Cephalopod fin

Fins project from the mantle and are often positioned dorsally. 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 fin cartilage (which reaches its greatest development in Octopodiformes). A fin may be attached to the internal shell or shell remnant (such as the gladius), to the opposite fin, to the mantle, or a combination of these.

Tail and secondary fins

Certain squid species possess a tail, which is an extension of the body past the fins. 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". This tail may be lost with age (as in most paralarval chiroteuthids) or remain through sexual maturity (as in Grimalditeuthis). Grimalditeuthis and larval Chiroteuthis are unusual in that they possess a pair of flotation devices or "secondary fins" attached to the tail. The vampire squid (Vampyroteuthis infernalis) also has two pairs of fins during a brief period of its ontogeny, and secondary fins have been reported in the extinct Trachyteuthis.