"Head foot." A cephalopod is a marine mollusc characterised by bilateral body symmetry, a prominent head, and a modification of the mollusk foot, a muscular hydrostat, into the form of arms or tentacles. The class contains two extant subclasses. In the Coleoidea, the mollusk shell has been internalized or is absent; this subclass includes the octopuses, squid, and cuttlefish. In the Nautiloidea the shell remains; this subclass includes the nautilus.