CHAPTER V. Crustacea (_continued_): Malacostraca: Leptostraca—Phyllocarida:

CRUSTACEA (CONTINUED): MALACOSTRACA: LEPTOSTRACA—PHYLLOCARIDA: EUMALACOSTRACA: SYNCARIDA—ANASPIDACEA: PERACARIDA—MYSIDACEA—CUMACEA— ISOPODA—AMPHIPODA: HOPLOCARIDA—STOMATOPODA

SUB-CLASS II.—MALACOSTRACA.

The Malacostraca are generally large Crustacea, and they are characterised by the presence of a definite and constant number of segments composing the body. In addition to the paired eyes we can distinguish two pairs of antennae, a mandibular segment, and two maxillary segments composing the head region proper; there then follow eight thoracic segments, the limbs belonging to the anterior thoracic segments being often turned forwards towards the mouth, and modified in structure to act as maxillipedes, while at any rate the last four are used in locomotion and are termed “pereiopods.” The abdomen is composed of six segments, which typically carry as many pairs of biramous “pleopods,” and the body terminates in a telson. Not counting the paired eyes or the telson, there are present nineteen segments. The excretory organs in the adult open at the bases of the second antennae, and are known as “green glands,” but in the larva maxillary glands may be present homologous to those which persist in the adult Entomostraca. This is the typical arrangement, but sometimes the maxillary glands persist in adult Malacostraca, e.g. Nebalia, Anaspides, and some Isopods.

The hepato-pancreatic diverticula are directed posteriorly, and not anteriorly as in most Entomostraca, and the stomach is often furnished with chitinous teeth and ridges forming an elaborate gastric mill, especially in the larger Decapods.

SERIES 1. LEPTOSTRACA.

=Division. Phyllocarida.=

FIG. 76.—Nebalia geoffroyi, ♀, × 20. A.1, A.2, 1st and 2nd antennae; Ab.1, Ab.6, 1st and 6th abdominal appendages; A.G. antennary gland; C, half of caudal fork; E, eye; G, ventral ganglionic chain; H, heart; I, intestine; L, upper liver-diverticulum; M, adductor muscle of halves of carapace; MX, palp of 1st maxilla; O, ovary; R, rostrum. (After Claus.) ]

The small shrimp-like Crustacean Nebalia, which is found burrowing in the superficial layers of sand in the littoral and sometimes the deeper regions of most seas, has been regarded, ever since its anatomy was made out by Claus, as a connecting link between Entomostraca and Malacostraca, and has been placed in a separate group Leptostraca.

The segmentation of the body is Malacostracan, save that two extra segments are present in the abdomen, and the paired compound eyes are borne upon stalks. The eight thoracic limbs are all very similar; they are built on the typical biramous plan, and each carries a bract; they have been compared, owing to their flattened, expanded shape, to the foliaceous limbs of the Phyllopods. The abdominal appendages are also biramous. The heart is greatly elongated, stretching through thorax and abdomen; there are present both the antennary excretory glands characteristic of adult Malacostraca and the maxillary glands characteristic of adult Entomostraca, and both the posterior and anterior livers characteristic of the two Orders respectively are present. This combination of characters justifies the belief that Nebalia represents a primitive form, standing to some extent in an intermediate position between Entomostraca and Malacostraca, but it may be doubted if the special relationship to the Phyllopoda, claimed on the strength of the foliaceous appearance of the thoracic limbs, can be legitimately pressed.

Nebalia shows the clearest signs of relationship to the other primitive Malacostraca, and especially to the Mysidae, which it resembles not only in general form and in the essentially biramous character of its appendages, but also in many embryological points and in the similarity in development of the brood-pouch.

A large number of very ancient palaeozoic fossils are known which are placed provisionally with Nebalia in the Division Phyllocarida, and some of these are no doubt closely related to the existing isolated genus. Hymenocaris from the Cambrian.

SERIES 2. EUMALACOSTRACA.

Before entering on a description of the members of this Series it is necessary to introduce and justify a new scheme of classification which has been proposed by Dr. W. T. Calman. This scheme necessitates the abandonment of the old Order Schizopoda, and also ignores the distinction which used to be considered fundamental between the sessile-eyed Crustacea (Edriophthalmata) and the stalk-eyed forms (Podophthalmata).

The old group of Schizopoda, to which Nebalia and the isolated form Anaspides, to be considered later, are undoubtedly related, represent very clearly the stem-forms from which the various branches of the Malacostracan stock diverge. No doubt they are themselves specialised in many directions, since they are a dominant group in present day seas, but their organisation is fundamentally of a primitive type. We see this especially in the comparative absence of fusion or reduction of the segments of the body externally and of the nervous system internally, and in the simple undifferentiated character of the trunk-limbs, all of which conform to the primitive biramous type. The most anterior thoracic limbs of the Schizopods are of particular interest. In the higher Malacostraca three of these limbs are usually turned forwards towards the mouth to act as maxillipedes, and the most anterior of all, the first maxillipede, is apt, especially in the Decapoda, to take on a flattened foliaceous form owing to the expansion of the basal segments to act as gnathobases (see Fig. 1, A, p. 10). Now this appendage in the Schizopods preserves its typical biramous character, and resembles the succeeding thoracic limbs, but in many of the species the basal joints show a tendency to be produced into biting blades (Fig. 1, E, p. 10), thus indicating the first step in the evolution of the foliaceous first maxillipede of the Decapoda. The primitive character of the Schizopods is also indicated by the fact that most of the Decapoda with uniramous limbs on the five hinder thoracic segments pass through what is known as the “Mysis stage” in development, when these limbs are biramous, the exopodites being subsequently lost in most cases.

The “Schizopoda” include a very large number of pelagic Crustacea of moderate size, which superficially appear to resemble one another very closely. The slender, elongated body, the presence of biramous limbs on all the thoracic and abdominal segments, and the possession of a single row of gills at the bases of the thoracic limbs, are, generally speaking, typical of the families Mysidae, Lophogastridae, Eucopiidae, and Euphausiidae, which go to make up the old Order Schizopoda.

It has, however, been pointed out first by Boas, and subsequently by Hansen and Calman, that the Euphausiidae are in many respects distinct from the other three families, and agree with the Decapoda, while the Eucopiidae, Lophogastridae, and Mysidae agree with the Cumacea, Isopoda, and Amphipoda.

