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The Cephalopods ("head-foot") are the mollusc class Cephalopoda characterized by bilateral body symmetry, a prominent head, and a modification of the mollusc foot into the form of arms or tentacles. Teuthology is the study of cephalopods, and teuthologists are the scientists who study them.
The class contains two extant subclasses. In the Coleoidea, the mollusc shell has been internalized or is absent; this subclass includes the octopuses, squids, and cuttlefish. In the Nautiloidea the shell remains; this subclass includes the nautilus. There are around 786 distinct living species of Cephalopods. Two important extinct subclasses are Ammonoidea, the ammonites and Belemnoidea, the belemnites.
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 and large brains. They have special skin cells called chromatophores that change color and are used for communication and camouflage. The nervous system of cephalopods is the most complex of the invertebrates. The giant nerve fibers of the cephalopod mantle have been a favorite experimental material of neurophysiologists for many years.
Cephalopods' primary method of movement is by jet propulsion. Oxygenated water is taken into the mantle cavity to the gills and through muscular contraction of this cavity, the spent water is expelled through the hyponome, created by a fold in the mantle. Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions.
Octopuses are also able to walk along the sea bed. Squids and cuttlefish can move short distances in any direction by rippling of a flap of muscle around the mantle.
Evolution
The class developed during the late Cambrian and were during the Paleozoic and Mesozoic dominant and diverse marine life forms. Early cephalopods were at the top of the food chain. The ancient (cohort Belemnoidea) and modern Coleoidea (cohort Neocoleoidea) 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.
Classification The classification as listed here (and on other cephalopod articles) follows primarily from Current Classification of Recent Cephalopoda (http://www.mnh.si.edu/cephs/newclass.pdf) (May 2001). Other classifications [1] (http://tolweb.org/tree?group=Coleoidea&contgroup=Cephalopoda) [2] (http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Protostoma/Mollusca/Cephalopoda/Coleoidea.htm) differ, primarily in how the various decapod orders are related, and whether they should be orders or families.
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