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A cephalopod is any member of the mollusc class Cephalopoda, characterized by bilateral body symmetry, a prominent head, and a modification of the mollusk foot, a muscular hydrostat, into the form of arms or tentacles. Teuthology, a branch of malacology, is the study of cephalopods.
The class contains two extant subclasses. In the Coleoidea, the mollusk shell has been internalized or is absent; this subclass includes the octopusi, squid, and cuttlefish. In the Nautiloidea, the shell remains; this subclass includes the nautilus. About 800 distinct living species of cephalopods have been identified.