It has, therefore, been suggested by these authors that the classification of the Malacostraca should be revised, and Calman (loc. cit.) has brought forward the following scheme:—

The division PERACARIDA, including the Eucopiidae, Lophogastridae, and Mysidae (= Mysidacea), the Cumacea, Isopoda, and Amphipoda, is characterised by the fact that when a carapace is present it leaves at least four of the thoracic segments free and uncoalesced: by the presence of a brood-pouch formed from the oostegites on the thoracic limbs of the female: by the elongated heart: by the few and simple hepatic caeca: by the filiform spermatozoa: and by the direct method of development without a complicated larval metamorphosis. The biting face of the mandible has a movable joint, the “lacinia mobilis.”

The division EUCARIDA, on the other hand, including the Euphausiidae and the Decapoda, shows the converse of these characters. The carapace coalesces with all the thoracic segments, there is never a brood-pouch formed from oostegites, the hepatic caeca are much ramified, the heart is short, the spermatozoa are spherical with radiating pseudopodia, the development is indirect with a complicated metamorphosis, and the mandible is without a lacinia mobilis.

Corresponding divisions are made by Calman to receive the other Malacostraca, namely, the PHYLLOCARIDA for Nebalia, the SYNCARIDA for Anaspides, and the HOPLOCARIDA for the Stomatopoda or Squillidae.

The important array of characters which separates the Euphausiidae from the other Schizopods and unites them with the Decapoda can no longer be neglected, and the consideration of Anaspides and its allies will further emphasise the extreme difficulty of retaining the Schizopoda as a natural group. In the sequel Calman’s proposed scheme will be adopted.

DIVISION 1. SYNCARIDA.

There is no carapace, and all the eight thoracic segments may be free and distinct. Eyes may be pedunculate or sessile. The mandible is without a lacinia mobilis. There is no brood-pouch, the eggs being deposited and hidden after fertilisation. The spermatozoa are filiform, the hepatic caeca very numerous, and the heart tubular and elongated, with ostia only in one place in the anterior thoracic region. The auditory organ is at the base of the first antennae.

=Order. Anaspidacea.=

=Fam. 1. Anaspididae.=—The mountain-shrimp of Tasmania, Anaspides tasmaniae, was first described by Thomson in 1893 from specimens taken in a little pool near the summit of Mount Wellington; it was redescribed by Calman, who drew attention to its remarkable resemblance to certain Carboniferous fossils of Europe and N. America (Gampsonyx, Palaeocaris, etc.).

The creature appears to be confined to the deep pools of the rivers and tarns on the mountains of the southern and western portions of Tasmania. The waters in which it occurs are always cold and absolutely clear, and there is no record of its living at altitudes much below 2000 feet, while it frequently occurs at 4000 feet. The body may attain upwards of two inches in length; it is deeply pigmented with black chromatophores, and it is held perfectly horizontal without any flexure. The animal rarely swims unless disturbed, usually walking about on stones and water-plants at the bottom of deep pools. In walking the endopodites of the thoracic limbs are chiefly instrumental, but they are assisted by the exopodites of the abdominal limbs.

When frightened the shrimp can dart rapidly forwards or sideways by the strokes of its powerful tail-fan, but it never jumps backwards as do the other Malacostraca. It appears to browse upon the algal slime covering the rocks and on the submerged liver-worts and mosses, but it does not refuse animal food, even feeding on the dead bodies of members of its own species. The thoracic limbs, which are all biramous except the last pair, carry a double series of remarkable plate-like gills on their coxopodites. The slender and setose exopodites of the thoracic limbs are respiratory in function, being kept in continual motion even when the animal is at rest, and serving to keep up a current of fresh water round the gills.

Anaspides shows a remarkable combination of structural characters, some of which are peculiar, while others are possessed in common with the Peracarida or Eucarida. The chief peculiar characters are the entire absence of a carapace, and the freedom of the eight thoracic segments, with eight free thoracic ganglia in the nerve-cord; the peculiar double series of plate-like gills; the structure of the alimentary canal; and the fact that the eggs, instead of being carried in a brood-pouch, or affixed to the abdominal limbs, are deposited under stones and among water-plants.

FIG. 77.—Anaspides tasmaniae in natural position for walking, × 1. The last two pereiopods point backwards and are overlapped by the first two pleopods. ]

The Peracaridan features, uniting it especially with the Mysidacea, are the structure of the elongated heart, the filiform spermatozoa, and the fact that no complicated metamorphosis is passed through, the young hatching out in a condition similar to, though possibly not identical with, the adult form.

The Eucaridan, especially Decapodan, features are the presence of an auditory sac on the basal joint of the antennules, and the modification of the endopodites of the first two abdominal appendages in the male to form a copulatory organ.

A type of a new genus of this family was found by me in the littoral zone of the Great Lake of Tasmania at an elevation of 3700 feet, and named Paranaspides lacustris.

This little shrimp (Fig. 78), which does not appear to grow to more than an inch in length, is totally different in appearance from Anaspides, being pale green and transparent, with a very marked dorsal hump as in Mysis, to which it bears a very striking superficial resemblance. It leads a more active swimming life than Anaspides, and with this habit is correlated the flexure of the body and the greater size of the tail-fan and the scale of the second antenna. The mandible is peculiar in being furnished with a four-jointed biramous palp, while that of Anaspides is three-jointed and uniramous, and the first thoracic appendage is provided with a setose biting lobe on the antepenultimate joint, thus more resembling a maxillipede. In other respects it agrees essentially in structure with Anaspides.

FIG. 78.—Paranaspides lacustris, × 4. a^1, a^2, First and second antennae; Ab.1, first abdominal segment; ep, epipodites or gills on the thoracic legs; md, mandible; Pl.1, first pleopod; T, telson; Th.8, eighth free thoracic segment; U, uropod, or sixth pleopod. ]

=Fam. 2. Koonungidae.=—The sole representative of this family, Koonunga cursor, has been recently described by Mr. O. A. Sayce, of Melbourne University, from a small stream some miles to the west of Melbourne. Although plainly belonging to the Anaspidacea, this interesting little animal, which only measures a few millimetres in length, and follows a similar habit to Anaspides, running about with its body unflexed, differs from all the other members of the Division in possessing sessile instead of stalked eyes, in the first thoracic segment being fixed to the head, and in a number of minor anatomical points.

It is impossible at present to assign the Carboniferous forms (Gampsonyx, Palaeocaris, etc.) to their exact position in the Division, but it seems that they agreed more closely with Anaspides than with the other two genera. From the position in which the fossils are preserved, it would appear that they followed a similar walking habit to Anaspides, and that the body was unflexed.

DIVISION 2. PERACARIDA.

The carapace, when present, leaves at least four of the thoracic somites distinct; the first thoracic segment is always fused with the head. The eyes are pedunculate or sessile.

