tissues of the crab just at the point where thorax and abdomen join, or a little below it, it causes the crab’s epithelium to degenerate, so that when the crab moults, a little hole is left in this region of the same size as the body of the Sacculina, owing to the failure of the epithelium to form chitin here; and thus the little parasite is pushed through this hole and comes to the exterior as the adolescent “Sacculina externa.” From this point onwards the crab, being inhibited in its growth through the action of the parasite, never moults again; so that the Sacculina occupies a safe position protruding from the crab’s abdomen, which laps over it and protects it. The remarkable features of this development are, firstly, the difficulty of understanding how the developing embryo is directed in its complicated wanderings so as always to reach the same spot where it is destined to come to the exterior; and, secondly, the loss after the Cypris stage of all the organs and the resumption of an embryonic undifferentiated state from which the adult is newly evolved. A certain parallel to this history is found in that of the Monstrillidae, described on pp. 64–66.

FIG. 69.—The mid-gut of Inachus mauritanicus with a young Sacculina overlying it, × 2. c.t, “Central tumour” of the parasite; d.i, d.s, inferior and superior diverticula of alimentary canal of host; n, “nucleus,” or body-rudiment of Sacculina; r, its roots; x, definitive position of the parasite. ]

FIG. 70.—Later stage in the development of the “Sacculina interna,” × 2. b, Body of Sacculina; c.t, “central tumour”; d.i, d.s, inferior and superior diverticula of alimentary canal of host; o, opening of perivisceral cavity of Sacculina; r, its roots. ]

FIG. 71.—Fourteen Cypris larvae fixed round the mantle-opening (o) of a young Sacculina externa, × 20. ]

The Rhizocephala are hermaphrodite with the possible exception of Sylon, which appears to be female and perhaps parthenogenetic, no male having been seen; but unlike most other hermaphrodite Cirripedes, they reproduce by a continual round of self-fertilisation. This is the more remarkable in that the vestiges of what appears to be a male sex are still found in Sacculina and Peltogaster; certain of the Cypris larvae in these genera, instead of fixing on and inoculating other crabs, become attached round the mantle-openings of young parasites of the same species as themselves, which have recently attained to the exterior of their hosts (Fig. 71). These larvae, which remind us of the complemental males in Scalpellum, etc., never produce spermatozoa, but rapidly degenerate where they are fixed, and appear never to play any rôle in the reproduction of their species. The nature of this remarkable phenomenon, together with the sexual condition of the Cirripedes in general, will be discussed in the next section.

Much remains to be elucidated in the life-histories of these curious animals, and it seems probable that intermediate stages may exist, showing us how the extreme discontinuity of development has been reached. Suggestive in this respect is the newly discovered parasite of the Isopod, Calathura, which the author has named Duplorbis calathurae. This animal does not appear to possess a root-system, but is attached to its host by a tube which passes right through the mesentery and opens into the mantle-cavity of the parasite. It may be suggested that this tube corresponds to the stalk of the normal Cirripede, but its exact mode of formation would certainly throw much light on the question of Rhizocephalan development.

=Phenomena of Growth and Sex in the Crustacea.=

In the foregoing account of the Cirripedia we have met with certain peculiar sexual relations in which closely allied species exhibit marked differences in regard to the distribution of the qualities of sex among their individuals; we have seen that the majority of species are hermaphrodite, unlike most Crustacea which, with the other exception of the parasitic Isopoda, are normally dioecious; and that in some species complemental males exist side by side with the hermaphrodites, while, in yet others, the individuals are either females or dwarf males.

Before examining the causes of these conditions, it will be opportune to consider a number of facts which throw light on the question of sex and hermaphroditism in general. We may then return to the discussion of the hermaphroditism found in particular in the Cirripedia and Isopoda.

=Parasitic Castration.=—Giard was the first to observe that a number of parasites exert a remarkable influence on the sexual characters of their host, such that the generative glands become reduced, or may completely degenerate, while the secondary sexual characters become materially altered. This was proved to occur in the most widely different hosts, affected by the most widely different parasites (e.g. Crustacea, Insecta, Worms). Moreover, it was apparent that the affection does not consist in the parasite merely destroying the generative organs, with which it often does not come into contact, but rather in the general disturbance of the metabolism set up by its presence.

