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] [Illustration: FIG. 52.--_Octopus lentus_ Baird, N. Atlantic, showing the peculiar formation of the hectocotylus arm, _h.a_. (After Verrill, × ½.)] It is believed that in the Tetrabranchiate Cephalopoda (_Nautilus_) a union of the four inner ventral arms may correspond functionally to the hectocotylising of the arm in the Dibranchiates.