Sometimes when banana slugs mate, they get stuck together, so they take turns gnawing off the penis to get unstuck!

Banana slugs are often associated with worms, but they’re actually part of the mollusk family; related to octopuses and oysters. So as not to dry out, it is covered in a film of slime. They have a hump on them like shoulders that works as primitive lungs. There’s a pore on the right side of the hump, called the mantle, which allows the air to circulate. It is basically the slug’s nose and it’s called the pneumostome.

Banana slugs mate at any time of the year, but most often when it’s nice out and they are out and about in the big old world. They find each other by following the slime trail and the slime likely carries chemicals that alert slugs that it is time to mate. Mating isn’t really a pleasant thing for the banana slug, as they often bite each other literally taking off chunks of flesh. Sometimes they even rise up and strike their partner like a venomous snake does. They do a sort of foreplay for sometimes hours to stimulate each other. It gets worse, too. Once they actually fertilize one another, they thrash about for sometimes hours trying to pull apart.

If they can’t disengage, they’ll turn to taking turns gnawing of the penis to get away from each other. Aren’t you glad you aren’t a banana slug! The only cool thing, is that they do have the ability to regenerate their penis, like a lizard can re-grow their tail.

(Source)