Based on the similar appearance of their shells, ammonites probably looked and behaved like modern nautiluses. The living nautilus has many (38 or more) tentacles, with which it catches food. The nautilus has small poorly developed eyes and a leather-like spotted hood. The nautilus animal inhabits the last and largest chamber in its shell. The animal grows new, larger chambers as it ages and grows bigger.

The shell of the nautilus protects the animal from predators. The nautilus can withdraw completely into its shell, almost like a turtle, and block the end of the shell with its hood. The nautilus uses the empty chambers in its shell to help maintain neutral buoyancy. This means the animal does not float or sink. The nautilus moves by "jet propulsion," by blowing a stream of water out of a tube between its shell. The nautilus lives in the open ocean near coral reefs. It is a predator, eating shrimp and small fish.

Two "uncoiled" ammonites

A nautilus shell - notice the 
smooth chamber walls