" MALACOSTRACA.

Series _LEPTOSTRACA_ - " Nebaliacea.

" _EUMALACOSTRACA._

Division _Syncarida_ - " Anaspidacea.

" _Peracarida_ - { " Mysidacea.

{ " c.u.macea.

{ " Tanaidacea.

{ " Isopoda.

{ " Amphipoda.

" _Eucarida_ - { " Euphausiacea.

{ " Decapoda.

" _Hoplocarida_ - " Stomatopoda.

[Ill.u.s.tration: FIG. 10--THE "FAIRY SHRIMP" (_Chirocephalus diapha.n.u.s_), MALE. 2. (After Baird.)]

Occasionally there may be found in rain-water puddles and the like, in the South of England, a beautiful, transparent, shrimp-like animal, an inch or more in length, to which the name of "Fairy Shrimp" has been given (Fig. 10). It is known in technical zoology as _Chirocephalus diapha.n.u.s_, and is a representative of the subcla.s.s BRANCHIOPODA. The members of this group are distinguished from other Crustacea by their flattened, leaf-like feet, each of which is divided into a number of lobes, and has a gill plate on the outer side. In _Chirocephalus_ there is no carapace, and the head is followed by eleven distinct body segments, each bearing a pair of leaf-like, or rather fin-like, feet.

The hinder part of the body has no appendages, and ends in a forked tail. In the female a large pouch hangs from the under-side of the body, just behind the limb-bearing part, and is often found filled with eggs.

In the male, a pair of remarkable-looking appendages, each shaped somewhat like a hand with webbed fingers, hang in front of the head.

These are connected with the antennae, and are known as the "claspers,"

from their function in seizing and holding the female. The eyes are set on movable stalks. Those Branchiopoda which, like _Chirocephalus_, have no carapace, form the order ANOSTRACA.

[Ill.u.s.tration: _PLATE II_

_Apus cancriformis_, FROM KIRKCUDBRIGHTSHIRE. (SLIGHTLY ENLARGED)]

A second order, the NOTOSTRACA, is represented by _Apus cancriformis_ (Plate II.), which occurs in many places in Europe in ponds and puddles, and very rarely indeed in Britain. In _Apus_ there is a large dorsal shield, or carapace, covering the greater part of the body, which consists of a large number of segments (about twenty-eight), and ends behind in a pair of long antenna-like filaments. The fin-like feet are also very numerous (about sixty-three pairs). The eyes are not stalked, but are set close together on the upper surface of the carapace.

[Ill.u.s.tration: FIG. 11--_Estheria obliqua_, ONE OF THE CONCHOSTRACA.

(After Sars, from Lankester's "Treatise on Zoology.")

A, Sh.e.l.l of female, from the side; B, male, from the side, after removal of one valve of the sh.e.l.l. (Enlarged.) _a'_, Antennule; _a''_, antenna; _ad_, muscle which draws together the valves of the sh.e.l.l; _f_, tail fork; _md_, mandible]

The third order of the Branchiopoda, the CONCHOSTRACA (Fig. 11), are not represented in Britain, though several species occur on the Continent of Europe. In these the carapace forms a bivalved sh.e.l.l, completely enclosing the body and limbs, and closely resembling that of a small Mollusc.

[Ill.u.s.tration: FIG. 12--_Daphnia pulex_, A COMMON SPECIES OF "WATER-FLEA." MUCH ENLARGED. (From British Museum Guide.)

Female carrying eggs in the brood-chamber]

The fourth order, the CLADOCERA, comprises the so-called "Water-fleas,"

which are abundant everywhere in ponds and lakes (Fig. 12). They are all of small size, almost or quite microscopic. The carapace, as in the Conchostraca, forms a bivalved sh.e.l.l, but does not enclose the head.

There is a single large eye, which really corresponds to two eyes fused together. A pair of large antennae, each with two branches, carrying long feathered hairs, project at the sides of the head, and are used in swimming with a peculiar jumping motion, from which the popular name of the animals is derived. There are not more than six pairs of feet. The "Water-fleas," of which _Daphnia pulex_ is one of the commonest species, are very beautiful and interesting objects for microscopic examination, on account of their transparency, which allows many details of their internal structure to be studied in the living animal.

[Ill.u.s.tration: FIG. 13--Sh.e.l.lS OF OSTRACODA. MUCH ENLARGED. (From Lankester's "Treatise on Zoology," after Brady and Norman, and Muller.)

