The hectocotylus is confined to the dibranchiate Cephalopoda, and its typical form, _i.e._ when part of the arm becomes disengaged and left with the female, occurs only in three genera of the Octopodidae, _viz._[_Argonauta_, _Ocythoe_ (_Philonexis_), and _Tremoctopus_. In all of these, the male is many sizes smaller than the female. In _Argonauta_ the third arm on the left side becomes hectocotylised. At first it is entirely enveloped in a kind of cyst, in such a way that only a small portion of the tip projects; subsequently the cyst parts asunder, and allows the arm to become expanded to its full length, which considerably exceeds that of the other arms. At a certain point the acetabula or suckers terminate, and the remainder of the arm consists of a very long, tapering, sometimes thread-like filament, which is pointed at the extreme tip. It is not yet known how the spermatophores find their way into the hectocotylus, or how the hectocotylus impregnates the ova of the female. The arm thus affected is not always the same. In _Tremoctopus_ it is the third of the right side, in the _Decapoda_ the modification usually affects the fourth of the left. [Illustration: FIG. 51.--Male of _Ocythoe tuberculata_ Raf. (= _Philonexis catenulatus_ Fér.), Mediterranean, showing three stages, =A=, =B=, and =C=, in the development of the hectocotylus arm: _h.cy_, hectocotylus still in the cyst; _c´y´_, spoon-shaped cyst at the end of the arm when freed; _th_, thread-like organ freed by the rupture of _c´y´_. Natural size. From specimens in the British Museum.] This singular property of the male Cephalopoda has only recently been satisfactorily explained. It is true that Aristotle, more than twenty-two centuries ago, distinctly stated that certain of the arms were modified for sexual purposes. Speaking of what he calls the polypus (which appears to represent the _Octopus vulgaris_ of the Mediterranean), he says: ‘It differs from the female in having what the fishermen call the white sexual organ _on its arm_;’ again, ‘Some say that the male has something of a sexual nature (αὶδοιῶδές τι) on one of its arms, that on which the largest suckers occur; that this is a kind of muscular appendage attached to the middle of the arm, and that it is entirely introduced within the funnel of the female’. Unfortunately the word translated by _introduced_ is corrupt, and can only be restored conjecturally. He again remarks, ‘The last of the arms, which tapers to a fine point and is the only whitish arm, it uses in sexual union.’[257]