Detachable Arms

I've been reading Thomas Mann's "The Magic Mountain". In the "Research" section (of Chapter V), the narrator describes a curious creature that I cannot identify:

In others [i.e., other animals] the male parent reached with his arm down the gullet of the female to deposit the semen within her; after which, bitten off and spat out, it ran away by itself upon its fingers, to the confusion of scientists, who for long had given it Greek and Latin names as an independent form of life.

Now, it's possible that this is just a flight of Mann's fancy, but I'm inclined to think that Mann was describing something that was actually being reported by scientists of the day; of course it also possible that the science of the time was wrong.

Nonetheless, I am curious and would like to learn what animal this is.

Googling for it, I came across an interesting article on BBC entitled Animal Sex that covers some of the same ground as "Research"; both writings, for example, describe the anglerfish's strange mating practices.

The BBC piece also describes some strange cephalopod practices that sound remarkably like the animal that Mann described. Some octopus species have an arm (3rd on the right) that doubles as a penis; they only mate once, and lose that arm in the process--it often remains stuffed inside the female. Some cuttlefish actually have a penis-arm that detaches itself and swims off on its own to empregnate a mate.

Now this all sounds remarkably like what Mann describes except for one key detail: cephalops do not have fingers!

So I ask you: What animal is Mann describing?