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This subclass includes the six species of living Nautilus, which are the only living cephalopods with a coiled external shell. The Nautilus shell is approximately 20 cm in diameter and coiled in a planispiral fashion, with ornamental banding of orange-brown stripes that are irregular and bi-laterally symmetrical. The shell is composed of internal gas-filled structures called camerae that are divided by septa. The animal lives within the last chamber, and each time a new septum is secreted, it moves forward on average about every two weeks. The chambers are connected by the siphuncle, a thin single tube that passes through the centre of each septum.

The head/foot region of Nautilus is complex, with tentacles surrounding the mouth and beak, and the eyes placed laterally. As in all cephalopods, the hyponome is used for locomotion and is located below the tentacles. It is funnel-shaped and can be turned in any direction. Water enters the mantle cavity through the inhalant siphon, passes over the gills and is then forced out under pressure through the hyponome propelling the animal in the opposite direction. The diagrams below come from E. Clarkson and show the internal structure.

Nautilus is a mobile feeder, grabbing prey with its tentacles, which it spreads out in a "cone of search". Once food is captured, the inner tentacles are used to manipulate it, while the powerful beak rips the animal, usually a fish or crustacean, apart. The food is then stored prior to digestion in an expanded oesophagus. All species of Nautilus are nocturnal, being active at night while during the day they sink to the seafloor where they rest; this is referred to as vertical diurnal migration.

Major Subclasses
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