The nautilus (from the Latin form of the original Greek ναυτίλος, ‘sailor’) is a pelagic marine mollusc of the cephalopod family nautilidae, the sole extant family of the superfamily nautilaceae and of its smaller but near equal suborder, nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus. Though it more specifically refers to species Nautilus pompilius, the name chambered nautilus is also used for any species of the nautilidae.

Nautilidae, both extant and extinct, are characterized by involute or slightly evolute shells that are generally smooth, with compressed or depressed whorl sections, straight to sinuous sutures, and a tubular, generally central siphuncle. Having survived relatively unchanged for millions of years, nautiluses represent the only living members of the subclass nautiloidea, and are often considered “living fossils.”

The name “nautilus” originally referred to the pelagic octopuses of the genus Argonauta, otherwise known as paper nautiluses, as the ancients believed these animals used their two expanded arms as sails.

The nautilus is similar in general form to other cephalopods, with a prominent head and tentacles, each composed of a long, soft, flexible cirrus and corresponding hardened sheath. Nautiluses typically have more tentacles than other cephalopods — up to ninety.

These tentacles are arranged into two circles and, unlike the tentacles of other cephalopods, they have no suckers, and are undifferentiated and retractable. The radula is wide and distinctively has nine teeth. There are two pairs of gills. These are the only remnants of the ancestral metamerism to be visible in extant cephalopods. The mouth consists of a parrot-like beak made up of two interlocking jaws capable of ripping the animal’s food—mostly crustaceans—from the rocks to which they are attached.

Males can be superficially differentiated from females by examining the arrangement of tentacles around the buccal cone: males have a spadix organ (shaped like a spike or shovel) located on the left side of the cone making it look irregular, whereas the buccal cone of the female is bilaterally symmetrical.

Like all cephalopods, the blood of the nautilus contains hemocyanin, which is blue in its oxygenated state. Unlike others, however, the nautilus does not have an ink sac and depends on its shell for protection from predators rather than on diversionary clouds of ink.

Nautilus pompilius is the largest species in the genus. One form from northwestern Australia, once called Nautilus repertus, may reach 26.8 centimetres (10.6 in) in diameter.

However, most nautilus species never exceed 20 centimetres (7.9 in). Nautilus macromphalus is the smallest species, usually measuring only 16 centimetres (6.3 in). A dwarf population from the Sulu Sea (Nautilus pompilius suluensis) is even smaller, with a mean shell diameter of 115.6 mm.

Nautiluses are the sole living cephalopods whose bony body structure is externalized as a shell. The animal can withdraw completely into its shell and close the opening with a leathery hood formed from two specially folded tentacles. The shell is coiled, aragonitic, nacreous and pressure resistant, imploding at a depth of about 800 metres (2,600 ft).

The nautilus shell is composed of two layers: a matte white outer layer, and a striking white iridescent inner layer. The innermost portion of the shell is a pearlescent blue-gray. The osmeña pearl, contrarily to its name, is not a pearl, but a jewellery product derived from this part of the shell.

Internally, the shell divides into camerae (chambers), the chambered section being called the phragmocone. The divisions are defined by septa, each of which is pierced in the middle by a duct, the siphuncle.

As the nautilus matures, it creates new, larger camerae, and moves its growing body into the larger space, sealing the vacated chamber with a new septum. The camerae increase in number from around four at the moment of hatching to thirty or more in adults.

The shell colouration also keeps the animal cryptic in the water. When seen from above, the shell is darker in color and marked with irregular stripes, which helps it blend into the dark water below. The underside is almost completely white, making the animal indistinguishable from brighter waters near the surface. This mode of camouflage is named countershading.

The nautilus shell presents one of the finest natural examples of a logarithmic spiral, although it is not a golden spiral. The use of nautilus shells in art and literature is covered at nautilus shell.

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