Coleoidea

Coleoidea or Dibranchiata is one of the two subclasses of cephalopod molluscs containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e. octopus, squid and cuttlefish). Unlike its sister groups, the shelled †Ammonoidea and Nautiloidea, the coleoids have at most an internal shell called cuttlebone or gladius that is used for buoyancy or as muscle attachment. Some species, notably the incirrate octopuses (including commonly known varieties living in the shallows), have lost their internal shell altogether, while in some it has been replaced by a chitinous support structure.