Page:The American Cyclopædia (1879) Volume XII.djvu/184

176 NAUTILUS genus nautilus (Linn.) has a discoid, symmet- rical, univalve shell, with simple aperture, su- tures, and siphuncle. The organization of the pearly nautilus (JV. pompilius, Linn.) was first made known by Prof. Owen in 1832, and after- ward by Gray, Grant, Be Blainville, Van der Hoeven, Valenciennes, and Huxley. The pos- terior portion of the body, containing the vis- cera, is soft, smooth, and adapted to the ante- rior chamber of the shell ; the anterior is mus- cular, 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 nu- cleus ; on the upper part of the head is a broad triangular muscular hood, the back part exca- vated 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 plates ; the eyes, large and prominent, are placed on short pedi- cels on the side of the head behind the digita- tions ; the subocular processes have no tenta- cles, 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 retrac- tile 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 pro- tected 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 back- ward by a succession of jerks occasioned by the 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 into the pericardium; the branchiaa are two pairs without branchial hearts, the larger bran- chia supporting 48 vascular folded plates on each side, the smaller 36 ; the large veins near the heart have clusters of follicles attached to them, according to Owen seeming to be ho- mologous with the so-called renal glands of lower mollusks ; by some they are considered as diverticula to relieve the circulation du- ring the varying pressures to which the ani- mal is subjected. The tongue is furnished with numerous papillae and spines. The nautilus, though the lowest of the cephalopods, ap- proaches 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 op- position of the jaws, the gustatory papillae of the tongue, and the cartilaginous cephalic skele- ton. 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 progres- sively vacated during the growth of the animal 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 de- serted 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 se- cretory 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 jewel- lers ; by removing the external coat by acids, the pearly surface is readily exposed, and shells thus treated and richly engraved were former- ly highly prized as ornaments for the mantle- piece and sideboard. This species is so com- mon 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 Pap- uan 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 am- monites the shell is coiled as in the nautilus, but is strengthened by arched ribs and dome-