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This article is about the sea animal. For other uses of "squid", see Squid (disambiguation).

Juvenile cephalopod from plankton
Scientific classification

The squid is a marine mollusc of the class Cephalopoda, subclass Coleoidea, order Teuthida, of which there are two major suborders, Myopsina and Oegopsina (including the giant squids like Architeuthis dux). Teuthida is the largest of the cephalopod orders, edging out the octopuses (order Octopoda) for total number of species, with 298 classified into 28 families.

The order Teuthida is a member of the superorder Decapodiformes (literally "ten legged"). Two other orders of decapodiform cephaopods are also called squid, although they are taxonomically disctinct from Teuthida and differ recognizably in their gross anatomical features. They are the single species Ram's Horn Squid (order Spirulida) and the bobtail squids of order Sepiolida. There is also one squid species grouped within the octopodiform ("eight legged") superclass: the Vampire Squid.

Like all cephalopods, squids are distinguished by having a distinct head, bilateral symmetry and tentacles with suckers; squid, like cuttlefish, have eight arms and two tentacles arranged in pairs. They also have chromatophores imbedded in their skin and the ability to expel ink if threatened. Being coleoids means that their bony structure is internalized (in the octopus it is nonexistent); in squid there is a single flat bone plate buried within the soft tissue structure. They have a specialized foot called the siphon, or hyponome, that enables them to move by expelling water under pressure. Squid are the most skilled of the coleoids at this form of motion. The mouth of the squid is beak-like and made of chitin, and contains the radula (the rough tongue common to all molluscs).

Squid have two gills and an extensive closed circulatory system with one major and two subsidiary hearts.

They are exclusively carnivorous, feeding on fish and other invertebrates. Squid usually have two elongated tentacles especially for the capture of food.

The majority of squid are no more than 60 cm in length, but the giant squid is reportedly up to 20m in length, which made it the largest invertebrate in the world, and it has the largest eyes of all. Recently, however, an even larger specimen of a poorly understood species, Mesonychoteuthis hamiltoni (the Colossal Squid) has been discovered.

Individual species of squid are found abundantly in certain areas and provide large catches for fisheries.

Squid is a popular food in many parts of the world, and finds its way into cuisines as widely separated as the Japanese and the Italian. In American fish markets and restaurants, it is usually known by the Greek plural calamari.