The nautilus is the only living cephalopod with an external shell.
Its smooth, coiled shell is divided into internal chambers connected by a tube called a siphuncle. By adjusting the fluid and gas in these chambers, the nautilus controls buoyancy and can move through the water without continuously swimming.
Nautiluses are cephalopods, the group that includes octopuses, squid and cuttlefish. Their living relatives generally have reduced internal shells or no shell at all. The nautilus therefore preserves an external feature that was common among many ancient cephalopods.
A nautilus has numerous simple tentacles without suckers and a relatively simple eye compared with the camera-like eyes of squid and octopuses. It usually lives on deep reef slopes in the Indo-Pacific and rises at night to feed on carrion and small animals.