NAUTILUS, a marine cephalopod of the genus Nautilus and family a few living species of which are the only survivors of the order or superorder Tetrabranchiota, which including bOth the nautiloid forms and the ammonites (q.v.) is divided into no less than 22 extinct families, so abundant were they in past ages. The nautiloids began in the Cambrian,became greatly differentiated and abundant in the Silurian and Carboniferous, and by the dose of the Pakeozoic Age had become almost entirely extinct. The more com plex ammonites, on the other hand, reached their full development only during the Meso zoic Age. The shell of the pearly or chambered nautilus, described below is typical of the I armature of the group. It is at first simple, but as the animal grows becomes coiled in a flat spiral and divided by internal trans verse septa into a succession of chambers, the last one of which is occupied by the ani mal. This process is continued until about 36 chambers have been formed, all but the last filled with gas which gives a certain buoyancy, but does not enable the animal to come to the surface. Externally the shell is porcella neous; internally is a layer of mother-of-pearl. The septa are perforated by an opening, through which a membranous tube or siphuncle passes from the posterior end of the animal to the apical or first chamber of the spire. Among other anatomical peculiarities the animal has four gills and four kidneys, calcareous instead of horny beaks, a funnel composed of two lobes instead of one tube, small stalked imperfect eyes and no ink bag. Quite unique are the tentacles, which number about 90. On each side of the head are a hood tentacle, two ocular tentacles and 17 brachial tentacles, and around the mouth four groups of 12 or 13 labial tentacles.
The best-known species is Nautilus pompilius which, with two others, inhabit the Indian Ocean and tropical parts of the south Pacific. They live exclusively on or near the bottom in relatively deep waters, are gregarious, more or less migratory and nocturnal. Although fond of any animal matter, they feed chiefly on prawns. In swimming they dart quickly back ward for short distances, with the tentacles either radiating or held together and stiffly projecting from the mouth of the shell; but they are inactive and rest most of the time on the bottom. Little is known of their breeding, which is supposed to take place in deep holes. The eggs are very large; including the capsule they measure nearly one and a half by three fourth inches. They are attached singly by the base, and the capsule is irregularly folded and fenestrated longitudinally in a most peculiar manner This species is very abundant in about 60 fathoms of water in the Philippines, and large numbers are captured by the fishermen in 'bamboo fish traps or baskets constructed on the principle of the lobster pots in use on the New England coast and baited with any kind of animal offal. The shells are somewhat used in the arts, but the flesh is little esteemed for food, as is that of the native species by the Fiji Islanders. Consult Willey, 'Zoological Results) (Part VI, Cambridge 1902), and La caze-Duthier, 'Arch. Zoologie Experimental& (1892).
The 'paper nautilus" is a very different animal, for which see ARGONAUT.