Nautilus (Gr. , from , a ship), a name applied to both the tetrabranchiate and dibranchiate orders of the cephalopod mollusks. In the former the true or pearly nautilus is the best known species of the only living genus representing the extinct chambered shells (such as ammonites, orthoceratites, turnhtes, etc.) which abounded during the primary and secondary geological ages; in the latter belongs the nautilus of the ancients (the paper nautilus of the moderns), more properly called argonaut. For the characters of the class and orders see Cephalopoda, and Mollusca. - The genus nautilus (Linn.) has a discoid, symmetrical univalve shell, with simple aperture, sutures, and siphunele. The organization of the pearlv nautilus (N. pompilius, Linn.) was first made known by Prof. Owen in 1832, and afterward by Gray, Grant, De Blainville, Van der Hoeven. Valenciennes, and Huxley. The posterior portion of the body, containing the viscera, is soft, smooth, and adapted to the anterior chamber of the shell; the anterior is muscular, including the organs of sense and loco-motion, and can be retracted 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, the back part excavated for the involuted convexity of the shell, protecting the head when retracted, and used as a foot for creeping at the bottom of the sea with the shell uppermost.

On each side of the head are 20 perforated digitated processes of a conical form, each containing a long finely ringed tentacle, whose inner surface is closely set with narrow transverse [dates; the eyes, large and prominent, are placed on short pedicels on the side of the head behind the digita-tions; the subocular processes have no tentacles, and are rudimentary external ears, their cavity extending to the auditory capsule. 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; the surrounding circular fleshy lip has 4 labial processes, each pierced by 12 canals, containing each a small retractile tentacle, making, with the 38 digital and 4 ophthalmic, 90 tentacles on and around the 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 fix 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 a succession of jerks occasioned by reaction of the ejected respiratory currents against the surrounding water.

The capacious crop opens into an oval muscular gizzard; the intestine terminates in the branchial cavity near the base of the funnel; the liver is bulky, and the bile is derived from arterial blood; there is no ink gland. Sea water is admitted the pericardium; the branchiae are two pairs without branchial hearts, the larger bran-cliia supporting 48 vascular folded plates on each side, the smaller 36; the large veins near the heart have dusters of follicles attached to them, according to Owen seeming to be homologous with the so-called renal glands of lower mollusks; by some they are considered as diverticula to relieve the circulation during the varying pressures to which the animal is subjected. The tongue is furnished with numerous papilla) and spines. The nautilus, though the lowest of the cephalopods, approaches the vertebrate type nearer than any other invertebrate, in the perfect symmetry of the organs, the larger proportion of muscle, the increased bulk and concentration of the nervous centres in and near the head, the vertical opposition of the jaws, the gustatory papillae of the tongue, and the cartilaginous cephalic skeleton. Its food consists of other mollusks and of crustaceans, showing that its natural habitat is the bottom of the sea, where it creeps about shell upward.

The parts of the shell progressively vacated during the growth of the auimal are successively partitioned off into air-tight chambers by thin smooth plates concave toward the opening, with sinuous margins, growing from the circumference toward the centre, and pierced by the membranous siphon. The young animal, before the shell becomes came-rated, cannot rise from the bottom; but the older ones can come to the surface by changes in the expansion of the soft parts, by a slight vacuum produced in the posterior part of the occupied chamber, and according to some by the exhalation of some light gas into the deserted chambers; they rise in the water as a balloon does in the air, with the ability also of directing the motions to a certain extent by means of the funnel; they float at the surface shell upward, and sink quickly by reversing the shell. The proportion of the air chambers to the dwelling chamber is such that the shell is nearly of the same specific gravity as the water; the siphon communicates with the pericardium, and is probably filled with fluid from that cavity; it conducts small vessels for the nutrition of the shell, and perhaps for secretory purposes.

A large and perfect shell will weigh 6 or 7 oz., and the soft parts 5 or 6 oz. more; the exterior crust of the shell is whitish with fawn-colored streaks and bands, and the interior has a beautiful pearly lustre, and is in request by cabinet makers and jewellers; by removing the external coat by acids, the pearly surface is readily exposed, and shells thus treated and richly engraved were formerly highly prized as ornaments for the mantle-piece and sideboard. This species is so common in the S. Pacific, that at certain seasons of the year they are carried by the winds and currents to the island shores, where they are used, when smoke-dried, for food; in the Papuan archipelago the shells are used as common utensils; they are found from the Persian gulf and Indian ocean to the Chinese seas and the Pacific. In the umbilicated nautilus (N, um-bilicatus, Lester) the last whorl of the shell does not envelop and conceal the others; the shape is ventricose, the surface reticulated, and the color dusky smoky, with numerous delicate chestnut flammules (five to the inch)-A nautilus extended in a straight line would be a shell like a fossil orthoceratite; in the ammonites the shell is coiled as in the nautilus, but is strengthened by arched ribs and domeshaped elevations on the convex surface. - The paper nautilus or argonaut belongs to the oc-topod group of the dibranchiate cephalopods, or to the acetabulifera of D'Orbigny, from the arms being provided with sucking disks.

