The Cephalopoda comprises a class of molluscs characterized by muscular arms, or tentacles, that project from the head and surround the mouth. Three divisions, or subclasses, are involved, the Ammonoidea which are extinct and the Nautiloidea and Coleoidea which have modern representatives. Coleoids differ in that their shells, if present, are internal. They include recent squid, octopus, and cuttlefish.