Modified arm of some cephalopods involved in sperm transfer. The degree of modification varies between the groups. In some species (Sepia and Loligo) several rows of suckers are smaller and form an adhesion area for the spermatophores. In octopods the tip of the arms carries a spoonlike depression and in other groups there is actually a cavity or chamber where the sperm is stored.
Alternative form for hectocotylus : hectocotylous.