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.
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]
The typical hectocotylus seems to have entirely escaped notice until early in the present century, when both Delle Chiaje and Cuvier described it, as detected within the female, as a parasite, the latter under the name of Hectocotylus octopodis. Kölliker, in 1845–49, regarded the Hectocotylus of Tremoctopus as the entire male animal, and went so far as to discern in it an intestine, heart, and reproductive system. It was not until 1851 that the investigations of Vérany and Filippi confirmed a suggestion of Dujardin,[258] while H. Müller, in 1853, completed the discovery by describing the entire male of Argonauta.
In all genera of dibranchiate Cephalopoda except Argonauta, Ocythoe, and Tremoctopus, one of the arms is sexually modified in various ways, but never becomes so much prolonged, and is never detached and left with the female. In Loligo Forbesii Stp. the fourth arm on the left has 23 pairs of regularly developed acetabula, which then lessen in size and disappear, being replaced by long pedunculated papillae, of which there are about 40 pairs. In Loligo vulgaris Lam. and L. Pleii Orb. 18 or 19 pairs of acetabula are regularly formed, and then occur 40 pairs of papillae, as in Forbesii. In other species of Loligo (gahi Orb., brevis Bl., brasiliensis Orb.) only the outer row of suckers becomes modified into papillae after about the 20th to the 22nd pair. In Sepioteuthis sepioides the modification is the same as in the Loligo last mentioned, but the corresponding arm on the right side is so covered with acetabula towards its extreme end, that it is thought that it in some way co-operates with the hectocotylised left arm.
In Octopus, the third arm on the right side is subject to modification. This arm is always shorter than the corresponding arm on the other side, and carries fewer suckers, but is furnished at the extreme tip with a peculiar kind of plate, which connects with the membrane at the base of the arm by a channel of skin, which probably conveys the spermatophores up to the tip.
In Octopus vulgaris, the species referred to by Aristotle, the hectocotylised arm is short, thin in its outer half and pointed at the extremity, while the fold of skin is very white, and gives the arm an appearance of being divided by a cleft at the side. At the same time, an unusual development of one or two suckers on the arm is not uncommon.[259]
Fig. 52.—Octopus lentus Baird, N. Atlantic, showing the peculiar formation of the hectocotylus arm, h.a. (After Verrill, × ½.)