Squid, a swift marine invertebrate (animal without a backbone). Squid are an important food in Asia and the Mediterranean area.

The squidThe squid has a beaklike mouth surrounded by ten arms lined with suckers.

The squid has a soft, torpedo-shaped body surrounded by a mantle, a muscular protective organ. At the posterior end, the mantle has fins, which are usually held close to the body; at the anterior end, it surrounds the head like a collar. The mantle is supported by a horny structure called a pen. The squid moves by jet propulsion—as the mantle opens, water is taken in; as the mantle closes, the water is expelled through the siphon, a nozzlelike structure below the eyes. By bending the siphon, the squid can swim in any direction; however, it usually swims backwards.

The beaklike mouth is surrounded by 10 arms, which are lined with suckers. Two of the arms are tentacles and are used to seize prey, such as fish or crustraceans, and transfer it to the other arms. The arms then hold it in place while the squid feeds.

The skin is studded with pigment-containing cells called chromatophores, which are red, pink, brown, blue, or yellow, depending on the species. These cells help the squid blend in with its surroundings. Some species have photophores, light-producing cells that blind prey. Like its relatives, the octopus and cuttlefish, the squid ejects an inky fluid, called sepia, to confuse predators as it escapes from danger.

The giant squid can reach more than 60 feet (18 m) in length (with the arms extended) and weigh up to 1,000 pounds (450 kg). It lives in open water and can swim at a speed of up to 15 miles per hour (24 km/h). Its chief predator is the sperm whale. Because of its size and appearance, it may have been the origin of one of the sea monster legends.

Common squid, or sea arrows, live in schools along the Atlantic coast. They are used as bait by fishermen. They are about eight inches (20 cm) long and have gray bodies with red spots.

Squid are valuable as laboratory animals because of their highly developed circulatory and nervous systems. Much of the knowledge of the action of human nerves is based on studies made on squid nerves.

Which Mollusks Are Giants?

It’s easy to see why giant squids have long been the stuff of legends. Giant squids are the largest invertebrates on Earth. Their huge, parrotlike beaks are so strong that they can crush bone. Each sucker on a giant squid’s arms may be as big as a dinner plate. Giant squids live about 3,000 feet (900 meters) down in the ocean depths. That’s so deep that scientists don’t know much about how they live. As a matter of fact, no one has ever seen a living giant squid.

Squid are of the phylum Mollusca, class Cephalopoda, order Decapoda. The giant squid is Architeuthis princeps ; the common squid are species of Loligo. L. pealei is a common species of the western Atlantic.