||Hec`to*cot"y*lus (?), n.; pl. Hectocotyli (#). [NL., fr. Gr. &?; a hundred + &?; a hollow vessel.] (Zoöl.) 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.