A cephalopod (family Nautilidae) with a smooth, coiled, chambered shell in which the edges roll inwards so that the final whorl envelops previous ones; most have a tube (siphuncle) running through all the chambers. A nautilus has up to 90 retractable tentacles with ridged surfaces used to seize prey, a radula with nine teeth, and a beak resembling that of a parrot. There are two genera with six species, found in all oceans.