The mandible possesses a lacinia mobilis. A brood-pouch is formed in the female from oostegites attached to the thoracic limbs. The hepatic caeca are few and simple; the heart is elongated and tubular; the spermatozoa are filiform, and development takes place without a complicated metamorphosis.

=Order I. Mysidacea.=

The Mysidacea, although pelagic, are not very often met with in the true plankton on the surface; they generally swim some way below the surface, going down in many cases into the abysses. For this reason they thrive excellently in aquaria, and the common Mysis vulgaris is often present in such numbers in the tanks at the Zoological station at Naples as to damage the other inmates by the mere press of numbers. The Mysidacea, like the majority of the Peracarida, undergo a direct development, and hatch out with the structure of the adult fully formed.

Many of the Mysidacea bear auditory sacs upon the sixth pair of pleopods, a characteristic not found in the Euphausiacea.

=Fam. 1. Eucopiidae.=—The curious form Eucopia australis (Fig. 79) described by Sars, may be chosen as an example of the Mysidacea.

The peculiarity of this form consists chiefly in the immense elongation of the endopodites of the fifth, sixth, and seventh thoracic appendages. Characteristic of the Mysidacea is the freedom of the hinder thoracic segments from fusion with the carapace, otherwise this animal is seen closely to resemble the Euphausia figured (Fig. 102). Eucopia australis, like so many of the Mysidacea, is a deep-sea animal, being brought up with the dredge from over 1000 fathoms; it is very widely distributed over the Atlantic Ocean.

FIG. 79.—Eucopia australis, young female, × 3. A, 1st antenna; Ab.1, 1st abdominal segment; Ab.6, 6th abdominal appendage; E, eye; T, telson; Th, 5th thoracic appendage. (After Sars.) ]

=Fam. 2. Lophogastridae.=—The members of this family (Lophogaster, Gnathophausia) agree with the Eucopiidae in the possession of branched gills on some of the thoracic limbs, in the absence of auditory sacs on the sixth pair of pleopods, in the presence of normally developed pleopods in both the male and female, and in the brood-lamellae being developed on all seven of the thoracic limbs. The endopodites of the posterior thoracic limbs are, however, of a normal size.

FIG. 80.—Dorsal view of male Diastylis stygia, × 12. A, 2nd antenna; Ab.6, 6th abdominal appendage. (After Sars.) ]

=Fam. 3. Mysidae.=—These differ from both the foregoing families in the absence of gills, in the presence of an auditory sac on the sixth pleopods, in the reduction of the other pleopods in the female, and in the brood-lamellae being developed only on the more posterior pairs of thoracic limbs. A number of closely related genera compose this family, of which Mysis, Boreomysis, and Siriella may be mentioned. Mysis oculata, var. relicta, is a fresh-water form from the lakes of northern and central Europe.

=Order II. Cumacea.=

The Cumacea are a group of small marine animals rarely attaining an inch in length, which agree with the Mysidacea in the characters noted above as diagnostic of the Division Peracarida; they possess, however, in addition a number of peculiar properties, and Sars believes them to be of a primitive nature showing relationship to Nebalia, and possibly to an ancestral Zoaea-like form. They follow a habit similar to that of the Mysidacea, being caught either in the surface-plankton or in great depths, many of the deep-sea forms being blind. They are, however, not true plankton forms, and they appear to attain a greater development both in point of variety and size in the seas of the northern hemisphere. The thoracic limbs may be biramous, but there is a tendency among many of the genera to lose the exopodites of some of the thoracic legs, an exopodite never being present on the last few thoracic limbs of the female and on the last in the male. In the Cumidae the four posterior pairs in both sexes have no exopodites. The first three thoracic appendages following the maxillae are distinguished as maxillipedes; they are uniramous, and the first pair carries an epipodite and a large gill upon the basal joints. Pleopods are only developed in the male sex.

The flagellum of the second antennae in the male may be enormously elongated, as in the Atlantic deep-sea species shown in Fig. 80, so as to exceed in length the rest of the body.

=Fam. 1. Cumidae.=—No sharp demarcation between thorax and abdomen. Four posterior pairs of legs in both sexes without exopodites. Male with five well-developed pleopods in addition to the uropods. Telson wanting. Cuma, Cyclaspis, etc.

=Fam. 2. Lampropidae.=—Body-form resembles that of Cumidae. All the thoracic limbs except the last have exopodites. The male has three pairs of pleopods. Telson present. Lamprops, Platyaspis, etc.

=Fam. 3. Leuconidae.=—Body-form similar to above. Male has only two pairs of pleopods. Mouth-parts peculiar, much less setose than in other families. Telson absent. Leucon, Eudorella.

=Fam. 4. Diastylidae.=—Anterior part of thorax sharply marked off from posterior part. Male has two pairs of pleopods. Telson present. Diastylis (Fig. 80). D. goodsiri from the Arctic ocean measures over an inch in length.

=Fam. 5. Pseudocumidae.=—Rather similar to Diastylidae, but differ in reduced size of telson and presence of exopodites on third and fourth thoracic legs of female. This family is represented by three very similar marine forms of the genus Pseudocuma; but, as Sars has shown, the Caspian Sea contains thirteen peculiar species, only one of which can be referred to the genus Pseudocuma, while the rest may be partitioned among four genera, Pterocuma, Stenocuma, Caspiocuma, Schizorhynchus.

=Order III. Isopoda.=

The Isopoda and the Amphipoda are frequently classed together as Arthrostraca or Edriophthalmata, owing to a number of features which they share in common, as, for instance, the sessile eyes which distinguish them from the podophthalmatous Schizopoda and Decapoda, the absence of a carapace, and the thoracic limbs which are uniramous throughout their whole existence. For the rest, in the presence of brood-plates and the other diagnostic characters, they are plainly allied to the other Peracarida, and an easy transition is effected from the Mysidacea to the Isopoda through the Chelifera or Anisopoda. Only one thoracic segment is usually fused with the head, the appendage of this segment being the maxillipede; in the Chelifera among Isopoda, and the Caprellidae among Amphipoda, two thoracic segments are fused with the head.

The Isopoda are distinguished from the Amphipoda by the dorso-ventral flattening of the body, as opposed to the lateral flattening in the Amphipoda, by the posterior position of the heart, and by the branchial organs being situated on the abdominal instead of on the thoracic limbs.

The Isopoda, following Sars’ classification, fall into six sub-orders—the Chelifera, Flabellifera, Valvifera, Asellota, Oniscoida, and Epicarida,—to which must be added the Phreatoicidea.

