cephalopod

cephalopod , member of the class Cephalopoda, the most highly organized group of mollusks (phylum Mollusca ), and including the squids , octopuses , cuttlefish , and nautiluses . The class as a whole has become adapted for a free-swimming existence. Cephalopods are able to move about rapidly, and most are aggressive carnivores. The part of the body that forms the foot in other mollusks is located anteriorly in cephalopods instead of ventrally. Part of the foot area surrounds the mouth and is modified into sucker-bearing tentacles, used to capture prey. The tentacles number 8 in octopuses, 10 in squids, and as many as 90 in nautiluses. The rest of the foot forms a muscular funnel, or siphon, which expels water from the mantle cavity, permitting cephalopods to move about by a kind of jet propulsion. Only one existing genus, the nautiluses, the sole survivors of an extinct group known as the nautiloids, possesses an external shell. In the squid and cuttlefish the shell has become internalized and reduced, and in the octopus it is completely absent. The cephalopod head is large and is equipped with prominent eyes that resemble those of vertebrate animals. The class Cephalopoda has a fossil record of 10,000 species, although only 600 exist today. The nautiloid group was dominant through Paleozoic times, and the ammonites flourished in the Mesozoic era.

cephalopod Any member of the advanced class of predatory marine molluscs Cephalopoda, including squid, nautilus, octopus, and cuttlefish. Each has eight or more arms surrounding the mouth, which has a parrot-like beak. The nervous system is well developed, permitting great speed and alertness; the large eyes have an image-forming ability equal to that of vertebrates. Most squirt an inky fluid to alarm attackers. Cephalopods move by squirting water from their mantle edge. Their heavily yolked eggs develop into larval young that resemble the adults. Members of this class vary dramatically in size from 4cm (1.5in) to the giant squid, which may reach 20m (65ft). There are more than 600 species.