Cephalopods are any member of the molluscan class Cephalopoda (Greek plural κεφαλόποδα (kephalópoda); "head-feet").
These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles (muscular hydrostats) modified from the primitive molluscan foot.
Fishermen sometimes call them inkfish, referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.
Evolutionary history and classification
Cephalopods became dominant during the Ordovician period (beginning 488.3 million years ago and ending 443.7 million years ago), represented by primitive nautiloids.
The class now contains two, only distantly related, extant subclasses:
- Coleoidea, which includes octopuses, squid, and cuttlefish; and
- Nautiloidea, represented by Nautilus and Allonautilus.
The differences between the two are:
In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains.
About 800 living species of cephalopods have been identified.
Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites).
Cephalopods include the order octopoda, commonly referred to as octopus.