=Sub-Order 1. Chelifera.=

The Chelifera, including the families (1) =Apseudidae= and (2) =Tanaidae=, are interesting in that they afford a transition between the ordinary Isopods and the Mysidacea. The important features in which they resemble the Mysidacea are, first, the fusion of the first two thoracic segments with the head, with the coincident formation of a kind of carapace in which the respiratory functions are discharged by a pair of branchial lamellae attached to the maxillipedes; and, second, the presence of very small exopodites on the first two thoracic appendages of the Apseudidae.

The second pair of thoracic limbs, i.e. the pair behind the maxillipedes, are developed both in the Apseudidae and Tanaidae into a pair of powerful chelae, and these frequently show marked sexual differences, being much more highly developed in the males than in the females. The biramous and flattened pleopods are purely natatory in function, and the uropods or pleopods of the sixth pair are terminal in position and slender.

FIG. 81.—Apseudes spinosus, ♂, × 15. A, 1st antenna; Ab, 6th abdominal appendage; T, 2nd thoracic appendage. (After Sars.) ]

Both families, of which the Apseudidae contain the larger forms, sometimes attaining to an inch in length, are littoral in habit, or occur in sand and ooze at considerable depths, many of the genera being blind. Many Tanaids (e.g. Leptochelia, Tanais, Heterotanais, etc.) live in the algal growths of the littoral zone, and being highly heliotropic they are easy to collect if a basinful of algae is placed in a strong light. The females carry the eggs about with them in a brood-pouch formed, as is usual in the Peracarida, by lamellae produced from the bases of the thoracic limbs. The males on coming to maturity do not appear to grow any more, or to take food, their mouth-parts frequently degenerating and the alimentary canal being devoid of food. They are thus in the position of insects which do not moult after coming to maturity; and, as in Insects, the males are apt to show a kind of high and low dimorphism—certain of the males being small with secondary sexual characters little different from those of the females, while others are large with these characters highly developed. Fritz Müller, in his Facts for Darwin, observes that in a Brazilian species of Leptochelia, apparently identical with the European L. dubia, the males occur under two totally distinct forms—one in which the chelae are greatly developed, and another in which the chelae resemble those of the female, but the antennae in this form are provided with far longer and more numerous sensory hairs than in the first form. Müller suggested that these two varieties were produced by natural selection, the characters of the one form compensating for the absence of the characters of the other. A general consideration of the sexual dimorphism in the Tanaidae lends some support to this view, since the smaller species with feeble chelae do appear to be compensated by a greater development of sensory hairs on the antennae, but the specific differences are so difficult to appreciate in the Tanaidae that it is possible that the two forms of the male in Müller’s supposed single species really belonged to two separate species.

=Sub-Order 2. Flabellifera.=

The Flabellifera include a number of rather heterogeneous families which resemble one another, however, in the uropods being lateral and not terminal, and being expanded together with the telson to form a caudal fan for swimming. The pleopods are sometimes natatory and sometimes branchial in function. Some of the families are parasitic or semi-parasitic in habit.

=Fam. 1. Anthuridae.=—These are elongated cylindrical creatures found in mud and among weeds upon the sea-bottom; their mouth-parts are evidently intended for piercing and sucking, but whether they are parasitic at certain periods on other animals is not exactly known. Anthura, Paranthura, Cruregens.

FIG. 82.—Gnathia maxillaris. =A=, Segmented larva, × 10; =B=, Praniza larva, × 5; =C=, gravid female, × 5; =D=, male, × 5. ]

=Fam. 2. Gnathiidae.=—These forms appear to be related to the Anthuridae; they are ectoparasitic on various kinds of fish during larval life, but on assuming the adult state they do not feed any more, subsisting merely on the nourishment amassed during the larval periods. The larvae themselves are continually leaving their hosts, and can be taken in great numbers living freely among weeds on the sea-bottom. The larvae, together with the adults of Gnathia maxillaris, are extremely abundant among the roots of the sea-weed Poseidonia cavolinii in the Bay of Naples. The young larvae hatch out from the body of the female in the state shown in Fig. 82, A. This minute larva fixes upon a fish, and after a time it is transformed into the so-called Praniza larva (B), in which the gut is so distended with the fluid sucked from the host that the segmentation in the hind part of the thorax is entirely lost. When this larva moults it may, however, reacquire temporarily its segmentation. After a certain period of this parasitic mode of life the Praniza finally abandons its host, and becomes transformed into the adult male or female. This may take place at very different stages in the growth of the larva, the range of variation in size of the adults being 1–8 mm., and it must be remembered that when once the adult condition is assumed growth entirely ceases. What it is that determines the stage of growth in each individual when it shall be transformed into the adult is not known. The males and females differ from one another so extraordinarily that it was for long denied that they were both derived from the Praniza larvae. This is nevertheless the case. The change from the Praniza to the female (Fig. 82, C) is not very great. The ovary absorbs all the nourishment in the gut and comes to occupy the whole of the body, all the other organs degenerating, including the alimentary canal and mouth-parts. Indeed, only the limbs with their muscles and the nervous system remain. The change to the male (D) is more radical. The food is here stored in the liver, which increases in the male just as the ovary does in the female. The segmentation is reacquired, and the massive square head is formed from the hinder part of the head in the Praniza, the anterior portion with its stylet-like appendages being thrown away. The powerful nippers of the male are not formed inside the cases of the old styliform mandibles, but are independent and possibly not homologous organs. The meaning of the marked sexual dimorphism and the use of the males’ nippers are not in the least known, though the animals are easy to keep under observation. In captivity the males never take the slightest notice of either larval or adult females.

=Fam. 3. Cymothoidae.=—This is a group of parasites more completely parasitic than the foregoing, but their outer organisation does not differ greatly from an ordinary Isopodan form. A great many very similar species are known which infest the gill-chambers, mouths, and skin of various fishes. The chief interest that attaches to them is found in the fact that a number of them, and perhaps all, are hermaphrodite, each individual acting as a male when free-swimming and young, and then subsequently settling down and becoming female. This condition is exactly the same as that occurring universally in the great group of parasitic Isopoda, the Epicarida, to be considered later. There is no evidence that the Cymothoidae are phyletically related to the Epicarida, so that the similar sexual organisation appears to be due to convergence resulting from similar conditions of life. The general question of hermaphroditism in the Crustacea has been shortly discussed on pp. 105–106. Cymothoa.

=Fam. 4. Cirolanidae.=—In this family is placed the largest Isopod known—the deep-sea Bathynomus giganteus, found in the Gulf of Mexico and the Indian Ocean, sometimes measuring a foot long by four inches broad. A common small littoral form is Cirolana.

