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Nautiloids are a large and diverse group of marine cephalopods that evolved in the Late Cambrian and are represented today by the living Nautilus and Allonautilus. Nautiloids flourished during the early Paleozoic Era where they were the main predatory animal. Some 2,500 species of fossil nautiloids are known but only a few survived to the present day. The animal is characterized by simple septa that are concave in the forward direction that produce simple sutures and a septa that connects the chambers. The septa form during growth spurts as the animal forms a new septum, adding another chamber while the forward part adds onto the shell. The sub-chambers are filled with gas for buoyancy similar to the atmosphere except it contains more nitrogen and less oxygen. The body of the animal occupies the newest chamber of the shell. The animal is free swimming and has ahead with two simple lens-free eyes and as many as 90 tentacles arranged around the mouth that is equipped with jaws that are horny and beak-like allowing it to feed on crustaceans. Nautili propel themselves by jets of water expelled from an elongated funnel called the hyponome which can be pointed in different directions. Nautiloids were at their height during the Ordovician, Silurian and Devonian periods with various straight, curved, and coiled shell forms coexisting. Nautiloids began to decline in numbers during the Devonian during which time their shells became increasingly tightly coiled. The massive extinctions at the end of the Permian were less damaging to the nautiloids with a few groups surviving into the early Mesozoic. There was a brief resurgence in the Tertiary that lasted up to the middle Cenozoic Era. During the global cooling during the Miocene and Pliocene, there was another decline in diversity. Today, there are only six living species, all belonging to two genera, Nautilus and Allonautilus.