The nautilus represents the only living member of the subclass Nautiloidea. The nautiluses are cephalopods that retain an outer shell unlike other distantly related animals such as squid and octopus.

 

Their beautiful shells have inspired many artists over the centuries, and they are also among the finest natural examples of a logarithmic spiral.

 

Because of their shells, the fossils of nautiluses are easier to come by than remains of other cephalopods, and fossil hunters have discovered ancient shells dating back at least 500 million years.