Nautiluses are members of a class of mollusks that also includes cuttlefish, octopuses, and squids. It is the only living cephalopod with a true external shell. The genus Nautilus comprises six similar species found in tropical Indo-Pacific waters from the surface to depths of more than two thousand feet. They are nocturnal and commonly feed on crabs and carrion. They have up to ninety arms that surround the head in two rings; males may have many fewer arms than females. Unlike octopuses, nautiluses lack ink sacs and suckers on their arms. The shell, which is coiled onto itself and covers the inner whorls, is lined with mother-of-pearl.