STRUCTURE. The Crustacea derive their name from the hard armor which in most of them covers the whole body, forming a. complex exo skeleton, calcareous and intermediate between shell and bone in the higher forms, while in the lower and smaller kinds it consists principally of chitin, and corresponds more nearly in its nature with the integuments of insects. It is secreted by the epidermis on its outer side, and serves tint merely for protection, but also for the attachment of the muscles. its color is dependent upon the absence, or presence and amount, of pig ment, which is also a product of the epidermis. and is formed during the growth of the skeleton. These pigment-cells are often very ornamental. especially under the microscope, as their shape is beautifully and very irregularly stellate. Usu ally, however, the pigment is so abundant as to make the body-wall opaque and the individual pigment-cells are no longer visible. The mineral matter of the skeleton is chiefly carbonate of lime. The smallest crustaceans have little or none in the skeleton, but in all of the larger forms it is present in greater or less abundance. In the so called 'stone-crabs' so much is present that the skeleton is really as hard as a rock. lit all eases, however. where the skeleton covers a joint it. is thin and free from mineral matter. thus permit ting perfect freedom of movement. The bristles and hairs often found on the outside of the skele ton are of the same essential structure as it, and have a ventral core of epidermis. In barnacles (q.v.), ostracods, water-fleas (q.v.), and a few other crustaceans, there is, in addition to the ex ternal skeleton, and formed as an outgrowth of it, a true shell, whose only function is protection. This shell may consist of several pieces, as in bar nacles, or be strictly bivalve, as in water-fleas, and it may be wholly chitinous or very firmly cal cified. Sometimes it is capable of inelosing the entire animal. but often it only seems to protect certain parts. It is usually hinged along the dor sal side of the animal, where it is attached to the skeleton, and is supplied with muscles for closing it.
The crustacean body is composed of segments, sonic very distinct, while others coalesce or are consolidated. of which the thorax of a crab af fords an excellent example. The first five rings are regarded as forming the head, the next eight the thorax. when that part of the body is separa ble from what follows.
Epidermis and Nolting.—The crustaceous cov ering has beneath it a true skin, and. like the epidermis, is cast off from time to time and sup plied anew. as the growth of the animal requires more room for the internal parts. In this molt ing 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 mem branes which connect the hard plates are split and torn. A period of apparent sickness pre• cedes and agitation accompanies the process: and the thick muscular parts of the limbs of crabs and lobsters become soft and flaccid, so as to be more easily extricated from their hard covering. The loss of a limb, which sometimes takes place in this process, and is otherwise a frequent occurrence, is easily repaired. for a new one grows in its stead: hut it is a curious cir cumstance that in order to this reproduction the limb must be broken off at a particular joint, the second from the body, thus leaving only a short stump: and when a limb is broken else where, the animal itself exercises the remarkable power of throwing it off by this joint.
Laconialire and Other Organs.—The principal organ of locomotion in many Crustacea, as in the lobster. shrimp. etc., is the abdomen, terminating in fan-like appendages: by bending the abdomen suddenly down under the body, the darts backward in the \rater. The limbs, which ate 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. Sonic have 'swimming-fret' or pleo
pods attached to the abdomen, often very differ ent from the thoracic legs. The legs of some aro fitted for burrowing. The first pair of legs ic not infrequently transformed into a pair of pow erful pincers or 'claws,' the last joint but one being prolonged 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 first pair of appendages are organs of touch called antennules: following these are the an tenna, also sense-organs: then come a pair of powerful jaws, the mandibles; and back of these are two pairs of accessory jaws. the maxilhe. The thoracie feet, which follow in regular order. are sometimes modified to function as jaws, and are then called maxillipeds. In some forms the mouth-parts are greatly modified to form suck ing instead of masticating organs. The digestive organs are very simple: there is a short but capacious gullet, a large stomach, and a straight and simple intestinal tube. A well-developed digestive gland called the liver is often present. The pyloric region of the stomach is sometimes furnished with a remarkable apparatus of hard tubercles or sharp teeth for grinding or tearing food. Many of the Crustacea feed on animal food, and are very voracious: many, however. feed on vegetable food. The nervous system of crustaceans agrees generally with that of other arthropods, and exhibits many gradations of con centration. The eyes are either simple (ocelli). aggregate (consisting of several oeelli under a common cornea), or compound: and the com pound eyes are often elevated on stalks. Besides eyes some crustaceans have ciliated pits or cavi ties that seem to be sense-organs. perhaps for smell or taste. Others have otoeysts or posi tional organs. hut it is doubtful whether any have the sense of hearing. In sonic Schlzopoda there are also 'accessory eyes' on the basal points of certain thoracic feet and in the middle line of the abdomen. The gills arc variously placed, on the sides of the body, or on the thoracic limbs, on the abdominal legs, etc. The heart is always in the middle line of the body on the dorsal side, is of variable 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.
REennoucrioN AND ECOLOGY. The sexes are distinct in most Crustacea, and all are ovipa rous. The eggs are almost invariably hatched in water, even those of terrestrial forms. and a sort of incubation often takes place. as the eggs are carried about under the abdomen or thorax of the female, attached to the pleopods or other appendages. Except in a few cases the develop ment takes place by metamorphosis. through a very remarkable series of larval stages. All the lower Crust:leen hostel as minute, oval, unseg mented creatures with three pairs of appendages, and this larva is called a nauplins. Although the higher Crustacea hatch as a more highly or ganized form called 'zoea,' and later pass through an lot larval form. the •megalops,' all Ira through a nauplins stage in the egg. See BARNACLE for illustration of •natiplins;' and see CRAB for illustrations of `zuea' and 'megalops.' More than 10.000 species of living Crustaeea are known. of which the greater number are marine; some inhabit fresh waters, running or stagnant ; vompa rat ively few are terrestrial. \lany exhibit a high degree of intelligence. The Crustacea constitute, in an economic sense, per haps the most important group of invertebrates. The myriads of the smaller forms that drift about the ocean and the great lakes furnish the greater part of the fare of the important food fishes. and are thus indirectly of value to man kind. In the economy of the ocean itself Crus tacea are also of great importance, for they act as the natural scavengers of the sea.