The most completely studied cases of parasitic castration are those of the Rhizocephalous Sacculina neglecta, parasitic on the spider-crab, Inachus mauritanicus, and of Peltogaster curvatus on the Hermit-crab, Eupagurus excavatus, var. meticulosa. The ordinary males of I. mauritanicus have the appearance shown in Fig. 72, A. The abdomen is small and bears a pair of copulatory styles, while the chelipedes are long and swollen. In the female (B) the abdomen is much larger and trough-shaped, and carries four pairs of ovigerous appendages; the chelae are small and narrow.

FIG. 72.—Illustrating the effect of parasitic Sacculina neglecta on Inachus mauritanicus, nat. size. =A=, Normal male; Inachus; =B=, normal female; =C=, male infested by Sacculina (final stage); =D=, abdomen of infested female; =E=, infested male in an early stage of its modification. ]

Now it is found that in about 70 per cent of males infected with Sacculina the body takes on to varying degrees the female characters, the abdomen becoming broad as in the female, with a tendency to develop the ovigerous appendages, while the chelae become reduced (Fig. 72, C). This assumption of the female characteristics by the male under the influence of the parasite may be so perfect that the abdomen and chelae become typically female in dimensions, while the abdomen develops not only the copulatory styles typical of the male, but also the four pairs of ovigerous appendages typical of the female. The parasitised females, on the other hand, though they may show a degenerate condition of the ovigerous appendages (Fig. 72, D), never develop a single positively male characteristic. On dissecting crabs of these various categories it is found that the generative organs are in varying conditions of degeneration and disintegration.

The most remarkable fact in this history is the subsequent behaviour of males which have assumed perfect female external characters, if the Sacculina drops off and the crabs recover from the disease. It is found that under these circumstances these males may regenerate from the remains of their gonads a perfect hermaphrodite gland, capable of producing mature ova and spermatozoa. The females appear quite incapable, on the other hand, of producing the male primary elements of sex on recovery, any more than they can produce the secondary. Exactly analogous facts have been observed in the case of the hermit-crabs parasitised by Peltogaster, but here the affected males produce small ova in their testes before the parasite is got rid of. Here, too, the females seem incapable of assuming male characters under the influence of the parasite.

To summarise shortly the conclusions to be deduced from these facts— certain animals react to the presence of parasites by altering their sexual condition. This alteration consists in the female sex in an arrest of reproductive activity, in the male sex in the arrest of reproductive activity coupled with the assumption of all the external characters proper to the female. But in these males it is not merely the external characters that have been altered; their capacity for subsequently developing hermaphrodite glands shows that their whole organisation has been converted towards the female state. That this alteration consists in a reorganisation of the metabolic activities of the body is clearly suggested, and in the succeeding paragraph we furnish some further evidence in support of this view.

FIG. 73.—Inachus mauritanicus, × 1. =A=, Low male; =B=, middle male; =C=, high male; the great chela of the right side is the only appendage represented. ]

=Partial and Temporary Hermaphroditism. High and Low Dimorphism.=

The reproductive phases of animals are frequently rhythmic, periods of growth alternating with periods of reproduction. This is well exemplified in the case of the ordinary males of Inachus mauritanicus, of some other Oxyrhynchous crabs, and of the Crayfish Cambarus. During the breeding season the males of I. mauritanicus fall into three chief categories: Small males with swollen chelae (Fig. 73, A), middle-sized males with flattened chelae (B), and large males with enormously swollen chelae (C). On dissecting specimens of the first and third categories it is found that the testes occupy a large part of the thoracic cavity and are full of spermatozoa, while in the middle-sized males with female-like chelae the testes appear shrivelled and contain few spermatozoa. These non-breeding crabs are, in fact, undergoing a period of active growth and sexual suppression before attaining the final state of development exhibited by the large breeding males. This phenomenon is obviously parallel to the “high and low dimorphism” so common in Lamellicorn beetles, where the males of many species are divided into two chief categories, viz. “low males” of small size in which the secondary sexual characters are poorly developed, and “high males” of large size in which these characters are proportionately much more highly developed than in the low males. The only difference between the two cases is that whereas in the beetles growth ceases on the attainment of maturity in the low degree, in the Crustacea the low male passes through a period of growth and sexual suppression to reach the high degree of development.

