The subphylum crustacea is one of the largest groups in the phylum Arthropoda. The subphylum is divided into several classes, the most important of which are Malacostraca and Cirripedia. The class
Malacostraca includes crayfish, crabs, lobsters, and shrimps. Typical features of malacostracans include a body divided into two sections (a combined head and thorax called a cephalothorax, and an abdomen); an exoskeleton (external skeleton) with a large plate (carapace) covering the cephalothorax; stalked, compound eyes; and two pairs of antennae. The class Cirripedia includes barnacles, which, unlike other crustaceans, spend their adult lives attached to a surface, such as a rock. Other characteristics of cirripedes include an exoskeleton of overlapping calcareous plates; a body consisting almost entirely of thorax (the abdomen and head are minute); and six pairs of thoracic appendages (cirri) used for filter feeding.
