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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. |
