Page:The New International Encyclopædia 1st ed. v. 05.djvu/725

* CRUSTACEA. 629 CRUSTACEA. covers the wliole body, forming a coiuiilu.x e.KO- skcletoii, euk-ari'oiis and internicdiatf bftwoon shell and bone in Uie hif;her forms, wliile in tin; lower and smaller kimls it consists principally of chitin. and corresponds more nearly in its nature with the integmnents of insects. It is secreted l)y the epidennis on its outer side, and serves not merely for protection, but al.so 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. The.se pigment-cells arc 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. In all cases, however, where the skeleton covers a joint it is thin and free from inineral 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 central core of epidermis. In barnacles (q.v. ), ostracods, water-iieas (q.v. ), and a few other crustaceans, there is. in addition to the e.x- 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 inclosing 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, some 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 Molting. — The crustaeeous cov- ering has beneath it a true skin, and. like the epidennis, 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 : but 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. Locomolive and Other OryuiDi. — The principal organ of locomotion in many t'rustacca, as in the lol)sler. shrimp, etc., is the abdonu>n. terminating in fan-like ap]iendagcs: by bending the abdomen suddenly down under the body, the animal darts backward 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 'swinuuing-feet' or pleo- pods attached to the abdomen, often very dillcr- ent from the thoracic legs. The legs of some are fitted for burrowing. The first pair of legs is not infrequently transformed into a pair of pow- erful pincers or 'claws,' the last joint biit 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 antcnnules : following these are the an- tenn.T. also sense-organs; then come a pair of powerful jaws, the mandibles ; and back of these are two pairs of accessory jaws, the maxillse. The thoracic 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 lultercles 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 ocelli 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 liave otocysts or posi- tional organs, but it is doubtful whether any have the sense of hearing. In some Sch'izopoda there are also 'accessorj' eyes' on the basal points of certain thoracic feet and in the middle line of the abdomen. The gills are 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 tlirough 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. P.EPRonucTioJf AXD Kcoi.ooY. 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 pleopcKls or other ap]>endages. Except in a few cases the develop- ment takes place by metamorphosis, through a verv remarkable series of larval stages. All the