Chapter IV.), it is sometimes known as the "nauplius eye." The female

_Cyclops_ carries her eggs until they hatch, in two oval packets attached to the sides of the body.

Forming a separate order (BRANCHIURA) apart from the more normal Copepoda (order EUCOPEPODA) is the little group of the Carp-lice, one of which, _Argulus foliaceus_, is common in England, living as a parasite on different species of fresh-water fish, and often found swimming free in ponds and rivers. It has a broad, flat, and very transparent body, about three-sixteenths of an inch in length. It differs from _Cyclops_ in a great many points, of which, perhaps, the most conspicuous is the possession of a pair of true compound eyes in addition to the median eye. On the under-side of the head are a pair of large round suckers, by means of which the animal fixes itself on to its prey. A study of their development shows that these suckers are really the maxillæ, which in the young animal are jointed limbs ending in a strong claw, but later become changed into the suckers of the adult. A sharp spine, which can be protruded in front of the mouth, is connected with what is believed to be a poison-gland. The eggs are not carried in packets by the female as in _Cyclops_, but are deposited on stones or water-weeds.

The fourth subclass, CIRRIPEDIA, comprises the Barnacles and Acorn-shells. These are very unlike any of the other Crustacea, and, in fact, they were long classed by naturalists with the Mollusca. It was not until their larval development was made known that they were recognized as Crustacea. The common Goose Barnacle (_Lepas anatifera_--Plate III.) is found adhering to the bottoms of ships and to floating timber. It has a fleshy stalk or peduncle which is fixed at one end to the supporting object, and bears at the other end a shell, made up of five separate plates, enclosing the body of the animal. The stalk corresponds to the front part of the head, and careful examination may discover at its end, among the hardened cement which fixes it to the support, the remains of the antennules by which the attachment of the young animal was first effected. The body of the animal within the carapace or shell bears the usual mandibles, maxillulæ, and maxillæ, close to the mouth, and six pairs of long, tendril-like feet. These feet have each two branches, composed of numerous short segments and fringed with long hairs. They can be protruded from the slit-like opening of the shell, forming a sort of "casting-net" for the capture of minute floating prey.

The Acorn-shells, of which one species (_Balanus balanoides_--Plate III.) is abundant everywhere on our coasts, covering rocks and stones just below high-water mark, differ from _Lepas_ and its allies in having no peduncle. The shell is cemented directly to the rock, and is conical in shape, like a small limpet, with a hole at the top which is closed by four movable valves.

The Stalked Barnacles, like _Lepas_ (suborder _Pedunculata_), and the Sessile Barnacles, or Acorn-shells, like _Balanus_ (suborder _Operculata_), together form the order THORACICA. Of the other orders which compose the subclass Cirripedia, the only one that need be mentioned here is the RHIZOCEPHALA, which comprises strangely degenerate parasites living on other Crustacea.

The Cirripedia are unlike nearly all other Crustacea in the fact that, with few exceptions, they are hermaphrodite, having both sexes united in each individual. In certain species of the Stalked Barnacles, however, there are minute male individuals that are attached, like parasites, to the large hermaphrodites. In a few species the large individuals only possess female organs, so that the separation of the sexes is complete.

The remarkable larval metamorphoses of Cirripedes and the modifications of structure presented by some parasitic forms will be described in later chapters.

The fifth and last subclass, that of the MALACOSTRACA, is by far the largest and most important, and will require to be considered in more detail than any of the others. The animals composing the various orders into which the subclass is divided differ very greatly in structure, but they all agree in having typically the same number of appendages as the Lobster--namely, nineteen pairs (or twenty, if the eye-stalks be included). They also agree in the very important character that the trunk limbs are divided into two sets, thoracic and abdominal, the former of eight, and the latter of six pairs.

The first order of the Malacostraca, the NEBALIACEA, comprises a few Crustacea of small size, which differ in some very important characters from all the other orders. _Nebalia bipes_ (Fig. 15), which occurs on the southern coasts of the British Isles, has a large bivalved carapace enclosing most of the limbs. In front, a small "rostral plate" is joined to the carapace by a movable hinge, and partly covers the stalked eyes. The eight pairs of thoracic feet are all alike, and are flattened and leaf-like in form, resembling those of the Branchiopoda. The first four pairs of abdominal limbs are large two-branched swimming feet, but the last two pairs are reduced to small vestiges. Two of the most important points in which the Nebaliacea differ from all the other Malacostraca are that there are seven instead of six somites in the abdomen (the last somite has no appendages), and that the telson has connected with it a pair of movable rods forming a "caudal fork" like that of the Branchiopoda. On account of the leaf-like thoracic feet and the possession of a caudal fork and other features, the Nebaliacea were formerly classified with the Branchiopoda, but a closer examination of their structure has shown that they are true Malacostraca. In having an additional somite in the abdomen and in other points, however, they may be regarded as forming a link between the Malacostraca and the lower forms of Crustacea, and for this reason they are set apart as a series LEPTOSTRACA, while the other orders form a series EUMALACOSTRACA.