=Fam. 5. Serolidae.=—The genus Serolis comprises flattened forms bearing a curious resemblance to Trilobites, which Milne Edwards considered more than superficial. The genus is confined to the littoral and deep waters of the southern hemisphere.

=Fam. 6. Sphaeromidae.=—These are flattened, broad-bodied forms, most commonly met with in the Mediterranean and warmer seas. Without being actually parasitic, they are frequently found as scavengers in decaying material, and they show some relationship to the parasitic Cymothoidae. In some of the genera, e.g. Cymodoce, the ovigerous female shows a degenerate condition of the mouth-parts, while the maxillipedes undergo an enlargement, and are used for causing a current through the brood-chamber.

=Sub-Order 3. Valvifera.=

FIG. 83.—Munnopsis typica (Munnopsidae), ♂, × 2. A, 2nd antenna; Ab, abdomen; T, 5th thoracic appendage or 4th leg. (After Sars.) ]

The Valvifera, illustrated by the =Idotheidae= and =Arcturidae=, are characterised by the uropods being turned back and expanded to form folding doors covering up the delicate pleopods, which are mostly respiratory in function, though the anterior pairs may serve as swimming organs. Arcturus is a typically deep sea genus, many species, remarkably furnished with spiny processes, having been taken by the Challenger in the southern hemisphere. The Idotheidae are more littoral forms, several species of Idothea being commonly met with off the British coasts, occasionally penetrating into brackish or even fresh water.

=Sub-Order 4. Asellota.=

In this group the abdominal segments are fused dorsally to form a shield-like caudal region; the pleopods are respiratory in function and reduced in numbers, the first pair being often expanded and produced backwards to form an operculum covering the rest. Several of the Asellota are fresh-water, Asellus aquaticus (=Asellidae=) being extremely abundant all over Europe in weed-grown ditches, the mud of slowly-moving streams, and even on the shores of large lakes. They are mostly sluggish in habit, but the marine =Munnopsidae= (Fig. 83, Munnopsis) are expert swimmers, the swimming organs being fashioned by the expansion and elongation of the thoracic legs.

=Sub-Order 5. Oniscoida.=

The Oniscoida are terrestrial forms in which the abdomen is fully segmented, the pleopods are respiratory, their endopodites being delicate branchiae, while their exopodites are plate-like and form protective opercula for the gills, and the uropods are biramous and not expanded. The epimera of the segments are greatly produced. The terrestrial Isopods, although air-breathers, are dependent on moisture, and are only found in damp situations. It seems probable that they have been derived from marine Isopods, since the more generalised of them, e.g., Ligia (Fig. 84), common on the English coasts, are only found in damp caves and crannies in the rocks.

FIG. 84.—Ligia oceanica, ventral and dorsal views, × 1. (From original drawings prepared for Professor Weldon.) ]

The related Ligidium is found far inland, but always in the neighbourhood of water. These two genera may be distinguished by the numerous joints in the flagellum of the second antennae, the flagellum being in all cases the portion of the antenna succeeding the long fifth joint. Philoscia muscorum occurs usually near the coast, but it is also found inland in England under trees in damp moss. This genus and the common Oniscus, found in woods, are distinguished by the presence of three joints in the flagellum of the second antenna. Philoscia can be distinguished from Oniscus by its narrower body and the pretty marbled appearance of its back. The genus Trichoniscus has four joints in the flagellum; various species are found in woods. In Porcellio and Armadillidium there are only two joints in the flagellum, while Armadillidium, the common garden wood-louse, can be distinguished from all others by the flattened shape of the uropods, and the habit of rolling up into a ball like an Armadillo.

There is also a very peculiar species, Platyarthrus hoffmannseggii, which occurs in England and Northern Europe, and always lives in ants’ nests. It is supposed that they serve as scavengers for the ants, which tend them carefully, and evidently treat them as domestic animals of some kind. The small creature is quite white and blind, and has exceedingly short antennae.

=Sub-Order 6. Epicarida.=

The Epicarida include an immense number of Isopods, parasitic upon other Crustacea. In the adult state they become greatly deformed, and offer very few characters of classificatory value, but they all pass through certain highly characteristic larval stages which are essentially similar in the different families. All the species are protandric hermaphrodites, each individual being male while in a larval state, and then losing its male organisation and becoming female as the parasitic habit is assumed.

Two series of families are recognised according to the larval stages passed through, the =Cryptoniscina=, in which the adult male organisation is assumed in the Cryptoniscus stage, and the female condition is imposed directly upon this form, and the =Bopyrina=, in which the Cryptoniscus passes into a further larval stage, the Bopyrus, which performs the function of the male, and upon which the female organisation is imposed as the parasitic habit is assumed.

The following is a list of the Epicarida with the Crustacea which serve as their hosts:—

Cryptoniscina │Microniscidae │on Copepoda. „ │Cryptoniscidae │on Ostracoda. „ │Liriopsidae │on Rhizocephala. „ │Hemioniscidae │on Cirripedia. „ │Cabiropsidae │on Isopoda. „ │Podasconidae │on Amphipoda. „ │Asconiscidae │on Schizopoda.

Bopyrina │Dajidae │on Decapoda „ │Phryxidae │ „ „ │Bopyridae │ „ „ │Entoniscidae │ „

FIG. 85.—Epicaridian larva, probably belonging to one of the Cryptoniscina. A, 2nd antenna; Ab, abdominal appendages; T, thoracic appendages. (From Bonnier, after Hansen.) ]

In all cases the first larval form which hatches out from the maternal brood-pouch is called the Epicaridian larva (Fig. 85).

This little larva has two pairs of antennae, a pair of curious frontal processes, and a pair of mandibles. The other mouth-parts are missing; there are only six thoracic limbs, but the full complement of six biramous pleopods are present, and at the end of the body there may be a long tube of unknown function.

As a type of the =Cryptoniscina= we may take the =Liriopsidae=, parasitic on the Rhizocephala, which are, of course, themselves parasitic on the Decapoda, the whole association forming a very remarkable study in Carcinology.

Almost every species of the Rhizocephala is subject to the attacks of Liriopsids, the latter fixing either on the Rhizocephala themselves, or else on the Decapod host at a point near the fixation of the Rhizocephalous parasite. An exceedingly common Liriopsid is Danalia curvata, parasitic on Sacculina neglecta, which is itself parasitic on the spider-crab, Inachus mauritanicus, at Naples. The adult Danalia is a mere curved bag full of eggs or developing embryos, and without any other recognisable organs except two pairs of spermathecae upon the ventral surface where the spermatozoa derived from the larval males are stored.

