Cephalopod fin

An extreme development of the cephalopod fin is seen in the bigfin squid of the family Magnapinnidae.

[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 most paralarval chiroteuthids)[6] 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.

[8] Fin placement in cephalopods is often termed normal, terminal, or subterminal, depending on their position with respect to the muscular mantle.

Standard measurements for squid, showing the location of the fins and tail in different configurations
Measurement of fin angle in a squid with sagittate fins
Time series showing up-and-down fin movement in an individual of the cirrate octopod Cirroteuthis muelleri
Adult Grimalditeuthis bonplandi with "secondary fins" supported on a well-developed tail