CRUSTACEANS, Crustacea, a class of articulated animals, agreeing with insects, arachnida, and myriapoda in having articulated limbs; but differing from them in important respects, and particularly from all of them in the adaptation of the organs of respiration to an aquatic life, even those of them which live on land being generally inhabitants of damp places, and breathing by a kind of gills. Sonic of the lowest and minute aquatic C., indeed, are not provided with gills; but the aeration of the blood is supposed to take place through the surface of the body.
The C. derive their name from the hard armor which in most of them covers the whole body, forming for it that sort of frame-work which is sometimes, not very cor rectly, called an external skeleton; and which, in those of highest organization, is very complex in its structure, and contains a large amount of calcareous matter—carbonate and phosphate of lime, so that it is in its substance intermediate between shell and bone —,-whilst in many of the lower and smaller kinds it consists principally of chime, and corresponds more nearly in its nature with the integuments of insects. The body of a crustacean is composed of rings (see ARTICULATA), generally 21 in number, and the crustaceous covering corresponds with it in this respect; the rings, alike of the body and its armor, being in some cases very distinct, whilst in others some of them in a great measure coalesce or are consolidated together, of which the thorax of a crab affords an excellent example. The first seven rings are regarded as forming the head, the next seven, the thorax; and the remainder the abdomen, corresponding with the head, thorax, and abdomen of insects. The crustaceous covering is considered as a peculiar epidermis, having beneath it a true skin, from which it is an inorganic exudation; and, like the epidermis, it is cast off from time to time, that its place may be supplied anew, as the growth of the animal requires more room for the internal soft parts. ]n this moulting, or casting of the shell, the animal divests itself of its covering not in separate parts, but in one piece, including the coverings of the limbs, and even of the antenna;, although the membranes which connect the hard plates are split and torn. A period of apparent sickness precedes and agitation accompanies the process; and the thick mus cular parts of the limbs of crabs and lobsters become soft and flaccid, so as to be much more easily extricated from their hard coverings. The loss of a limb, which sometimes takes place on such an occasion, and is otherwise a frequent occurrence, is easily repaired, for a new one grows in its stead; but it is a curious circumstance that in order to this reproduction, the limb must ho broken off at a particular joint, the second from the body, thus leaving only a short stump; and when a limb is broken elsewhere, the animal itself exercises the remarkable power of throwing it off by this joint.
The principal organ of locomotion in many C., as In the lobster, shrimp, etc., is the abdomen, terminating in fan-like appelidages; by betiding the abdomen suddenly down under the thorax, they dart backwards in the water. The limbs—which are connected with the thoracic rings—are, in some, organs of swimming; those of others are used for walking at the bottom of the water or on dry ground. Some have what arc called false
legs or pro-legs attached to the abdomen, often very different from the thoracic legs. The legs of some are fitted for burrowing. The first pair of legs is not unfrequently transformed into a pair of powerful claws or pincers—the last joint but one being pro longed so as to oppose the last joint, which becomes attached as to the side of it; and these are used for seizing and tearing food. The limbs of the first thoracic rings are, in many organs still more intimately connected with the mouth, and have received the. name of foot-jaws, the transition from the true mandibles and maxilla: to the organs of locomotion being often very gradual. The mouth of some small parasitic C. is, how ever, formed for sucking, and not for tearing and masticating food. The digestive organs are very simple in all; there is a short but capacious gullet, a large stomach, and a straight and simply intestinal tube. The pyloric region of the stomach, however, is furnished with a remarkable apparatus of hard tubercles or sharp teeth for grinding or tearing food, supplementary to the external organs of the mouth. Almost all of the C. feed on animal food, and they are very voracious. A few feed on vegetable food. The nervous system of C. agrees generally with that of insects, and exhibits many grada tions of division and concentration. C. in general, appear to possess all the five senses. Their eyes are either simple (stemmatic), aggregate (consisting of several stemmata under a common cornea), or compound. The compound eyes are often on foot-stalks. The gills are variously placed; in the internal cavity, under the enlargement of the plate of a single rim., which covers the thoracic rings in crabs, etc.; on the tho racic limbs; on the abdominal or false legs, etc. The heart is always in the middle line of the body, is of various form, and distributes the blood by a number of trunks through the system; but the blood returns to venous sinuses, from which, and not from the heart, it is sent into the gills, and it is not until after its aeration in the gills that it comes to the heart again; not, however, without being mixed with venous blood which has not undergone the sante aeration. The sexes are distinct in most of the C.; and they are all oviparous. A sort of incubation of the eggs takes place, in order to which they are carried under the abdomen or under the thorax of the female, attached to the false legs or to sonic of the thoracic appendages. It has recently been discovered, con trary to former belief, that C.—or at least many of them—undergo metamorphoses; and the curious creatures known by the name wea have been found to be the young of crabs.
The greater number of C. are marine: some inhabit fresh waters, running or stag nant; comparatively few are terrestrial.
Cuvier divided C. into two sections, malacodraca and entamostraca; the former sec tion containing the orders decapoda, stomapoda, lcemodipoda,amphipoda, and isopoda; the latter the branehiapoda and pcecilapoda. Another division has been more recently pro posed by Milne Edwards, and very generally adopted into ayphosura (the genus limulus alone), having a mouth destitute of jaws, and for which legs perform the office of jaws; maralosa, or masticating C.; and edentata, or suctorial crustaceans.