Nautilus, the one living genus of Cephalopoda belonging to the sub-class Tetrabranchiata, and the type of the order Nautiloidea (q.v.). The animal is included in a strong coiled shell composed of many different chambers; these are cut off from one anothei by transverse calcareous plates, or septa, which are curved and have the convex surface directed forwards. The traces of these septa on the external wall of the shell are gently wavy or straight, and not complicated by a great series of foldings as in the Ammonites. The septa are perforated in the centre by a membranous tube known as the siphuncle, which is protected as it passes through the septa by a number of backwardly directed calcareous processes known as the collars. Though the shells of the Nautilus are extremely common, the animal is rare. Its anatomy is of great interest, as the genus is the only known living member of the order. The body is internal, and occupies only the last body-chamber of the shell. It differs from all the other living Cephalopods [Cuttlefish] by the absence of an ink-bag, of auricles to the heart, and of the expansions at the base of the gills known as "branchial hearts." The foot is divided into lobes arranged around the mouth; each of these lobes corresponds to one of the arms of the Octopus (q.v.); the suckers on the arms of other Cephalopods are here represented by a series of tentacles, of which there are over sixty. The funnel, by the ejection of water from which the animal can swim backwards, consists only of a loose slit, and not a closed jet, as in the other Cephalopoda. The eyes are extremely simple, and consist only of a pair of simple pits. There are two pairs of gills (whence the name Tetrabranchiata). As there are also two pairs of nephridia, it h thought that this may indicate a trace of ancestral segmentation. The genus is marine, and occurs widely distributed through tropical seas. As at present defined, it dates only from the Liassic period.