FIG. 86.—Inachus mauritanicus, ♀, × 1, carrying two Sacculina neglecta (a, b), and a Danalia curvata (c), the latter bearing two dwarf males. ]

In Fig. 86 is represented a female of Inachus mauritanicus which carried upon it two Sacculinae and a Danalia curvata, and upon the latter are seen two minute larval males in the act of fertilising the adult Danalia. The eggs develop into the Epicaridian stage, after which the larva passes into the Cryptoniscus stage (Fig. 87). In this larval form the segments are clearly delimited; the only mouth-parts present are the mandibles, but there are seven pairs of thoracic limbs and the full number of pleopods. This Cryptoniscus stage is found in all the Epicarida, and only differs in detail in the various families.

FIG. 87.—Ventral view of Cryptoniscus larva of Danalia curvata, ♂, × 25. ]

In the Cryptoniscina the Cryptoniscus larva is the male, and at this stage possesses a pair of large testes in the thorax. The ovaries are also present at this stage as very small bodies applied to the anterior ends of the testes. The larval males in this state seek out adult fixed Danaliae and fertilise them; and, when this is accomplished, they themselves become fixed to the host and begin to develop into the adult female condition. The limbs are all lost, and out of the mouth grows a long proboscis (Fig. 88, P), which penetrates the tissues of the host. The ovaries begin to grow, and a remarkable process of absorption in the testes takes place. These organs, when fixation occurs, are never empty of spermatozoa, and are frequently crammed with them. After fixation some large cells at the interior borders of the testes begin to feed upon the remains of these organs and to grow enormously in size and to multiply by amitosis. These phagocytes, as they really are, attain an enormous size, but they are doomed to degeneration, the chromatin becoming dispersed through the cytoplasm, and the nuclei dividing first by amitosis and then breaking up and disappearing. As the parasite grows, the heart at the posterior end of the body ceases to beat; the ovaries increase enormously at the expense of the alimentary canal, and on the ventral surface two pairs of spermathecae are invaginated ready to receive the spermatozoa of a larval male. In the adult condition, after fertilisation has taken place and the ovaries occupy almost the whole of the body, the remains of the phagocytic cells can be seen on the dorsal surface in a degenerate state. They evidently are not used as food, and their sole function is to make away with the male organisation when it has become useless.

FIG. 88.—Side view of Danalia curvata, × 15, shortly after fixation and loss of larval appendages. A, Alimentary canal; E, eye; H, heart; N, phagocytic cells; O, ovary; P, proboscis. ]

FIG. 89.—Optical section (dorsal view) of Danalia curvata, in the same stage as Fig. 88. A, Alimentary canal; Ec, ectoderm; H, heart; N, phagocytic cells; O, ovaries; P, proboscis. ]

In the series =Bopyrina=, after the free-living Epicaridian and Cryptoniscus stages, a further larval state is assumed, called the Bopyrus, which is the functional male, and, after performing this function, passes on to the adult female condition.

The family =Bopyridae= is parasitic in the branchial chamber of Decapoda, especially Macrura and Anomura. When one of these Decapods is infested with an adult Bopyrid the gill-chamber in which it is situated is greatly swollen, as shown in Fig. 90. A very common Bopyrid is Bopyrus fougerouxi, parasitic in the gill-chambers of Palaemon serratus. The Bopyrus larva or functional male has the appearance shown in Fig. 91. It differs from the Cryptoniscus stage in possessing a rudimentary pair of anterior thoracic limbs and seven pairs normally developed, while the abdominal limbs are plate-like and branchial in function. The male can often be found attached to the female beneath the last pair of incubatory lamellae.

FIG. 90.—Galathea intermedia, with a Pleurocrypta microbranchiata under its left branchiostegite (B), × 1. (After Sars.) ]

FIG. 91.—Ventral view of male Bopyrus fougerouxi, × 30. A, 1st and 2nd antennae; T, 8th (last) thoracic appendage. (After Bonnier.) ]

The adult female condition, which is assumed after the Bopyrid stage is passed through, is illustrated in Fig. 92. The body acquires a remarkable asymmetry, due to the unequal pressure exerted by the walls of the gill-chamber. The antennae and mandibles (Fig. 92, B) are entirely covered up by the largely expanded maxillipedes; maxillae are, as usual, entirely absent. Very large lamellae grow out from the bases of the thoracic limbs to form a brood-pouch, and in this manner the adult condition is attained.

The final complication in the life-histories of these Isopoda is reached by the family =Entoniscidae=, which are parasitic when adult inside the thoracic cavity of Brachyura and Paguridae. The cephalothorax of a Carcinus maenas, which contains an adult Portunion maenadis (P), is shown in Fig. 93. The parasite is of a reddish colour when alive.

FIG. 92.—Bopyrus fougerouxi. =A=, Ventral view of female carrying a male (M) between her abdominal appendages, × 8; =B=, ventral view of part of head of female, the maxillipedes and the left mandible having been removed. A.1, A.2, 1st and 2nd antennae; M, male; Mn, right mandible; Mx, left maxillipede; O, oostegite; T, left 4th thoracic appendage or 3rd leg. (After Bonnier.) ]

FIG. 93.—Cephalothorax of Carcinus maenas, seen from the ventral side, containing a parasitic Portunion maenadis (P), × ½. (After Bonnier.) ]

The Entoniscidae pass through a free living Epicaridian and Cryptoniscus stage, and become adult males in the Bopyrus stage. It is stated, however, by Giard and Bonnier that these individuals, which actually function as males, never grow up into adult females, though all the adult females have passed through a male stage in which the male genital ducts are not formed. The hermaphroditism, therefore, in these animals at any rate is absolutely useless from a reproductive point of view, and this justifies our looking for some other explanation of it, such as was suggested on p. 105.

FIG. 94.—Portunion maenadis, ♀:—=A=, Young, × 10; =B=, older, × 5; =C=, adult, before the eggs are laid, × 3. A, 2nd antenna; Ab, abdomen; B, anterior lobe of brood-pouch; B′, its lateral lobe; H, head; 1, 2, 1st and 2nd incubatory lamellae (oostegites). (After Giard and Bonnier.) ]

The Bopyrus fixes in the gill-chamber of the host and becomes converted into the adult female by a series of transformations. As these changes take place it invaginates the wall of the gill-chamber and pushes its way into the thoracic cavity of the crab, though it lies all the time enveloped in the invaginated wall of the gill-chamber, and not free in the body-cavity of the crab. The transformations which it undergoes are shown in Fig. 94. The body first assumes a grub-like appearance (A), and two pairs of incubatory lamellae (1, 2) grow out from the first and second thoracic segments. In the next stage (B) these lamellae assume gigantic proportions, and four pairs of branchiae grow out from the abdominal segments (Ab). In the final stage (C) the incubatory lamellae have further increased in size, and constitute the main bulk of the body; the enormous mass of eggs is passed into the incubatory pouch, and all that remains of the rest of the body is the small head (H) and the abdomen (Ab), furnished with its branchiae. Communication with the external world is kept up through an aperture which leads from the brood-pouch into the gill-chamber of the host, and through this aperture the young are hatched out when they are developed sufficiently.

