CEPHALOPOD

Squid (Illex coindeti) swimming forward any marine mollusk of the class Cephalopoda (phylum Mollusca), including the cuttlefish, nautilus, octopus, and squid (see photograph). Some representatives of the group rank as the most active and largest invertebrate animals in existence, as, for example, the giant squid Architeuthis, which reaches nearly 18 m (59 feet), including its tentacles. Cephalopods are exclusively marine forms. Cuttlefish are primarily found along the continental shelf; nautilus and squid inhabit the open seas, often at considerable depths; and most octopods live near shore in rocky and coralline habitats, though some occur in deep-sea environments. Cephalopods are bilaterally symmetrical and characteristically have a highly developed centralized nervous system embedded in cartilage. Their eyes are generally image-forming organs similar in structure (though not in derivation) to the eyes of vertebrates. The head is armed with prehensile tentacles that have rows of round suction disks. The number and structure of the tentacular arms vary from group to group. The octopus, for example, has eight equisized arms, whereas squid and cuttlefish have eight such arms plus two longer, slender tentacles with expanded ends. All cephalopods are able to swim by means of a jet propulsioni.e., they fill the folds in the body walls with water and then squeeze it through a flexible, muscular funnel, or siphon, on the ventral side. The force of the expelled water propels the animals backward. Most cephalopods can readily change skin colour to blend in with their surroundings for protection against predators. Many species also squirt clouds of dark inklike fluid at potential attackers. The genus Nautilus has a large, chambered external shell, but other cephalopods have only reduced internal shells (e.g., the flat, chalky shell of the cuttlefish and the thin, elongated shell of the squid) or none at all, as in the case of the octopus. Most cephalopods are predaceous carnivores that feed on fish, crustaceans, and other mollusks. They use a radula, a tonguelike structure covered with rows of numerous small teeth, and horny jaws to tear apart their prey. Sexes are separate, and eggs are generally laid in grapelike festoons attached to a hard surface. See also cuttlefish; nautilus; octopus; squid. Squid (Illex coindeti) swimming forward any member of the class Cephalopoda, of the phylum Mollusca, a small group of highly advanced and organized, exclusively marine animals. The octopus, squid (see photograph), cuttlefish, and chambered nautilus are familiar representatives. The extinct forms outnumber the living, the class having attained great diversity in late Paleozoic and Mesozoic times. The extinct cephalopods are the ammonites, belemnites, and nautiloids, except for five living species of Nautilus. Additional reading General treatises on the biology, evolution, ecology, physiology, and behaviour of the cephalopod group include Marion Nixon and J.B. Messenger (eds.), The Biology of Cephalopods (1977); P.R. Boyle (ed.), Cephalopod Life Cycles, 2 vol. (198387); and Clyde F.E. Roper, Michael J. Sweeney, and Cornelia E. Nauen, Cephalopods of the World: An Annotated and Illustrated Catalogue of Species of Interest to Fisheries (1984), which also has an illustrated glossary and keys to identification of orders and families. K.N. Nesis, Cephalopods of the World: Squids, Cuttlefishes, Octopuses, and Allies (1987; originally published in Russian, 1982), is a review of anatomy, with keys to the identification of species. W. Bruce Saunders and Neil H. Landman (eds.), Nautilus: The Biology and Paleobiology of a Living Fossil (1987), is a comprehensive, scholarly treatise on the evolution, distribution, ecology, physiology, biology, shell structure, and aquarium maintenance of Nautilus. Peter Douglas Ward, The Natural History of Nautilus (1987), compares the growth, buoyancy, physiology, and ecology of Nautilus with those of other living cephalopods. Clyde F.E. Roper and Kenneth J. Boss, The Giant Squid, Scientific American, 246(4):96100, 104105 (April 1982), is a nontechnical summary of current knowledge. M.J. Wells, Octopus: Physiology and Behaviour of an Advanced Invertebrate (1978), is a highly technical but very readable review.