Apophallation is the biting off of the penis, known to occur in terrestrial slugs, which are hermaphroditic gastropod mollusks.
It has been reported in some species of banana slugs, Ariolimax, and in the distantly related Deroceras laeve.
[1][2] In the mating of banana slugs, the penis is inserted into the body of the partner.
The penis may become trapped, perhaps because of the action of a special muscle,[clarification needed] in which case the penis is gnawed off by either the partner or the owner.
[1] One paper reports that individuals of Deroceras laeve sometimes bite off their own penis after mating, which is then eaten by the partner.