Joseph Thomas Cunningham, 1911 Encyclopædia Britannica, Volume 19… (1911)
“ The separation of one of the arms of the male for purposes of reproduction is one of the most remarkable peculiarities of the Octopoda. It does not occur, however, in octopus nor in many other members of the group. One arm is always considerably modified in structure and employed in copulation, but it is only in three genera, one of which is Argonauta, that the arm spontaneously separates. The detached arm is found still alive and moving in the mantle cavity of the female, and when first discovered in these circumstances was naturally regarded by the older naturalists as a parasite. ”