The presence of these parasites, although they are never in actual contact with the internal organs of the crab, calls forth the same phenomenon of parasitic castration as was observed in the Rhizocephala. A remarkable association is also found to exist between the Entoniscidae and Rhizocephala, of such a kind that, on the whole, a crab infested with a Rhizocephalan is more likely to harbour an Entoniscid than one without. The explanation of this association is probably that a crab with a Sacculina inside it is prevented from moulting as often as an uninfected crab, and, in consequence, the larval stages of the Entoniscid in the crab’s gill-chamber are more safely passed through.

=Sub-Order 7. Phreatoicidea.=

The members of this sub-order, although agreeing with the Isopoda in the essentials of their anatomy, resemble the Amphipoda in being rather laterally compressed, and in having the hand of the first free thoracic limb enlarged and subchelate. The abdomen is greatly produced laterally by expansions of the segments. In fact, the shape of the body and of the limbs is very Amphipodan.—Phreatoicus from New Zealand, Southern Australia, and Tasmania. Phreatoicopsis, a very large form from Gippsland, Victoria. Only one family exists, =Phreatoicidae=.

=Order IV. Amphipoda.=

In this order the body is flattened laterally, the heart is anterior in position, and the branchial organs are attached to the thoracic limbs.

There are three well defined sub-orders, (i.) the Crevettina, including a vast assemblage of very similar animals, of which the common Gammarus and Orchestia may serve as examples; (ii.) the Laemodipoda or Caprellids, and (iii.) the Hyperina.

We cannot do more than touch on the organisation of these sub-orders.

=Sub-Order 1. Crevettina.=

In this sub-order only one thoracic segment is fused with the head; the basal joints of the thoracic limbs are expanded to form broad lateral plates, and the abdomen is well developed, with six pairs of pleopods, the last three pairs being always turned backwards, and stiffened to act as uropods.

This group has numerous fresh-water representatives, e.g. Gammarus of several species, the blind well-shrimp Niphargus, and the S. American Hyalella; but the vast majority of the species are marine, and are found especially in the littoral zone wherever the rocks are covered with a rich growth of algae, Polyzoa, etc. The Talitridae or “Sand-hoppers” have deserted the waters and live entirely in the sand and under rocks on the shore, and one common European species, Orchestia gammarellus, penetrates far inland, and may be found in gardens where the soil is moist many miles from the sea.

The Rev. T. R. R. Stebbing, in his standard work on this group, recognises forty-one families, and more than 1000 species, so that we can only mention a few of the families, many of which, indeed, differ from one another in small characters.

=Fam. Lysianassidae.=—The first joint of the first antenna is short, with an accessory flagellum. Mandible with a palp, and with an almost smooth cutting edge. The third joint of the second gnathopod is elongated. This family is entirely marine, comprising forty-eight genera, with species distributed in all seas. One genus, Pseudalibrotus, inhabits the brackish water of the Caspian Sea. Lysianassa has several common British and Mediterranean species.

=Fam. Haustoriidae.=—The members of this family are specially adapted for burrowing, the joints of the hinder thoracic limbs being expanded, and furnished with spines for digging. Some of the species are common on the British coasts, e.g. Haustorius arenarius. Pontoporeia has an interesting distribution, one species, P. femorata, being entirely marine, in the Arctic and North Atlantic, P. affinis inhabiting the Atlantic, and also fresh-water lakes in Europe and North America, P. microphthalma being confined to the Caspian Sea, and P. loyi to Lakes Superior and Michigan.

=Fam. Gammaridae.=—Includes fifty-two genera. The first antennae are slender, with the accessory flagellum very variable. The mandibles have a dentate cutting edge, spine-row, and molar surface, and a three-jointed palp. The first two thoracic limbs are subchelate. This family includes a few marine, but mostly brackish and fresh-water species. Crangonyx is entirely subterranean in habitat, as is Niphargus, N. forelii occurring, however, in the deep waters of Lake Geneva. Both these genera are blind. Gammarus has thirty species, G. locusta being the common species on the North Atlantic coasts, and G. pulex the common fresh-water species of streams and lakes in Europe. A number of Gammaridae inhabit the Caspian Sea, e.g. Boeckia, Gmelina, Niphargoides, etc., while the enormous Gammarid fauna of Lake Baikal, constituting numerous genera, showing a great variety of structure, some of them being blind, belong to this family, e.g. Macrohectopus (Constantia), Acanthogammarus, Heterogammarus, etc.

FIG. 95.—Gammarus locusta, ♂ (above) and ♀ (below), × 4. Abd.1, First abdominal segment; T, telson; Th, seventh free thoracic segment (= 8th thoracic segment); U, third uropod. (After Della Valle.) ]

=Fam. Talitridae.=—This family may be distinguished by the absence of a palp on the mandible, and by one ramus of the uropods being very small or wanting. The various kinds of “Sand-hoppers” belong here, familiar creatures on every sandy coast between tide-marks. The genera Talitrus and Talorchestia always frequent sand, while Orchestia is generally found under stones and among weed. Some species of Orchestia, e.g. O. gammarellus, live inland in moist places at some distance from the sea; one species of Talitrus (T. sylvaticus) occurs at great elevations in forests in Southern Australia.

Hyale is a coastal genus, and is also found on floating objects in the Sargasso Sea. Hyalella is confined to Lake Titicaca and the fresh waters of South America. Chiltonia from S. Australasia.

=Fam. Corophiidae.=—The members of this family have a rather flattened body and small abdomen, and the side-plates on the thorax are small. The uropods are also small and weak. Some species of the genus Corophium are characteristic of the Caspian Sea.

=Sub-Order 2. Laemodipoda.=

FIG. 96.—Caprella grandimana, × 4. a, Abdomen; g, gills; t, 3rd (first free) thoracic segment; t′, 8th thoracic segment. (After P. Mayer.) ]

=Fam. 1. Caprellidae= are also chiefly littoral forms, swarming among rocks covered by algae, though they are by no means so easy to detect as the Gammaridae and Tanaidae which haunt similar situations. In a basinful of algae or Polyzoa taken from the rocks fringing the Bay of Naples, the latter are easily collected, the Tanaidae always crawling out of the weeds in the direction of the light, while the Gammarids dart about in all directions; but the Caprellidae, with their branching stick-like forms, harmonise so well with their surroundings that it requires an experienced eye to detect them. The body is elongated and thin, resembling that of a stick-insect. The first two thoracic segments are more or less completely fused with the head; the second and third thoracic limbs end in claws; the two following thoracic limbs are normal in the genus Proto, rudimentary in Protella, and absent in the remaining genera, though their gills remain as conspicuous flabellate structures. The three hind legs are normal, and the abdomen is reduced to a tiny wart at the hind end of the greatly elongated thorax.

