Cephalopod fossils have been found in rocks of many ages, and numerous representatives are alive today. Squids, octopuses, cuttlefish, and the pearly nautilus are among the cephalopods living in modern seas. All are members of the large phylum Mullusca.
Cephalopods are the most advanced class of the mulluskes.
They have a distinct head, a highly developed nervous system, and eyes much like those of humans. The cephalopod's mouth is surrounded by long tentacles commonly armed with suckers. Beneath the tentacles is a tube, the siphon, through which the animal can force a jet of water and thus move about by jet propulsion. Coiled cephalopods live today only in the South Pacific, but in the geologic past they were scattered throughout the world. Modern squids live in shallow coastal waters over much of the globe.