THE CHAMBERED NAUTILUS - FAMILY NAUTILIDAE. Shell with few whorls, overlapping more or less ; septa simple ; siphuncle nearly central ; aperture wide.
Five fossil genera and one living genus constitute this family.
Genus NAUTILUS, Linn.
Shell a flat spiral, pearly, with yellowish outer layer, cross banded with brown; chambers small, except outer one; septa pearly, concave; siphuncle, a membranous tube, central, passing through each septum; body, size of a fist; head conical ; jaws two, strong; tentacles nearly one hundred, unlike, in four groups on thick bases ; hood formed by union of two tentacles ; siphon formed of two overlapping lobes; eyes large, lateral; organs of smell, small tentacles. Distribution world-wide in deep seas.
Of this genus alone, six hundred extinct species are already distinguished by fossil remains. Only six species are living to-day. Of these but one is abundant or well known.
The Chambered Nautilus (N. Pompilius, Linn.) has a large flattened spiral shell, four to six inches in diameter, which in adults has two and one-half coils. It is gracefully turned and deli cately built, pearly within and porcellanous outside, the yellowish ground of the exterior marked with reddish-brown cross-bands or stripes, variously branched. Opposite the opening of the shell the coil bears a large patch of black. There is a narrow band of black lining the edge of the opening.
In New Guinea the market value of bright, perfect shells is fully appreciated. Curio-hunters will often discover that by pay ing a good price they have rewarded the wily and industrious native for his pains in restoring, by judicious use of paints and dyes, the faded glory of a wave-worn shell. Oftener the shell is cleaned by having all the outer coating removed, so as to show only pearl throughout.
459 The Chambered Nautilus The floor of the cavernous outer chamber is of pearl, secreted by the mantle, which lies next to it. As growth extends the edge of the shell, giving more room, the body is drawn forward, and a pearly wall is formed one-half inch or more outside the last one. It is shaped like a deep saucer, with a little pit in the
centre. The siphuncle, a membranous extension of the body, like a piece of delicate rubber hose, passes through all the par titions, connecting the innermost chamber of the shell with the outermost one. The siphuncle is supported and protected at each passage.
Some investigators assert with confidence that air is admitted by it to the shell chambers, and that the buoyancy thus acquired enables the Nautilus to float, and to carry the shell with ease when it walks on the deep sea bottom. Analysis shows the gas to be a little richer in nitrogen than the atmosphere, but other wise to be much the same. "To maintain the relative weight of the growing animal and its shell with the water," is the opinion of many. The last word has not been said upon this interesting question.
The shell is brightest and most lustrous when inhabited by its living tenant. The body is white as curds, except for brown and yellowish trimmings which harmonise with the markings on the shell. Stout muscle masses attach the body to the walls of the outer chamber.
"A Nautilus shell with a cauliflower sticking out of its mouth"— thus one surprised scientist characterised the living mollusk. The extended body shows a complicated system of tentacles of varying sizes, with bases of swollen fleshy ridges which conceal the mouth. Three dozen tentacles of greatest length form the outer whorl. These each consist of a stocky, dark brown, basal part surmounted by a slender, white "cirrus" or whip, whose function is prehensile. They are the hands and feet and fins of the Nautilus. There are no suckers. The tentacles cling firmly but delicately to objects, seizing with avidity soft bits of decaying animal matter. The mantle rim twitches while the tentacles are active. When a live animal like a crab is seized, the head protrudes, the two horny parts of the beak are seen to tear the creature's tough body cover, or cut it like a pair of shears, and the rasping mollusk tongue comminutes the flesh.