From Wikipedia, the free encyclopedia:
The cephalopods (Greek plural ?????????? (kephalópoda); "head-feet") are 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 octopuses, squid, and
cuttlefish. In the Nautiloidea, the shell remains; this subclass includes the
nautilus. About 800 distinct living species of cephalopods have been identified.
Two important extinct taxa are Ammonoidea, the ammonites, and Belemnoidea, the
Cephalopods are found in all the oceans of Earth, at all depths. None of them can
tolerate freshwater, but a few species[clarification needed] tolerate more or
less[weasel words] brackish water.
Cephalopods have been around from the Early Paleozoic to present day.