Nautilus is a sole living cephalopod whose bony body structure is externalized as a shell.
Having survived relatively unchanged for millions of years, nautiluses represent the only remaining members of the subclass Nautiloidea, and are often considered to be “living fossils”. Extinct relatives of the nautilus include ammonites, such as the baculites and goniatites.
The eye structure of the Nautilus is highly developed but lacks a solid lens. They have a simple “pinhole” lens through which water can pass. This represents a working transitional stage in the possible evolution of the eye.