The posterior portion of the body containing the viscera is soft, smooth and adapted to the anterior chamber of the shell; the anterior portion is muscular, including the organs of sense and locomotion, and can be drawn within the shell. The mantle is very thin behind, and prolonged through the calcareous tube of the occupied chamber as a membranous siphon, and through all the divisions of the shell to the central nucleus; on the upper part of the head is a broad, triangular, muscular hood, protecting the head when retracted and used as a foot for creeping at the bottom of the sea, with the shell uppermost. … The mouth has two horny mandibles, like the beak of a parrot reversed, the lower overlapping the upper, moving vertically, and implanted in thick, muscular walls. … There are ninety tentacles about the labial processes and head. The internal cartilaginous skeleton is confined to the lower surface of the head, a part of the cephalic nervous system being protected in a groove on its upper surface, and the two great muscles which fasten the body to the shell are attached to it. The funnel is very muscular and is the principal organ of free locomotion, the animal being propelled backward by the reaction of the ejected respiratory current against the water before it. … The nautilus, though the lowest of the Cephalopods, offers a nearer approach to the vertebrate animals than does any other invertebrate, in the perfect symmetry of the organs, the larger proportion of muscle, the increased bulk and concentration of the nervous centers in and near the head, and in the cartilaginous cephalic skeleton. The mature nautilus occupies but a small part of the shell, the parts progressively vacated during its growth are one after another partitioned off by their smooth plates into air tight chambers, the plates growing from the circumference toward the center, and pierced by the membranous siphon. The young animal before the formation of these chambers can not rise from the bottom of the sea, but the older ones come to the surface by the expansion and protrusion of their bodies producing a slight vacuum in the posterior part of the chamber unoccupied, and, some say, by the exhalation of some light gas into the other deserted chambers. They rise in the water as a balloon does in the air, because lighter than the element surrounding them. They float on the surface with the shell upward, and sink quickly by reversing it. By a nice adjustment, in the completed structure, between the air chambers and the dwelling chamber, the house and its inhabitant are nearly of the same specific gravity as water. … In parts of the Southern Pacific, at certain seasons of the year, fleets of these little ships are carried by the winds and currents to the island shores, where they are captured and used for food. [Illustration: ARGONAUT.]