The first order of the Peracarida, the MYSIDACEA, consists of small, free-swimming, shrimp-like animals (Fig. 16). Many species are common in the sea round the British coasts, and from their possession of a brood-pouch, in which the young are carried, they are sometimes known as "Opossum Shrimps." The eyes are stalked, and the carapace is well developed, although it does not unite with all the thoracic somites. The antennae have a flattened, scale-like exopodite, probably of use for keeping the animal balanced in swimming. Only one pair of the thoracic limbs are modified to form maxillipeds, and all the legs (as in the larval Lobster) have exopodites which form the chief swimming organs.

The uropods and telson form a "tail-fan." One of the most curious points in the organization of some Mysidacea is the possession of a pair of statocysts in the endopodites of the uropods. Each statocyst consists of a small cavity containing a cake-shaped concretion known as a "statolith," resting on a group of sensory hairs. There is reason to believe that these organs have the same function as the statocysts of the Lobster, although they are placed at the other end of the body. The statolith serves the same purpose as the sand-grains found in the Lobster's statocyst, although, unlike these, it is not introduced from outside, but is formed in position by secretion from the walls of the sac.

[Ill.u.s.tration: FIG. 17--_Gnathophausia willemoesii_, ONE OF THE DEEP-SEA MYSIDACEA. HALF NATURAL SIZE. (From Lankester's "Treatise on Zoology,"

after Sars.)

_gr_, A groove dividing the last abdominal somite]

Most of the Mysidacea have no special organs of respiration, that function being discharged (as in many of the smaller Crustacea) by the general surface of the body, and especially by the thin carapace; but certain deep-sea Mysidacea (Fig. 17) have tufted gills attached at the base of the thoracic legs. In all cases the maxilliped has a plate-like epipodite, which lies under the side-fold of the carapace, and no doubt a.s.sists respiration, causing by its movements a current of water to flow under the carapace.

[Ill.u.s.tration: FIG. 18--_Diastylis goodsiri_, ONE OF THE c.u.mACEA.

ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.)

_a'_, Antennule; _l_1-_l_5, the five pairs of walking legs; _m_, brood-pouch; _ps_, "pseudo-rostrum" formed by lateral plates of the carapace; _t_, telson; _ur_, uropods]

The members of the second order of the Peracarida, the c.u.mACEA (Fig.

18), are small marine Crustacea in which the anterior part of the body is generally stout, while the abdomen is slender and very mobile. The short carapace does not cover more than the first three or four of the thoracic somites. The eyes are not stalked, and are usually fused together to form a single organ on the front part of the head. Swimming branches (exopodites) are usually present on some of the thoracic legs, at least in the males, which are more active swimmers than the females.

In the males, also, the swimmerets of the abdomen are often more or less developed, but they are always absent in the females. The uropods do not form a tail-fan, but are slender forked rods carrying comb-like rows of spines, said to be used in cleaning the anterior appendages from the mud among which these animals generally live. The telson is often absent, or, rather, it is coalesced with the last somite of the abdomen. Under the side-fold of the carapace on each side lies, as in the Mysidacea, the epipodite of the maxilliped; but in this order it forms a gill, and usually carries a row of flattened gill lobes.

[Ill.u.s.tration: FIG. 19--_Apseudes spinosus_, ONE OF THE TANAIDACEA.

ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.)

_ex_, Vestiges of exopodites on second and third thoracic limbs; _oc_, the small and immovable eye-stalks; _sc_, scale or exopodite of antenna; _ur_, uropod]

The third Order, that of the TANAIDACEA (Fig. 19), is of special interest, since in many respects it forms a transition to the next. It comprises a number of minute Crustacea, generally found burrowing in mud in the sea. They have a small carapace, which only involves the first two thoracic somites, the rest of the somites being distinct. The side-folds of the carapace enclose a pair of small cavities, within which lie, as in the case of the last two orders, the epipodites of the maxillipeds. The eyes are not movable, although they are set on little side-lobes of the head, representing the vestiges of eye-stalks. The first pair of thoracic limbs are maxillipeds, and the second pair are very large, and form pincer-claws (chelae). Minute vestiges of exopodites are sometimes found on the second and third pairs, but they are not used for swimming, and only help to keep a current of water flowing through the gill cavities. The abdomen is very short, with small swimmerets, and the telson is not separated from the last somite. The uropods are generally very small, and do not form a tail-fan.

[Ill.u.s.tration: FIG. 20--A WOODLOUSE (_Porcellio scaber_), ONE OF THE ISOPODA. ENLARGED. (From Lankester's "Treatise on Zoology," after Sars.)]

Unlike the Tanaidacea, the ISOPODA, which form the fourth order of the Peracarida, are very numerous in species, and very varied in structure and habits. The most familiar are the Woodlice, or Slaters, which are commonly found in damp places, under stones and the like. Besides these, however, the order includes a vast number of forms living in the sea and a few that live in fresh water. The examination of a common Woodlouse, such as _Oniscus_ or _Porcellio_ (Fig. 20), will give a general idea of the form and structure of a typical Isopod, although many curious modifications are found, some of which will be mentioned in later chapters.

