(From crusta, a -shell, and
) are animals which have the external parts firm and hard, but contain afleshy soft substance within. The firm part consists of a semicalcareous crust, forming one very large and several small pieces, or a series of rings nearly equal. They have their heads furnished with horns and other appendages; numerous feet obliquely bent and articulated; two arms called claws, notched like a forceps, and breathe by means of distinct gills. Ancient naturalists united them with fish, or in a separate class after fish, or after the molluscae, Pliny comprehends all crustaceous animals under the name of crabs. Linnaeus classes them among insects without wings, under the generical name of crabs. - Since the time of the Swedish naturalist they have-been arranged with insects, though their structure is very different, since they breathe by gills, and have a muscular heart. Cuvier and La Marck have agreed, therefore, in separating them from insects. The former places them as a distinct class between worms and insects; the latter between the molluscae and arachnides, a class formed by him to connect the Crustacea; with insects. They differ, therefore, from fish and molluscae in having articulated limbs; and from insects by having a muscular heart, and breathing by gills.
Dr. Cullen takes notice in general of the lobster, crab, prawn, and shrimp only; of which he says the two former hardly differ in any quality from one another: and from the small proportion of volatile alkali that is obtained from their entire substance, or extract, he concludes they contain less animal matter than the flesh of quadrupeds, birds, or even the amphibia. They appear to be more easy of digestion than animal food, or aliment of any other kind. See Aliment.