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nautilus, cephalopod mollusk belonging to the sole surviving genus (Nautilus) of a subclass that flourished 200 million years ago, known as the nautiloids. The spirally coiled shell consists of a series of chambers; as the nautilus grows it secretes larger chambers, sealing off the old ones with thin septa. The animal lives in the largest and newest chamber, with a tubular elongation of the body, known as the siphuncle, extending through the septa to the apex of the shell. The siphuncle removes liquid from the chambers and replaces it with gas, giving the animal the buoyancy that permits it to swim (backwards except when feeding), which it accomplishes by ejecting water through a funnel. The nautilus breathes by means of two pairs of gills; it feeds on crabs and other animals, which it catches with its long, slender tentacles (numbering more than 90) that encircle the mouth. There is a thickened area over the head, called the hood, that acts as a protective lid when the animal withdraws into the shell. The nautilus lives in deep water in the S Pacific and Indian oceans. It is active at night, when it seizes crabs and fish as food; during the day it stays hidden in coral crevices. The paper nautilus, which is not a true nautilus, is a close relative of the octopus, belonging to the order Octopoda. The true nautilus is classified in the phylum Mollusca, class Cephalopoda, order Nautilida, family Nautilidae.
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