Crustacea

(-ta'se-ah,) or CRUSTACEANS. [ L. cru sta, a shell.] (Zool.) A cla of animals belonging to the branch Articulata, and including the Crab (Figs. 229, 47,99), Lobster, and Prawn. It takes its name from the crust in which the animals are oncased, an integument strengthened with carbonate of lime. The body is divided into several segments, to some of which articulated limbs are attached. There are three principal divisions of the body-head, thorax, and abdomen. The thorax is covered by a large shield called the carapace. The abdomen is sometimes small, and soldered to the under side of the thorax; at others it is elongated, and an important agent of locomotion, as in the 9 37 6 8 9 5 8 10 2 4 1 Fig. 229. - ANATOMY OF A CRAB. 1, Heart; 2. artery of the eyes; 3, artery of the abdomen; 4, bran chiæ: 5, raised bran chiæ; 6, stomach; 7, muscles of the stomach; 8, liver; 9, a part of the skin which covers the inside of the carapace; 10, side. lobster, when it is furnished with swimming-plates at the end. There are usually two pairs of an tennæ, one pair being considered by some naturalists the organs of hearing, the other of smell. The great majority of crustaceans are aquatic, some living in fresh water, but the greatest number in the sea, and their blood isaërated by means of gills. Their anatomy may be easily understood with the help of the accompanying diagram. All the species lay eggs, which the female usually carries about with her, under its tail, until the time for hatching arrives. They periodically cast their cases as they grow in size, a new case having been prepared beneath the old one, and this is at first a mere soft skin, but it soon becomes as hard as the preceding integument. The forms of these animals are very curiously varied, now one and now another part being transformed or suppre ed. They consequently afford very interesting studies to the naturalist. The crustaceans are divided in this work, after Danna, into the 3 orders, Decapods, Tetra decapods, and Entomostraca. [s. 248]