The condition of the middle-sized males may be looked upon as one of partial hermaphroditism, indications of the female state being found in the flattened chelae and in the reduced state of the testes. This interpretation is greatly strengthened by the state of affairs observed in the life-history of the male Sand-hoppers, Amphipods of the genus Orchestia. In the young males of several species of this genus, at the time of year when they are not actively breeding, small ova are developed in the upper part of the testes of more than half of the male individuals, these ova being broken down and reabsorbed as the breeding season reaches its height. Nor is this phenomenon confined to this genus; in the males of a number of widely different Crustacea these small ova are found in the testes at certain periods of the life-history (e.g. Astacus), when the animal is not breeding.

The foregoing facts indicate unmistakably that the males of a number of Crustacea under certain metabolic conditions, i.e. when a stage of active growth as opposed to a stage of reproductive activity is initiated, alter their sexual constitution in such a way that the latent female characteristics are developed, and the organism appears as a partial hermaphrodite. In the preceding paragraph we saw that the males of a number of animals, especially Crustacea, react to the metabolic disturbance set up by the presence of a parasite in exactly the same way, i.e. by developing into partial or total hermaphrodites. From these two converging bodies of facts we may conclude, firstly, that sex and metabolism are two closely connected phenomena; and, secondly, that the male sex is especially liable to assume hermaphrodite characters whenever its metabolic requirements are conservative, assimilatory, or in a preponderating degree anabolic, as when a phase of active growth is initiated, or the drain on the system, due to the presence of a parasite, is to be made good.

=Normal Hermaphroditism in Cirripedia and Isopoda Epicarida.=

The above-mentioned groups contain the only normally hermaphrodite Crustacea, and since they are in most respects highly specialised, we may be certain that they have been secondarily derived from dioecious ancestors. They both lead a sessile or parasitic life, and it is noteworthy that this habit is often associated with hermaphroditism, e.g. in Tunicates. A sessile or parasitic mode of life is one in which the metabolic functions are vegetative and assimilatory rather than actively kinetic or metabolic. It is in this state that we have seen the males of a number of Crustacea taking on a temporary or partial hermaphroditism. We may, therefore, inquire, whether in these cases of normal hermaphroditism there is any evidence to show that here too the hermaphroditism has been acquired by the male sex as a response to the change in the metabolic conditions. In the parasitic Isopoda Epicarida (see pp. 129–136) the hermaphroditism is of a very simple kind; all the individuals are at first males, whose function it is to fix on and fertilise the adult parasites. These subsequently develop into females which are in their turn cross-fertilised by the young larvae derived from a previous generation. All the individuals being alike, it seems probable that they have been derived from one sex, and the general nature of hermaphroditism deduced above may lead us to suppose that that sex was originally male, the female having been suppressed. In certain Cirripedia, e.g. most species of Scalpellum, there exist, besides the hermaphrodite individuals, complemental males, so that here a superficial conclusion might be drawn that the hermaphrodites represent the female sex. But if we can suggest that the complemental males are in reality similar in derivation to the hermaphrodite individuals, we shall be in a position to claim that the hermaphrodite Cirripedes are similar to the Isopoda Epicarida, and have probably also been derived from the male sex. There is decided evidence pointing to this conclusion. In the first place, the complemental males of at least one species of Scalpellum, S. peronii, do show an incipient hermaphroditism in the presence of small ova in their generative glands, which, however, never come to maturity.

The condition of the degenerate males in the Rhizocephala may also be interpreted in the same manner. These never pass beyond the Cypris stage of development, in which they resemble in detail the Cypris larvae of the ordinary hermaphrodite individuals, and they are quite useless in the propagation of their species.

It is more reasonable to suppose that these Cypris larvae, which fix on the mantle-openings of adult parasites, are in reality identical with the ordinary Cypris which infest crabs and develop into the hermaphrodites, than that they represent a whole male sex doomed beforehand to uselessness and degeneration. If we suppose that the Cirripedes have passed through a state of protandric hermaphroditism similar to that of the Isopoda Epicarida, it is plain that all the larvae must have originally possessed the instinct of first fixing on the adult parasites, and we may suppose that this instinct has been retained in the Rhizocephala, but is now only actually fulfilled by a certain proportion of the larvae, which, under existing circumstances, are useless and fail to develop further; while the rest of the larvae, not finding an adult parasite to fix upon, go straight on to infect their hosts and develop into the adult hermaphrodites.

The same explanation would apply to the complemental males in Scalpellum, etc., these individuals being also potential hermaphrodites, which are arrested in development, though not so completely as in the Rhizocephala, owing to the position they have taken up.