There is no distinct carapace, but the last vestige of one may be indicated by the fact that the first thoracic somite is completely fused with the head. All the other somites of the body are distinct (in some Isopods, however, the abdominal somites are coalesced), but the telson is not separate from the last somite. The eyes are not stalked, but are sessile on the sides of the head. The antennules have only a single branch, and in the Woodlice are very small. The antennae have no exopodite, although in a few other Isopods a minute vestige is present.

The thoracic limbs never have any trace of exopodites. The first pair are maxillipeds, and if they carry an epipodite it is never enclosed in a gill cavity, as in Tanaidacea. The swimmerets form one of the most characteristic features of the Isopoda, for they are always flattened into thin plates, which act as gills. In the Woodlice, which breathe air, certain curious modifications of the swimmerets are found, which will be described in a later chapter. In some Isopods that live as parasites on fish or on other Crustacea, each individual is at first a male, and later becomes a female. They are almost the only Crustacea, except the Cirripedes already mentioned, which are normally hermaphrodite.

[Ill.u.s.tration: FIG. 21--AN AMPHIPOD (_Gammarus locusta_). ENLARGED.

(From Lankester's "Treatise on Zoology," after Sars.)

_a'_, Antennule; _a''_, antenna; _acc_, accessory (inner) flagellum of antennule; _br_, gill plate; _cx_, c.o.xal plate (the expanded first segment of the leg); _gn_, the two pairs of gnathopods (prehensile legs); _plp'''_, abdominal appendage of third pair; _prp'_, _prp''_, first and second peraeopods, or walking-legs; _t_, telson; _ur_, uropod; II, VIII, second and eighth thoracic somites; 1, 6, first and sixth abdominal somites]

The fifth order of the Peracarida, the AMPHIPODA, is also a very large one. The "Sand-hoppers," which are very common on sandy coasts, belong to this order, as do also a very large number of other forms found in the sea and in fresh water, which have no popular names. A common species is _Gammarus pulex_, sometimes called the "Fresh-water Shrimp,"

which is found everywhere in streams and ditches. Several closely allied species, such as _G. locusta_ (Fig. 21), are found in the sea. The body is flattened from side to side, and the abdomen is generally bent upon itself. There is no carapace, but, as in the Isopods, the first thoracic somite is fused with the head. The eyes are sessile on the sides of the head. The antennules have a small inner branch, and the antenna have no exopodites. The thoracic limbs, of which the first pair form maxillipeds, have no exopodites, and are partly hidden by a row of shield-like plates along the sides of the thorax. These plates are formed by the enlarged and flattened basal segments of the limbs themselves, and on the inner side they carry a series of oval plates, which are the gills. The abdominal appendages are divided into two sets: the first three pairs have each two slender, many-jointed branches, and are used in swimming; the last three pairs are short, stiff, and directed backwards, and are used in pushing the animal through mud or among water-weeds. In many Amphipods, such as the Sand-hoppers, the last three pairs of abdominal limbs are used in jumping by sudden backward strokes of the abdomen.

[Ill.u.s.tration: FIG. 22--TWO SPECIES OF CAPRELLIDae. (From Lankester's "Treatise on Zoology," after Sars.)

A, _Phtisica marina_, a species which retains the fourth and fifth pairs of thoracic limbs (_prp'_, _prp''_); B, _Caprella linearis_, in which these limbs are represented only by the gills (_br_). (Enlarged.) _a'_, Antennule; _a''_, antenna; _abd_, vestigial abdomen; _gn_, gnathopods; _m_, brood-pouch; IV, V, fourth and fifth thoracic somites]

[Ill.u.s.tration: FIG. 23--_Paracyamus boopis_, THE WHALE-LOUSE OF THE HUMPBACK WHALE. (From Lankester's "Treatise on Zoology," after Sars.)

A, Male, dorsal view, enlarged; B, the maxillipeds detached and further enlarged. _a'_, Antennule; _a''_, antenna; _abd_, vestigial abdomen; _br_, gills; _gn_, gnathopods; IV, V, fourth and fifth thoracic somites]

Two families of the Amphipoda differ so much in general appearance from the others that they deserve mention. The Caprellidae (Fig. 22) have the body drawn out to a thread-like slenderness, and the abdomen reduced to a mere vestige. The fourth and fifth pairs of thoracic limbs are generally absent, though the corresponding gills remain. The animals live in the sea, clambering among sea-weeds or zoophytes in a fashion which recalls the movements of "looper" caterpillars. The _Cyamidae_, or "Whale-lice" (Fig. 23), are, as the name implies, parasites on the skin of whales, and are closely related to the Caprellidae. They have, however, a broad, flattened body, more like that of an Isopod than an ordinary Amphipod, and their legs have strong curved claws with which they cling to the skin of their host.

