The nautilus and the ammonite are similar organisms. Both are aquatic molluscs with spiral shells. Ammonites, however, have been extinct since the KT event that killed the dinosaurs 65 million years ago while the nautilus still roams the seas. There are numerous other differences between the two creatures, most of which are minor.
The nautilus is a living cephalopod (like ocotopi, cuttlefish, squid) mollusc with a spiral shell and large, distinct eyes. It uses jet propulsion to move through the water, taking in liquid and then expelling it through a funnel for locomotion. Ammonites were the predecessors to living nautiluses and had a nearly identical structure with minor differences that distinguish the two families.
The siphuncle is a tube that runs through the spiralled shell of ammonites and nautiluses. This tube is used for expelling water and gases as the shell grows or to decrease the density of the animals' interior so that it floats upward. The siphuncle ran along the outer edge of the shell in ammonites, while it runs straight through the centre of the shell in modern nautiluses.
The shells of ammonites and nautiluses are divided into various chambers that can fill with gas or water depending on whether the animal needs to sink or float. Ammonites had 26 chambers while modern nautiluses have 30. These chambers are separated by walls known as sutures. Sutures are uniformly curved in the modern nautilus, however they tend to undulate in the extinct ammonites. The undulation of the sutures creates a "ribbed" look to the ammonite's shell that is absent in nautiluses, which have smooth shells.
One other minor difference between nautiluses and ammonites is their defensive behaviours. The ammonites had the ability to suck their bodies into their shells for protection. A flap called the aptychus would close at the head of their shells to protect the animals. Nautiluses cannot retract into their shells. They use a leathery hood atop their heads for protection. Modern nautiluses use dark colours atop their shells and light colours on the bottom for camouflage, but the colours of the ammonites are unknown since all that exist of them are fossils.