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Cephalopoda ("head-foot") is a class of the phylum Mollusca. It is characterized by bilateral body symmetry, a prominent head, and a modification of the mollusc foot into the form of tentacles. It contains the subclass Coleoidea, where the mollusc shell has been internalized or is absent, of the octopus, squid, and cuttlefish and the shelled subclass Nautiloidea of the nautilus. There are around 650 distinct living species of Cephalopods. An important extinct class is Ammonoidea, the ammonites.

Cephalopods are found in all the oceans of the world and at all depths. They are regarded as the most intelligent of the invertebrates and have well developed senses. They have special skin cells call chromatophores that change color and are used for communication and camouflage.

Evolution The class developed during the late Cambrian and were during the Paleozoic and Mesozoic dominant and diverse marine life forms. Early cephalopds were at the top of the food chain. The modern Coleoidea diverged from the external shelled Nautiloidea around 425 million years ago. Unlike most modern cephalopods, ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species. Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous.

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