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

The squid is a marine mollusc of the class Cephalopoda, subclass Coleoidea, order Teuthida, of which there are two major suborders, Myopsina and Oegopsina (the latter includes Architeuthis dux, the giant squid).

Like all cephalopods, they are distinguished by having a distinct head, bilateral symmetry and tentacles with suckers; squid, like cuttlefish, have ten tentacles arranged in pairs. They also have chromatophores imbedded in their skin and the ability to expel ink if threatened. Being coleoidea 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 coleoidea 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 known 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.