The Collaborative International Dictionary of English v.0.48:

Crustacea \Crus*ta"ce*a\ (kr?s-t?"sh?-?), n. pl. [Neut. pl. of
   NL. crustaceus pert. to the crust or shell, from L. crusta
   the hard surfsce of a body, rind, shell.] (Zool.)
   One of the classes of the arthropods, including lobsters and
   crabs; -- so called from the crustlike shell with which they
   are covered.
   [1913 Webster]

   Note: 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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Crustacea
    n 1: class of mandibulate arthropods including: lobsters; crabs;
         shrimps; woodlice; barnacles; decapods; water fleas [syn:
         Crustacea, class Crustacea]