Highly specialized marine molluscs divided into two distinctive subclasses, the shelled subclass Nautiloidea and the subclass Coleoidea, in which the shell is reduced and internal or lacking entirely. About 600 species worldwide, typically nektonic or bentho-pelagic, fastmoving, predatory. Foot modified to form grasping arms, intimately associated with the head, which has greatly developed eyes, a marked development of the cerebral ganglia, and a powerful beak.