A, _Philomedes brenda_ (Myodocopa); B, _Cypris fuscata_ (Podocopa); C, _Cythereis ornata_ (Podocopa). _n_, Notch characteristic of the Myodocopa; _e_, the median eye; _a_, mark of attachment of the muscle connecting the two valves of the sh.e.l.l. A and C are marine species; B is from fresh water]

The OSTRACODA (Fig. 13), which form the second subcla.s.s in the system of cla.s.sification here adopted, are nearly all microscopic animals, and are found abundantly in fresh water as well as in the sea. The carapace forms a bivalved sh.e.l.l, which completely encloses the body and limbs, and is often sculptured in an elegant fashion. The Ostracoda are remarkable for the very small number of their appendages. There are not more than two pairs of limbs behind the maxilla. Most of the species are included in two orders, the _Myodocopa_ and the _Podocopa_, of which the former may generally be distinguished by a notch in the anterior part of the margin of the sh.e.l.l (Fig. 13, A, _n_). In the _Podocopa_ the margin is entire.

[Ill.u.s.tration: FIG. 14--_Cyclops albidus_, A SPECIES OF COPEPOD FOUND IN FRESH WATER. (After Schmeil.)

Female specimen carrying a pair of egg-packets. The actual length is about one tenth of an inch]

The subcla.s.s COPEPODA comprises animals, for the most part of microscopic size, which are abundant in fresh water and in the sea. The common fresh-water genus _Cyclops_ (Fig. 14) furnishes a good example of the type of structure characteristic of the cla.s.s. The body is somewhat pear-shaped, with a narrow abdomen ending in a "caudal fork." The body is divided into somites, and there is no overlapping carapace, although the head and the first two thoracic somites are coalesced. There are four pairs of two-branched, oar-like, swimming feet, and a fifth pair, found in some other Copepoda, is represented in _Cyclops_ by minute vestiges on the first segment of the narrow posterior part of the body.

The antennules are very large, unbranched and composed of numerous segments; the antennae are much smaller. In addition to the usual mandibles, maxillulae, and maxillae, there is a pair of maxillipeds which really represent the first pair of trunk limbs. There is a single red eye in the middle of the front of the head. This eye is not formed, like the single eye of the Cladocera, by fusion of a pair of eyes, but it corresponds to a median eye of simple structure which is found in the Branchiopoda, Ostracoda, and many other Crustacea, in addition to the paired compound eyes. From the fact that this median eye is the only one present in the earliest larval stage of Crustacea, the Nauplius (see 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 maxillae, 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 subcla.s.s, CIRRIPEDIA, comprises the Barnacles and Acorn-sh.e.l.ls. These are very unlike any of the other Crustacea, and, in fact, they were long cla.s.sed 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 sh.e.l.l, 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 sh.e.l.l bears the usual mandibles, maxillulae, and maxillae, 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 sh.e.l.l, forming a sort of "casting-net" for the capture of minute floating prey.

The Acorn-sh.e.l.ls, of which one species (_Bala.n.u.s 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 sh.e.l.l 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.

[Ill.u.s.tration: _PLATE III_

GROUP OF SPECIMENS OF THE GOOSE-BARNACLE (_Lepas anatifera_), ONE SHOWING THE CIRRI EXTENDED AS IN LIFE. (NATURAL SIZE)

(_From Brit. Mus. Guide_)

GROUP OF A COMMON SPECIES OF ACORN-Sh.e.l.l OR ROCK BARNACLE (_Bala.n.u.s balanoides_)(NATURAL SIZE)]

The Stalked Barnacles, like _Lepas_ (suborder _Pedunculata_), and the Sessile Barnacles, or Acorn-sh.e.l.ls, like _Bala.n.u.s_ (suborder _Operculata_), together form the order THORACICA. Of the other orders which compose the subcla.s.s 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 s.e.xes 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 s.e.xes 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 subcla.s.s, 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 subcla.s.s 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.

[Ill.u.s.tration: FIG. 15--_Nebalia bipes._ ENLARGED. (From Lankester's "Treatise on Zoology," after Claus.)

_a'_, Antennule; _a''_, antenna; _ab_1-_ab_6, the abdominal limbs; _ad_, muscle joining the two valves of the sh.e.l.l; _f_, tail-fork; _p_, palp of maxillula; _r_, rostral plate; _t_, telson; 1-7, the seven somites of the abdomen]

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 cla.s.sified 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 cla.s.sification, 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 antennae 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 s.e.x, 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.

[Ill.u.s.tration: FIG. 16--_Mysis relicta_, ONE OF THE MYSIDACEA. ENLARGED.

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

_cs_, Cervical groove of the carapace; _m_, brood-pouch]