Crus-tā'ce-ā, n. pl.

(krus-tā'-she-ā), n. pl. [N. Lat., from Lat. crusta, the hard surface of a body, rind, shell. See supra, and cf. CRUSTACEOUS.] One of the classes of the Articulata, or articulated animals (see ANIMAL KINGDOM), including lobsters, shrimps, and crabs; so called from the crust-like shell with which the body and legs are covered.

The body consists of an anterior part, more or less jointed, made up of the head and thorax combined, and called the cephalo-thorax, and of a posterior jointed part called the abdomen, or (improperly) tail. A long-tailed crustacean is one having an elongated abdomen, like the lobsters. The species are divided as follows:—

I. DECAPODS, having ten feet, and mostly of large size; including, 1. Brachyural, or short-tailed species; the abdomen being very small, and in the male, without appendages, as the common crab. 2. Anomoural species; the abdomen being abnormal, as the soldier or hermit-crab, &c.; it constitutes a species intermediate between the other two. 3. Macroural, or long-tailed species; the abdomen being long, and having a regular series of legs or appendages in both sexes, as the lobster, cray-fish, and shrimp. 4. Anomobranchiata species, resembling the macroural in form, but having the gills external or else wanting, as the Squilla and Mysis.

II. TETRADECAPODS, having fourteen feet, and of small size, seldom exceeding an inch. The species are divided as follows: 1. Isopods, in which the abdomen is very short, and there are branchial but no leg-appendages, as the sow-bug. 2. Anisopods, constituting a group intermediate between the Isopods and Amphipods, having the legs of the former and abdomen of the latter. 3. Amphipods, having the abdomen elongate, and having swimming appendages, as the sand-flea.

III. ENTOMOSTRACANS, including species of inferior grade, irregular in the number of legs, and without abdominal appendages; some being of large size, as the horse-shoe, or Limulus; others minute, as the Daphnia, Cyclops, &c.

IV. CIRRIPEDS, or the barnacle tribe. See CIRRIPEDE.

V. ROTIFERS, or animalcular species, destitute of limbs and moved by cilia. Dana.