This theory throws light on another dark feature of Cirripede life-history, namely, the gregarious instinct. The associations of Cirripedes are not formed by a number of Cypris larvae fixing together on the same spot, but rather by the Cypris larvae seeking out adolescent individuals of their own species and fixing on or near them. Now, if we suppose that the Cirripedes have passed through a condition of protandric hermaphroditism similar to that of the Isopoda Epicarida, it is clear that a slight modification of the sexual instinct of the larvae would lead to the gregarious habit, while its retention in some individuals in its original form accounts for their finding their way to the mantles of adult individuals and developing into the so-called complemental males.

Certain Cirripedes, viz. certain species of Scalpellum and Ibla and all the Acrothoracica, are dioecious. It is impossible to decide at present whether these species retain the primitive dioecious condition of the ancestral Cirripedes, or whether they too have been derived from an hermaphrodite state, but in the present state of knowledge they hardly affect the validity of the theory that has been proposed to account for the nature of the complemental males and the hermaphrodite individuals.

=Order IV. Ostracoda.=

The Ostracoda are small Crustacea, the body consisting of very few—about eight—segments, and being completely enclosed in a carapace, which has the form of a bivalve shell. Development is direct, without a Nauplius stage.

The Ostracoda are marine and fresh-water animals that can be divided into several families, differing slightly in habits and in structures correlated with those habits.

FIG. 74.—Candona reptans. =A=, Natural size; =B=, X 15. a, 1st antennae; b, 2nd antennae; c, walking legs. (After Baird.) ]

The =Cypridae= and =Cytheridae= include all the fresh-water and a vast majority of marine genera, adapted for a sluggish life among water-plants, though some can swim with considerable activity. The common Cypris and Candona of our ponds and streams are familiar instances. The movements of these animals are effected by means of the two pairs of uniramous pediform antennae which move together and in a vertical straight line. In the Cypridae (Fig. 74) there are, besides the mandibles, two pairs of maxillae, a pair of walking legs, and, lastly, a pair of appendages, which are doubled up into the carapace, and are used for cleaning purposes. In the marine Cytheridae there is only one maxilla, the last three appendages being pediform and used in walking. The telson in the Cytheridae is rudimentary, but is well developed in the Cypridae. The heart is altogether absent.

In many of the fresh-water forms, e.g. common species of Candona and Cypris, males are never found, and parthenogenetic reproduction by the females appears to proceed uninterruptedly. Weismann kept females of Cypris reptans breeding parthenogenetically for eight years. He also remarks on the fact that these, and indeed all parthenogenetic female Ostracoda, retain the receptaculum seminis, used normally for storing the spermatozoa derived from the male, unimpaired.

Some of the Cytheridae occur in deep water. Thus Cythere dictyon was frequently taken by the Challenger in depths of over 1000 fathoms, but the majority prefer shallow water.

FIG. 75.—Asterope oblonga, ♀, removed from its carapace, × 25. A, Alimentary canal; A{1}, A{2}, 1st and 2nd antennae; E, eye; G, gills; G.O, generative opening; H, heart; M, mandible; T, 6th appendage; T′, last appendage (cleaning foot). (After Claus.) ]

The =Halocypridae= and =Cypridinidae= comprise marine genera of a pelagic habit. The first antennae are chiefly sensory, but the second antennae are biramous, and they do not merely move up and down, as in the preceding families, but sideways like oars, the valves of the shells being excavated to admit of free movements. There are two pairs of maxillae; the succeeding limbs differ in the two families. In the Cypridinidae, e.g. Asterope (Fig. 75), the first leg (T) is lamelliform and is used as an accessory maxilla, while the second leg (T’) is turned upwards into the shell as a cleaning organ. In the Halocypridae the first leg is pediform, and differs in the two sexes, while the second leg is rudimentary and points backwards. In Asterope peculiar branchial organs (G) are present on the back. Both families possess a heart; the Halocypridae are blind, while the Cypridinidae possess eyes.

The =Polycopidae= and =Cytherellidae= are curious marine families of a pelagic habit, with biramous second antennae well adapted for swimming, and very broad. The first maxilla in the Polycopidae is also employed in swimming, while the second is modified into a branchial organ; the maxillae of the Cytherellidae are more normal in structure, but both carry branchial lamellae. The posterior limbs are altogether absent in Polycopidae, and in the Cytherellidae are only represented by the copulatory organs of the male.