Fossil range: Late Cambrian - Recent
A variety of cephalopod forms from Ernst Haeckel’s 1904
Kunstformen der Natur
The cephalopods (Greek plural Κεφαλόποδα
(kephalópoda); "head-feet") are the mollusc
class Cephalopoda characterized by bilater-
al 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 in-
cludes the octopuses, squid, and cuttlefish. In
the Nautiloidea the shell remains; this sub-
class includes the nautilus. About 800 dis-
tinct living species of cephalopods have been
identified. Two important extinct taxa are
Ammonoidea, the ammonites, and Belem-
noidea, the belemnites.
Cephalopods are found in all the oceans of
Earth, at all depths. None of them can toler-
ate freshwater, but a few species tolerate
more or less brackish water.
There are around 800 extant species of ceph-
alopod, although new species continue to
be described. It is estimated that around
11,000 extinct taxa have been described,
although the soft-bodied nature of cephalo-
pods means that they are not easily fossil-
Cephalopods occupy most of the depth of
the ocean, from hydrothermal vents to the
sea surface. Their diversity is greatest near
the equator (~40 species retrieved in nets at
11°N by a diversity study) and decreases to-
wards the poles (~5 species captured at
Nervous system and
See also: Cephalopod intelligence and squid
Cephalopods are widely regarded as the most
intelligent of the invertebrates and have well
developed senses and large brains; larger
than the brains of gastropods or bivalves.
The nervous system of cephalopods is the
most complex of