Inkfish

Cephalopods Temporal range: Late Cambrian – Present PreЄ Є O S D C P T J K Pg N
Bigfin reef squid (Sepioteuthis lessoniana)
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda Cuvier, 1797
Subclasses and orders
Subclass Orthoceratoidea † Orthocerida † Pseudorthocerida † Ascocerida † Lituitida † Dissidocerida † Subclass Nautiloidea Plectronocerida † Ellesmerocerida † Actinocerida † Endocerida † Tarphycerida † Oncocerida † Discosorida † Nautilida Bactritida † Subclass Ammonoidea † Goniatitida † Prolecanitida † Ceratitida † Ammonitida † Subclass Coleoidea Belemnoidea † Aulacocerida † Belemnitida † Hematitida † Phragmoteuthida † Neocoleoidea (most living cephalopods) ?Boletzkyida † Sepiida Sepiolida Spirulida Myopsida Oegopsida Octopoda Vampyromorphida

A cephalopod (/ˈsɛfələpɒd, ˈkɛf-/) is any member of the molluscan class Cephalopoda (Greek plural κεφαλόποδα, kephalópoda; "head-feet") such as a squid, octopus or nautilus. 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.

Cephalopods became dominant during the Ordovician period, 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. 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).