CRUSTACEA, Icrtis-ti'shd-a, a primary group (phylum) of animals represented by the barnacle, lobster, crayfish, shrimp and crab. Cnistacea differ from other arthropod animals. The body consists of about 20 segments which in the more specialized forms are grouped into two regions, the head-thorax (cephalothorax) and hind-body or abdomen. The segments of the cephalothorax are fused together so that the limits between the segments are lost, and the whole mass is protected by the shield or carai pace. The skin is thick and rendered solid by the deposition of lime (carbonate and phos phate), so that the integument forms a dense crust, hence the name Crustacea, They differ from trilobites and king crabs in having two pairs of 'antennae, while they breathe by means of gills attached to the legs. Like the other marine arthropods named, they have legs which are divided into two divisions, an outer (exopodite) and an inner (endopodite). Crtis tacea differ also from the Palgeopoda (trilo bites, merostomes and arachnids) in the high degree of specialization of their appendages, there being three to six kinds, with correspond ing functions, while in the trilobites, so far as we know, the single pair of antenna are suc ceeded by numerous (over 20) pairs of legs, all of the same shape and functions. In the head-thorax, besides the antennae, there is on each side the mouth a pair of mandibles, each with a palpus, two pairs of maxilla or accessory jaws, which are flat, divided into lobes, and of unequal size; three pairs of foot jaws (maxillipedes), which differ from the max illa in having gills like those on the five follow ing pairs of legs. There are thus 13 pairs of cephalothoracic appendages, indicating that there are 13 corresponding segments; these, with the seven abdominal segments, indicate that there are 20 segments in a typical crustacean. There are six pairs of swimming legs (swim merets), the last very broad in the lobster and shrimp, with the telson forming the The Crustacea as a rule respire by gills. These, as in the lobster and crab, are com posed of a series of little filaments into which the blood flows to be aerated. The filaments branch out from a common stalk which grows out of the basal joint of the five pairs of legs and the three pairs of foot-jaws. These gills are folded up toward the back, and are contained in a sort of chamber made in part by the carapace. In shrimps, lobsters and crabs the sea-water passing into the cavity between the body and the free edge of the carapace is afterward scooped out through an opening or passage on each side of the head by the movements of mem branous flaps called °gill-bailers.)) The diges
tive organs are well developed, especially the fore stomach, in the hinder part of which are several very large calcareous teeth for crushing the food, serving, when closed together, as a strainer through which the partly digested food presses into the long slender straight intes tine, which ends in the telson. The liver is very large, as in, all marine arthropods, or in such terrestrial types as the scorpions and spiders, which are derived from the king crabs. The brain of the higher Crustacea is very com plex, corresponding with the complicated reflex movements of an animal composed of so many segments, and bearing such a complicated series of appendages devoted to so great a variety of functions. The eyes are usually compound or many-faceted, and are mounted on freely mov able stalks. The ear is a sac in the basal joint of the smaller or second pair of antenna. The organs of smell are usually well developed, as Crustacea mainly depend on this sense in find ing their food. These consist of minute delicate sensory rods on the smaller antenna. The hairs fringing the mouth-parts and legs are often deli cate tactile organs. The green glands in the head function as kidneys, and open out at the base of the larger antenna.
With only a single known exception (Squilla), Crustacea carry their eggs about attached to the swimming or other legs. The eggs of some crabs (Neptunus) are minute and excessively numerous, their number amount ing to millions, while the lobster may produce from 20,000 to 80,000 eggs. Crustacea pass through a well-marked metamorphosis, nearly all (except the amphipods and isopods) hatching from the egg as a larva called (nauplius,* which has an oval non-segmented body, with three pairs of appendages, by which it swims about at the surface of the sea. After a series of molts, at each of which new segments with their appendages arise, it finally reaches matu rity. The shrimps and crabs hatch in a more advanced larval stage called the nauplius stage being partly suppressed and thrust back into the embryo period. The zoea has a head and abdomen, but no thorax: this, however, is developed later, and after a series of molts the parent form is attained.