It differs from the true nautilus in the arms of larger size and more complicated structure, partially connected by membrane at the base; in the larger and more complex eyes, not pedunculated but lodged in orbits; in the gills being only two in number, each with a branchial heart; in the funnel being an entire tube; and in the presence of an ink gland and bag for its secretion. In the genus argonauta (Linn.), in the females, which alone have a shell as an egg receptacle, the first or dorsal pair of the eight arms are dilated into broad thin membranes, which secrete and sustain the very light, paper-like, calcareous, symmetrical, and single-chambered shell; like the other arms, these are provided with two rows of suctorial disks, extending around the whole circumference, by means of which the animal retains the shell in position; the six non-palmated arms serve as organs of prehension and locomotion, as the animal drags itself along the bottom or climbs the rocks in search of food, and as anchors; the shell, as in the nautilus, is carried above the body. The arms are attached to the anterior part of the cephalic cartilage; the suckers are completely under the control of the animal, which can fasten or relax them instantly.

Swimming is effected in a retrograde manner by the ejected currents from the funnel. The skin is soft and tender, and includes a great number of cells containing pigment matter of different colors, whose contractions and expansions, with the surface movements, give it a remarkable power of rapidly changing its tints. There is no internal shell, and it is now ascertained that the external shell is peculiar to the female, and is only an incubating and protective nest for the eggs; it is not the homologue of the internal rudimentary shell of the cuttle fish, nor of the external chambered shell of the nautilus, but rather answers to the cocoon of leeches and other articulates, or to the egg-float of the delicate gasteropod janthina; the eggs are attached by thread-like stalks to the involuted spire of the shell, behind and beneath the "body of the female. The best known species, the A. argo (Linn.), inhabits the Atlantic, Pacific, and Indian oceans, and the Mediterranean, especially about Sicily. In the last named locality Mme. Jeanette Power made the experiments which determined that the argonaut is the maker of its own shell, and not a parasitic occupant like the hermit crab; this question arose from the fact that the animal has no muscular or other attachment to the shell, and has been known voluntarily to quit it, and survive in captivity a considerable time without any attempt to return to it; it also repairs the shell when broken by the agency of the pal-mated arms.

For an account of the arguments for and against parasitism (among the advocates of the former being Lamarck, Leach, De Blainville, Broderip, and Sowerby, and among those of the latter Cuvier, Duvernoy, Ferussac, and D'Orbigny), and for an extensive bibliography on this animal, see "Proceedings of the Boston Society of Natural History," vol. v., pp.369-'81 (1856). Leach, who considered the animal a parasite, described it as the genus ocythoe. The sexes are distinct; the specimens usually found are all females, the males hav-ving been until recently described as parasites under the name of hectocotylns; this is a wormlike body, resembling the arm of a cuttle fish, the under surface bordered with 40 or 50 pairs of alternating suckers; for a long time regarded as a parasitic annelid, it is now known to be the spermatophorous arm of the male argonaut, deciduous during sexual congress, and attaching itself within the mantle of the female; in this genus it is the third arm of the left side which is thus deciduous and hollowed for the spermatic receptacle. The male argonaut has no shell and no palmated arms, and is only about one eighth of the size of the female.

The argonaut, according to Pang, rises to the surface shell upward, turning it downward when it floats on the water; by retracting the six arms within the shell and placing the palmated ones on the outside, it can quickly sink, explaining why the animal is so rarely taken with the shell. The shell is flexible in the water, but very fragile when dry; after having been soaked in water for some time it may be bent as before. A specimen, one of the largest known, in the cabinet of the Boston society of natural history, is 10 in. long, 6½ broad, and the opening 4 in. wide; it cost the donor $500. Many species are described.

Pearly Nautilus (Nautilus pompilius).

Pearly Nautilus (Nautilus pompilius).

Paper Nautilus (Argonauta argo).

Paper Nautilus (Argonauta argo).