nautilus - n.
(pl. nautiluses or nautili)
1 any cephalopod of the genus Nautilus with a light brittle spiral shell, esp. (pearly nautilus) one having a chambered shell with nacreous septa.
2 (in full paper nautilus) any small floating octopus of the genus Argonauta, of which the female has a very thin shell and webbed sail-like arms. [L f. Gk nautilos, lit. sailor (as NAUTICAL)]