The Collaborative International Dictionary of English v.0.48:

Hectocotylus \Hec`to*cot"y*lus\, n.; pl. Hectocotyli. [NL.,
   fr. Gr. ? a hundred + ? a hollow vessel.] (Zool.)
   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.
   [1913 Webster]