A Peculiar Squid

A cross section of a ram's horn squid
Ewald Rübsamen from Die Cephalopoden

Coiled within the ram's horn squid is its shell. While most squid have a flattened shell remnant called a pen, the ram's horn squid has an internal coiled shell that they use to control buoyancy like the nautilus. They are the only cephalopod with an internal, coiled shell.