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.
Most of the cephalopods we find as fossils had a calcareous outer shell. Some were loosely coiled, some tightly coiled, and others were shaped like a tapered tube. During the Ordovician Period, between 490 and 443 million years ago, some straight cephalopods grew to be as long as 19 feet, although most were much shorter.