One of the classes of the arthropods, including lobsters and crabs; -- so called from the crustlike shell with which they are covered. [1913 Webster
" The body usually consists of an anterior part, made up of the head and thorax combined, called the cephalothorax, and of a posterior jointed part called the abdomen, postabdomen, and (improperly) tail. They breathe by means of gills variously attached to some of the limbs or to the sides the body, according to the group. They are divisible into two subclasses, Entomostraca and Malacostraca, each of which includes several orders."