Cephalopoda or cephalopods are extraordinary molluscs equipped with vertebrate-like intelligence and a unique buoyancy system for locomotion. Cephalopods evolved during the Cambrian where external shell was modified into a chambered buoyancy apparatus. During the mid-Palaeozoic cephalopods diverged into nautiloids and the presently dominant coleoids. Coleoids (i.e. squids, cuttlefish and octopods) internalised their shells which appears to be a unique evolutionary event. The general tendency of shell reduction reflects a trend towards active modes of life and much more complex behaviour.
Cephalopods are exclusively marine animals. They 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. About 800 living species of cephalopods have been identified. Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites).