Class of mulluscs. Exclusively marine. All are predatory and have at least 8 arms, a parrot-like beak, siphon mantle, complex eyes and highly developed nervous system. Regarded as the most intelligent of the invertebrates. Three main clades: Ammonoidea (extinct and shelled), Nautiloidea and Coleoidea (squids, cuttlefish, octopuses and belemnites). Shells contain interconnected, internal chambers. Straight to planispiral (coiled in a single plane) shells. Bilateral body symmetry. Earliest cephalopods appear in the late Cambrian. Ammonoids go extinct at the end of the Cretaceous. Only one surviving Nautiloid clad survives through the end of the Cretaceous extinction event.