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Class CRUSTACEA.
Forming part of the great division of the animal kingdom, the
Annulosa, and immediately following the Insects, comes the important class of animals called CRUSTACEA.
The length to which the article "Insects" has necessarily extended, must be our apology for the summary manner in which this most interesting class must be treated.
Crustaceous animals are true members of the
Articulata, having, like insects, no internal skeleton, but generally, on the contrary, an external tegumentary skeleton, composed of flexible rings or joints, and the body provided with articulated or jointed organs of locomotion.
They differ, however, from insects in having their respiration carried on by means of gills or branchiæ, and in having a double or complete circulation of blood. Their body is in general covered with a hard external shell or crust (
crusta), from which circumstance they derive their name. This shell has for its base a peculiar substance, which is found also in the covering of insects, called
chitine. It is commonly called the carapax or carapace; and in many of the species, as the common crab and lobster, is extremely hard, almost stony, being composed of a large portion of carbonate of lime with a small quantity of chitine; whilst in others, as in the Entomostracous species, it is of a horny consistence, the composition partaking largely of chitine and albumen, and slightly of carbonate of lime. This carapace being divided into rings or joints, the animals, even in those which have it hardest, possess a considerable degree of freedom of motion. The normal number of these rings or joints is twenty-one, but two or three are often blended together or soldered into one. To each of these joints except the last, there is attached a pair of members, the forms and uses of which vary much in the different species, according to their age, &c. These members are always divided into joints or articulations, the number of which is very various, and are covered with the same envelop or crust as the body itself. Crustacea possess the power of casting off or shedding their carapace at certain periods.