The orders of the Eumalacostraca, again, are grouped, as shown in the table of classification, into four divisions. The first of these, the SYNCARIDA, includes only one order, comprising a few small Crustacea (see Fig. 84, p. 264) which have recently been discovered in fresh water in Tasmania and Australia. They have no carapace, and all the thoracic somites, or all but the first, are distinct. The antennules are two-branched, the antennæ may have a scale-like exopodite, and the last pair of abdominal appendages form, with the telson, a tail-fan. The eyes are sometimes stalked, but in one species they are sessile. The thoracic limbs, which are not clearly divided into maxillipeds and legs, carry a double series of plate-like gills or epipodites. As will be shown later, the living Syncarida are especially interesting on account of their resemblance to certain very ancient fossil Crustacea.

The second division of the Eumalacostraca, the PERACARIDA, includes five orders, the members of which differ very greatly in appearance. They all agree, however, in certain important points of structure, of which the most conspicuous is the possession, in the female sex, of a brood-pouch for carrying the eggs and young. This brood-pouch is formed by a series of overlapping plates attached to the bases of the thoracic limbs.

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 antennæ 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.

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 assists respiration, causing by its movements a current of water to flow under the carapace.

The members of the second order of the Peracarida, the CUMACEA (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.

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 (chelæ). 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.

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 antennæ 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.

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.

Two families of the Amphipoda differ so much in general appearance from the others that they deserve mention. The Caprellidæ (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 _Cyamidæ_, or "Whale-lice" (Fig. 23), are, as the name implies, parasites on the skin of whales, and are closely related to the Caprellidæ. 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.

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 antennæ 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 class. 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 illustrated 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 classified (in the arrangement adopted here) in two suborders, which are again subdivided into sections and tribes.

ORDER DECAPODA.

Suborder NATANTIA - { Tribe Penæidea. { " Stenopidea. { " Caridea.

" REPTANTIA.

Section _Palinura_ - { " Scyllaridea. { " Eryonidea.

" _Astacura_ - " Nephropsidea.

" _Anomura_ - { " Galatheidea. { " Thalassinidea. { " 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 _Penæidea_ include the large Prawns of tropical seas (_Penæus_--Plate IV.), which have the first three pairs of legs provided with chelæ, and not differing greatly in size. The _Stenopidea_ are a small group of forms resembling the _Penæidea_ in having chelæ 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.

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 sex. 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.

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.

The third section of the Reptantia, the _Anomura_, comprises forms in which the abdomen is variously modified, being either bent upon itself or, if extended, more or less soft and feebly armoured. The last pair of legs are commonly reduced in size, and not used in walking. The members of the four tribes composing the section differ widely in their general appearance.

The _Galatheidea_ (Plate VI.) are small, flattened, lobster-like animals which have the abdomen bent under the body. In one family (_Porcellanidæ_) the animals have quite the appearance of little Crabs (see Fig. 41, p. 113), but they may be distinguished from the true Crabs (Brachyura) by the fact that there are only three pairs of walking legs behind the great chelæ, the last pair of legs being very small and carried folded up at the sides of the body, or even within the gill chambers.

The _Thalassinidea_ are small lobster-like animals which burrow in sand and mud, and have generally a more or less soft abdomen (see Fig. 38, p. 103).

The tribe _Paguridea_ includes the Hermit Crabs (_Paguridæ_) and their allies. The typical Hermit Crabs (Plate VII.), which are familiar objects in seaside rock-pools, live in the empty shells of Whelks and other Gasteropod Molluscs, which they carry about with them as portable shelters. The structure of the animals is modified in adaptation to this curious habit. The abdomen, which is protected during life by the borrowed shell, is soft and unarmoured, and is spirally twisted. The swimmerets, which have only the function of carrying the eggs in the female, are much reduced, and are usually present only on one side of the body. The uropods no longer form a tail-fan, but are adapted for firmly wedging the hind part of the body into the coils of the shell. One of the chelipeds is much larger than the other, and serves to block up the opening when the animal withdraws into its shelter. In tropical countries certain Hermit Crabs (_Coenobitidæ_) have become adapted to a life on land, and one of these, the well-known Coconut Crab, or Robber Crab (_Birgus latro_), which is the largest species of the tribe, has given up the habit of protecting itself with a shell, and its abdomen has again acquired a strong armour on the upper side. The marine _Lithodidæ_--to which the British Stone Crab, _Lithodes maia_ (Plate VIII.) belongs--seem at first sight to have little resemblance to the Hermit Crabs, for they have the abdomen very small, and tucked up under the body as in the true Crabs. Like the Porcellanidæ, mentioned above, however, the Lithodidæ have only three pairs of walking legs behind the chelipeds, the last pair being feeble and usually folded out of sight within the gill chambers. The relationship of the Lithodidæ to the Hermit Crabs is shown by the abdomen, which is more or less twisted to one side, and has swimmerets only on one side in the female, and quite wanting in the male.