P. Mayer has described cases of external hermaphroditism as being fairly common in certain species, e.g. Caprella acutifrons, and this is interesting if we take into consideration the frequent partial hermaphroditism exhibited by the gonad of Orchestia at certain times of year (see p. 104).

=Fam. 2. Cyamidae.=—These are closely related to the Caprellidae in the form of the limbs and the reduced state of the abdomen. Cyamus ceti, which lives ectoparasitically on the skin of whales, has the body expanded laterally instead of being elongated, as in the Caprellids.

=Sub-Order 3. Hyperina.=

FIG. 97.—Phronima sedentaria, ♀, in a Pyrosoma colony, × 1. (After Claus, from Gerstaecker and Ortmann.) ]

These are an equally distinct and curious group of Amphipods, characterised by the large size of the head and the transparency of the body. Instead of haunting the littoral zone they are pelagic in habit, and many of them live inside transparent pelagic Molluscs, Tunicates, or Jellyfish. A well known form is Phronima sedentaria, which inhabits the glassy barrel-like cases of the Tunicate Pyrosoma in the Mediterranean. The female is often taken in the plankton together with her brood in one of these curious glass houses; the zooids of the Pyrosoma colony are completely eaten away and the external surface of the case, instead of being rough with the tentacles of the zooids, is worn to a smooth, glass-like surface. It has been observed that the female actively navigates her house upon the surface of the sea; she clings on with her thoracic legs inside, while the abdomen is pushed out through an opening of the Pyrosoma case behind, and by its alternate flexion and extension drives the boat forwards, the water being thus made to enter at the front aperture and supply the female and her brood with nourishment.

DIVISION 3. HOPLOCARIDA.

The carapace leaves at least four of the thoracic somites distinct. The eyes are pedunculate. The mandibles are without a lacinia mobilis; there are no oostegites, the eggs being carried in a chamber formed by the maxillipedes. The hepatic caeca are much ramified, the heart is greatly elongated, stretching through thorax and abdomen, with a pair of ostia in each segment. The spermatozoa are spherical, and there is a complicated and peculiar metamorphosis.

=Order. Stomatopoda.=

FIG. 98.—Lateral view of Squilla sp., × 1. A.1, A.2, 1st and 2nd antennae; Ab.1, 1st abdominal segment; Ab.6, 6th abdominal appendage; C, cephalothorax, consisting of the head fused with the first five thoracic segments; E, eye; M, 2nd maxillipede; T, telson. (After Gerstaecker and Ortmann.) ]

The Stomatopoda are rather large animals, occasionally reaching a foot in length, all of which exhibit a very similar structure; Squilla mantis and S. desmaresti are found on the south coast of England not very frequently; but they are very common in the Mediterranean, living in holes or in the sand within the littoral zone of shallow water. They differ from all the other Malacostraca by a combination of characters, and Calman proposes the term HOPLOCARIDA for a division equivalent to the Peracarida, Eucarida, etc.

The abdomen is very broad and well developed, ending in a widely expanded telson. There is a carapace which covers the four anterior thoracic segments, leaving the four posterior segments free. The portion of the head carrying the stalked eyes constitutes an apparently separate segment articulated to the head. The antennae, mandibles, and maxillae are normal; there then follow five pairs of uniramous thoracic limbs turned forwards as maxillipedes and ending in claws; the second pair of these is modified into a huge raptorial arm, exactly resembling that of a Praying Mantis (cf. vol. v. p. 242), by means of which the Squilla seizes its prey. The last three thoracic limbs are small and biramous. The pleopods are powerful, flattened, biramous swimming organs with small hooks or “retinaculae” upon their endopodites, which link together each member of a pair in the middle, and with large branching gills upon the exopodites.

The internal anatomy exhibits several primitive features. The nervous system is not at all concentrated, there being a separate ganglion for each segment; and the heart stretches right through thorax and abdomen, with a pair of ostia in each segment. There are also ten hepatic diverticula given off segmentally from the intestine.

The female has the curious habit of carrying the developing eggs in a chamber improvised by the apposition of the maxillipedes, so that it looks rather as if she were in the act of devouring her own brood.

The metamorphosis of the larvae, despite the work of Claus and Brooks, is not very accurately known, especially uncertain being the identification of the different larvae with their adult forms. The chief interest consists in the fact that certain of the anterior thoracic limbs develop in their normal order and degenerate, to be reformed later, just as in the Phyllosoma larva of the Loricata (see pp. 165, 166).

In one series of larvae, probably not of Squilla itself, but of related genera, the young hatch out as “Erichthoidina” (Fig. 99), with the thoracic appendages developed as biramous organs as far as the fifth pair, and with a single abdominal pair of limbs.

FIG. 99.—Erichthoidina larva of a Stomatopod, with five pairs of maxillipedes, and the first pair of abdominal appendages, × 10. (From Balfour, after Claus.) ]

The abdominal series of limbs is next completed; the second thoracic limb assumes its adult raptorial structure, but the succeeding three limbs become greatly reduced and may entirely degenerate, leaving the posterior six thoracic segments without limbs.

Usually the anterior three pairs are only reduced, and then redevelop side by side with the small posterior limbs as they appear. This larva is then termed the “Erichthus” (Fig. 100); but when they completely disappear the larva is called a “Pseudozoaea,” owing to its resemblance to the Zoaea stage of the Decapoda, which is also characterised by the suppressed development of the thoracic segments.

FIG. 100.—Older Erichthus larva, with six pairs of abdominal appendages, × 15. (From Balfour, after Claus.) ]

The so-called “Alima” larva of Squilla is also a Pseudozoaea, but it is apparently arrived at directly without the previous formation and degeneration of the anterior thoracic limbs, the larva hatching out from the egg in the Pseudozoaeal stage.

=Fam. Squillidae.=—Of the six known genera none extend into the cold subarctic seas; the majority are characteristic of the warm or tropical seas (Gonodactylus), some of the species having very wide ranges, e.g. G. chiragra, which is completely circumtropical, and appears to have entered the Mediterranean at some period, though it is very rare there.