Anatomy
The nautilus is similar in general form to other cephalopods, with a prominent head and tentacles. 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.
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.
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