The Collaborative International Dictionary
Hectocotylus \Hec`to*cot"y*lus\, n.; pl. Hectocotyli. [NL., fr. Gr. ? a hundred + ? a hollow vessel.] (Zo["o]l.) One of the arms of the male of most kinds of cephalopods, which is specially modified in various ways to effect the fertilization of the eggs. In a special sense, the greatly modified arm of Argonauta and allied genera, which, after receiving the spermatophores, becomes detached from the male, and attaches itself to the female for reproductive purposes.
Wiktionary
n. (context biology English) A modified arm of any of several male cephalopods that functions as a reproductive organ by transfering sperm to the mantle of the female
Wikipedia
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.
Usage examples of "hectocotylus".
And then his hectocotylus reached for her, striking swiftly, and lodged his needlelike spermatophore among the roots of her arms.
And already he was holding out his hectocotylus toward her, the modified arm bearing the clutch of spermatophores at its tip.