The _Hippidea_ are curious little Crabs found burrowing in sandy beaches in the warmer seas. They have the abdomen tucked under the body, and the legs flattened for shovelling the sand.

The BRACHYURA, or true Crabs, form the fourth section of the Reptantia, and are distinguished by having the abdomen reduced to a tail-flap, which is doubled up under the cephalothorax, and is usually without any trace of the uropods which are present in all the groups already mentioned, with the single exception of the Lithodidæ. At the sides of the head the side-plates of the carapace become firmly soldered to the "epistome," a plate which lies in front of the mouth, and in this way there is formed the "mouth-frame," within which lie the jaws, covered in by a pair of "folding-doors" formed by the flattened third maxillipeds.

The first tribe of the Brachyura, the _Dromiacea_, comprises a number of Crabs that in many points of structure resemble the Lobsters, and are regarded as the most primitive members of the section. _Dromia vulgaris_ (Plate IX.), a furry, clumsy-looking Crab, occasionally found on our southern coasts, has the last two pairs of legs short and carried up over the back, where they are used for holding a mass of living sponge which the Crab uses as a cloak to protect and conceal itself. At the sides of the abdomen, wedged in between the telson and the last somite, a pair of small plates may be seen, which are the last vestiges of the uropods. These are wanting in the other tribes of the Brachyura.

The _Oxystomata_ (Plate X.), which form the second tribe of the Brachyura, are distinguished by the form of the mouth-frame, which is narrowed in front so as to be triangular instead of square in outline. The passages through which the water passes out from the gills, which in other Crabs open at the front corners of the mouth-frame, are carried forwards to the front of the head. The Oxystomata are most abundant in tropical seas, but are represented on the British coasts by species of _Ebalia_, small and compact Crabs which are not unlike pebbles of the gravel among which they live.

The remaining Crabs form the tribe _Brachygnatha_, in which the mouth-frame and the maxillipeds that close it are more or less quadrilateral in shape. The tribe is divided into two subtribes, which may be recognized by the general shape of the carapace. In the _Brachyrhyncha_ this is generally rounded or square-cut in front, without a projecting rostrum. In this subtribe are included the great majority of Crabs. The Edible Crab and the Shore Crab (Plate IX.) are familiar examples. In the _Oxyrhyncha_, on the other hand, the carapace is generally narrowed in front, with a projecting rostrum, either simple or forked, and is often armed with spines. In this subtribe are included the long-legged Spider Crabs, several species of which are common on our coasts. The Giant Spider Crab of Japan (Plate XI.) is the largest of living Crustacea.

The last division of the Eumalacostraca, the HOPLOCARIDA (Plate XII.), is one of very small extent, comprising only a single order (_Stomatopoda_) of very remarkable Crustacea which are common in tropical seas, and of which at least one species, _Squilla desmarestii_, is occasionally captured on the south coast of England. The Stomatopoda are prawn-like Crustaceans, usually with a flattened body, and are easily recognized by the form of the large claws (the second pair of thoracic limbs), in which the last segment shuts down, like the blade of a pocket-knife, on the preceding segment, and forms a very efficient weapon, so that the larger species are not to be handled without caution. The resemblance of these claws to those of the mantis-insect of Southern Europe led to a common Mediterranean species receiving long ago the name _Squilla mantis_ (Plate XII.).

The Stomatopoda have a small carapace, which does not cover the last four thoracic somites, and has in front a small flattened rostrum, attached by a movable hinge, like that of the Leptostraca. The eyes are stalked, and, like the antennules, are attached to a separate movable segment of the front part of the head--a peculiarity not found in any other Crustacea. There are small plate-like gills attached to the bases of some of the thoracic limbs, but the chief organs of respiration are large feathery gills attached to the pleopods or swimmerets.

The Stomatopoda are all found in the sea, generally in shallow water, burrowing in sand or hiding in crevices of rocks or corals. Some species are more than a foot in length.