Anatomical diagram of an adult female chambered nautilus, the best known species of nautilus, a "living fossil" related to the octopuses. The animal has a primitive brain that forms a ring around its oesophagus, has four gills (all other cephalopods have only two), and can only move shell-first (seemingly "backwards") by pumping water out through its funnel. The shell and tentacles are shown here as shadows.Diagram: K.D. Schroeder
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