The nautiluses (Nautilidae) are the close cousins of the octopuses, cuttlefish and squids. Nautiluses form one of the two groups of cephalopods
(the other being the Coleoidea, which include octopuses, squids and cuttlefish). The Nautilidae includes six species, of which the most familiar is the chambered nautilus, Nautilus pompilius
. Nautiluses are unique among living cephalopods in that they are the only group to have retained an external shell.