Squid (Teuthida) is a subdivision of the class Cephalopoda. The body is elongated, torpedo-shaped, which allows them to move at high speed "tail" forward, the main mode of movement - reactive. Along the squid's body runs a cartilaginous "arrow" that supports the body. It is called the gladius and is a vestige of the shell. The length is usually 25-50 cm, sometimes up to 18 m. Giant squid of the genus Architeuthis can reach 20 meters (counting tentacles) and are the largest invertebrates. They have 8 arms and 2 tentacles. Suction cups on arms in two rows, on tentacles in 4 or more rows. Coloration is varied, some deep-sea squid species have transparent bodies. Nektonnye animals. Over 250 species. Widespread, live mainly in tropical waters. In Russian waters - about 30 species, most of them are predators. Many Teuthida (Todarodes, Loligo) are objects of fishing. Squid are traditionally harvested in the southern seas of Asian countries: Vietnam, China, Japan and others.

All members of the group have a radula for scraping and crushing food, and an ink gland. The secretion secreted by her, colors the water and hides squid when attacked by predators. Swim with the help of fins and reactive - sharply eject water from their mantle cavity through a funnel, turning which they are able to maneuver, move backward and forward. Eggs (up to 200 thousand) are laid in capsules, less often one by one, in the water or on the bottom. Most abundant and diverse in tropical waters. They are found both in the surface layers and at great depths. Some species make long-distance feeding and spawning migrations. A number of species can change coloration. Small planktonic species are characterized by poorly developed muscles and gelatinous body, and regulate their buoyancy by changing the concentration of ammonium chloride in their bodies. Deep-sea species are often transparent or colored in dark red tones, many possess luminescent organs (photophores). Squid serve as a food base for many fish, toothed whales, seabirds. They are an object of fishing, occupying the first place among all groups of mollusks by volume of catch. They are used by humans for food, serve as a raw material for the pharmaceutical industry and perfumery.