What is Nautilus?
{{Taxobox | name = Nautilus | image = Nautilus pompilius.jpg | image_width = 280px | image_caption = Chambered nautilus: ''Nautilus pompilius'' | regnum = [[Animal]]ia | phylum = [[Mollusca]] | classis = [[Cephalopod]]a | subclassis = [[Nautiloid]]ea | ordo = Nautilida | subordo_authority = [[Louis Agassiz|Agassiz]], 1847 | familia = '''Nautilidae''' | familia_authority = [[Henri Marie Ducrotay de Blainville|Blainville]], 1825 | subdivision_ranks = [[Genus|Genera]] | subdivision = ''Allonautilus''<br> ''Nautilus''}} [[File:Nautilus diagram-en.svg|thumb|right|280px|Anatomy of a nautilus]] '''Nautilus'''<ref>from [[Greek language|Greek]] ''nautilos'', 'sailor'</ref> is the common name for [[cephalopod]]s of the [[family (biology)|family]] '''Nautilidae'''. They are the only living members of the subclass [[Nautiloidea]].<ref>Sweeney M.J. 2002. [http://tolweb.org/accessory/Nautilidae_Taxa?acc_id=2324 Taxa associated with the Family Nautilidae Blainville, 1825.] {{Webarchive|url=https://web.archive.org/web/20070930204032/http://tolweb.org/accessory/Nautilidae_Taxa?acc_id=2324 |date=2007-09-30 }} Tree of Life web project.</ref> In appearance, they have not changed much in millions of years. The term ''chambered nautilus'' is also used for any species of the Nautilidae. Many [[biologist]]s see them as '[[living fossil]]s', because they are the only living descendents of the group which gave rise to the [[ammonites]] in the [[Palaeozoic]] [[era]]. Today, there are six living [[species]] in two [[genera]]. The species ''Nautilus pompilius'', from [[Western Australia]] is the largest and best known. It get up to 27&nbsp;cm in diameter. However, most other nautiluses never exceed 20&nbsp;cm. The Bellybutton nautilus, ''Nautilus macromphalus'', is the smallest species, usually measuring only 16&nbsp;cm. The [[radula]] is wide and has exactly nine teeth. There are two pairs of [[gill]]s. ==The shell== [[Image:Nautilus species shells.png|thumb|right|200px|Nautilus shells: ''N. macromphalus'' (left), ''A. scrobiculatus'