Cephalopods are characterized by bilateral body symmetry, a prominent head, and a modification of the mollusc foot into the form of tentacles. The class contains two subclasses. In the Coleoidea, the mollusc shell has been internalized or is absent; this subclass includes the octopus, squid, and cuttlefish. In the Nautiloidea the shell remains; this subclass includes 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.
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.