Hectocotylus

A hectocotylus (plural: hectocotyli) is basically a cephalopod phallus. One of the arms of the male of most kinds of cephalopods is modified and specialized to transfer sperm to the female and fertilize her eggs. It is a muscular hydrostat that serves as a spermatophore. Depending on the species, the male may use it merely to store and transfer sperm to the female, or he may wrench it off and present it to the female. Males generally form a new hectocotylus in each new season.