Hectocotylus

A hectocotylus (plural: hectocotyli) is one of the arms of male cephalopods that is specialized to store and transfer spermatophores to the female. Structurally, hectocotyli are muscular hydrostats. Depending on the species, the male may use it merely as a conduit to the female, or he may wrench it off and present it to the female.

The name hectocotylus was devised by Georges Cuvier, who first found one embedded in the mantle of a female argonaut. Supposing it to be a parasitic worm, Cuvier gave it a generic name. The hectocotyl arm was first described in the biological works of Aristotle; although he knew of its use in mating, he was doubtful that a tentacle could deliver sperm.

Anatomy

Generalized anatomy of squid and octopod hectocotyli:

Variability

Hectocotyli are shaped in many distinctive ways, and vary considerably between species. The shape of the tip of the hectocotylus has been much used in octopus systematics.