Cephalopods

A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, 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. Fishers sometimes call cephalopods "inkfish", referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.

The Tetrabranchiata are members of a now discontinued taxonomic order of cephalopod, whose bodyplan included four gills, four hearts, four kidneys, and an external shell, today represented only by the nautilus. They were placed in opposition to the Dibranchiata, those cephalopods with only two gills, three hearts, two kidneys, and no external shell. The distinction was eventually proven to be erroneous.