Cephalopod fins, sometimes known aswings,^[1] are pairedflap-like locomotory appendages. They are found inten-limbed cephalopods (includingsquid,bobtail squid,cuttlefish, andSpirula) as well as in theeight-limbedcirrate octopuses andvampire squid. Many extinct cephalopod groups also possessed fins.Nautiluses and the more familiarincirrate octopuses lack swimming fins. An extreme development of the cephalopod fin is seen in thebigfin squid of the familyMagnapinnidae.^[2]

Fins project from themantle and are often positioneddorsally. In most cephalopods, the fins are restricted to the posterior end of the mantle, but in cuttlefish and some squid they span the mantle's entire length.

Fin attachment varies greatly among cephalopods, though in all cases it involves specialised fincartilage (which reaches its greatest development inOctopodiformes).^[3] A fin may be attached to the internal shell or shell remnant (such as thegladius), to the opposite fin, to the mantle, or a combination of these.^[4]

Certain squid species possess atail, which is an extension of the body past the fins.^[5] The tail may be said to start at "the point where a hypothetical line, continuous with the broad posterior edge of the fin, crosses the midline of the body".^[1] This tail may be lost with age (as in mostparalarvalchiroteuthids)^[6] or remain throughsexual maturity (as inGrimalditeuthis).Grimalditeuthis and larvalChiroteuthis are unusual in that they possess a pair offlotation devices or "secondary fins" attached to the tail. Thevampire squid (Vampyroteuthis infernalis) also has two pairs of fins during a brief period of itsontogeny, and secondary fins have been reported in the extinctTrachyteuthis.^[7]

Cephalopod fin morphology is highly variable. The fins may be large and muscular, extending for the entire length of the mantle, or greatly reduced (sometimes less than 10% ML) and restricted to the mantle's posterior end.^[8] Fin placement in cephalopods is often termednormal,terminal, orsubterminal, depending on their position with respect to the muscular mantle.^[5]

Eight major fin shapes can be distinguished among theDecapodiformes: sagittate (the most common shape in squid), rhomboid, circular/elliptical, lanceolate, ear-shaped, ribbed, lobate, and skirt-like.^[9]

• Cephalopod zootomy