hectocotylus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A modified arm of the male of certain cephalopods, such as most octopuses, functioning as a reproductive organ in transferring sperm to the mantle cavity of the female.

from The Century Dictionary.

• noun [capitalized] In zoology, a spurious genus of parasitic organisms, the same as the Trichocephalus of Delle Chiaje; in reality, the detached male arm of a cephalopod, attached to the female, and mistaken for a parasite.
• noun In biology, the metamorphosed reproductive arm of certain of the male cephalopods, as the argonaut, which becomes detached and is deposited within the mantle-cavity of the female for the purpose of conveying the sperm-cells to her. Also hectocotyle. See cut under Argonautidæ.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology A modified arm of any of several male cephalopods that functions as a reproductive organ by transfering sperm to the mantle of the female