[Ill.u.s.tration: FIG. 24--_Meganyctiphanes norvegica_, ONE OF THE EUPHAUSIACEA. TWICE NATURAL SIZE. (From Lankester's "Treatise on Zoology.")]

The third division of the Malacostraca, the EUCARIDA, consists of two orders of very unequal interest and importance. The first of these, the order EUPHAUSIACEA (Fig. 24), comprises only a single family of small, shrimp-like Crustacea found swimming freely at the surface or in the depths of the sea. In these the carapace fuses with all the thoracic somites, the eyes are stalked, the antennules have two flagella, and the antennae have a broad scale. None of the thoracic limbs are modified into maxillipeds, and all carry swimming exopodites. The uropods and telson form a tail-fan. A single series of feathery gills are attached to the bases of the thoracic limbs. Nearly all the Euphausiacea possess the power of emitting light, and are furnished for the purpose with a number of organs which were formerly supposed to be "accessory eyes."

The second order of the Eucarida, the DECAPODA, is by far the largest of the orders of Crustacea, and it includes all the larger and more familiar members of the cla.s.s. It is necessary, therefore, to give a considerably fuller account of its subdivisions than has been given in the case of the other orders. The typical characters of the Decapoda are well ill.u.s.trated by the Lobster, which has been already described. As in the Euphausiacea, the eyes are stalked, and the carapace fuses with all the thoracic somites. From the Euphausiacea the Decapoda differ in the fact that three pairs of the thoracic limbs are modified as maxillipeds, the remaining five pairs forming the "ten legs" to which the name of the order alludes. Further, the gills are arranged in more than one series, not all attached to the bases of the legs, as in the Euphausiacea, and covered over by the side-flaps of the carapace instead of being freely exposed. While agreeing in these essential characters, however, the members of the order Decapoda differ very widely among themselves in structure and in general form, and they are cla.s.sified (in the arrangement adopted here) in two suborders, which are again subdivided into sections and tribes.

ORDER DECAPODA.

Suborder NATANTIA - { Tribe Penaeidea.

{ " Stenopidea.

{ " Caridea.

" REPTANTIA.

Section _Palinura_ - { " Scyllaridea.

{ " Eryonidea.

" _Astacura_ - " Nephropsidea.

" _Anomura_ - { " Galatheidea.

{ " Thala.s.sinidea.

{ " Paguridea.

{ " Hippidea.

" _Brachyura_ - { " Dromiacea.

{ " Oxystomata.

{ " Brachygnatha.

Subtribe Brachyrhyncha.

" Oxyrhyncha.

The suborder NATANTIA includes the numerous species of what are commonly known as Prawns and Shrimps. These are characteristically powerful swimmers, with lightly armoured bodies, more or less flattened from side to side, with a thin, saw-edged rostrum, and with large swimmerets which are the chief organs of swimming; in addition, some of the more primitive Natantia have swimming branches, or exopodites, like those of the Euphausiacea, on the thoracic legs. This suborder is divided into three tribes. The _Penaeidea_ include the large Prawns of tropical seas (_Penaeus_--Plate IV.), which have the first three pairs of legs provided with chelae, and not differing greatly in size. The _Stenopidea_ are a small group of forms resembling the _Penaeidea_ in having chelae on the first three pairs of legs, but the third pair are much larger than the others. The _Caridea_ comprise our common Prawns (_Leander, Pandalus_) and Shrimps (_Crangon_), besides a host of less generally known forms; in these the third legs are never chelate, although the first and second often are.

[Ill.u.s.tration: _PLATE IV_

_Penaeus caramote_ FROM THE MEDITERRANEAN. (ABOUT HALF NATURAL SIZE)

(_From Brit. Mus. Guide_)]

The second suborder, that of the REPTANTIA, is much more diversified, but the animals composing it are united by certain characteristics, of which the most obvious are their creeping habits (although some species can swim well), their heavily armoured bodies, often more or less flattened from above downwards, with the rostrum never thin and saw-edged, and the swimmerets not used to any great extent for swimming.

The first section of the Reptantia, the _Palinura_, includes the Spiny Lobsters, Rock Lobsters, or Sea-Crawfish, and their allies, forming the tribe _Scyllaridea_. They are distinguished by having no large pincer-claws, though the last pair of legs may have small pincers in the female s.e.x. One species, the Common Spiny Lobster (Plate V.), is found on the southern and western coasts of the British Islands. The other tribe belonging to this section is the _Eryonidea_, comprising a number of small lobster-like forms living in the deep sea. They have pincer-claws on the first four, or on all five, pairs of legs, and they are of special interest on account of their geological antiquity.

[Ill.u.s.tration: _PLATE V_

THE COMMON SPINY LOBSTER (_Palinurus vulgaris_). (MUCH REDUCED)

(_From Brit. Mus. Guide_)]

The section _Astacura_ contains only a single tribe, _Nephropsidea_, formed by the true Lobsters and the fresh-water Crayfishes. They have pincer-claws on the first three pairs of legs, and the first pair are much larger than the others.