page_title: Nautilus
section_title: Shell
hierarchical_section_title: Nautilus / Anatomy / Shell
context_page_description: The nautilus 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 of the Nautilidae. All are protected under CITES Appendix II.
  Nautilidae, both extant and extinct, are characterized by involute or more or less convolute 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 hundreds of millions of years, nautiluses represent the only living members of the subclass nautiloidea, and are often considered "living fossils".
  The word nautilus is derived from the Greek ναυτίλος nautílos and originally referred to the paper nautiluses of the genus Argonauta, which are actually octopuses. The word nautílos literally means "sailor", as paper nautiluses were thought to use two of their arms as sails.
context_section_description: Nautiluses are the sole living cephalopods whose bony body structure is externalized as a planispiral 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 m (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 4 at the moment of hatching to 30 or more in adults.
  The shell coloration 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 called 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.
caption: Nautilus half-shell showing the camerae in a logarithmic spiral
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