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Well, these slugs are hermaphrodites, which means that they can act as both males and females at the same time. When they mate, they insert their penises into each other at the same time. The unusual thing (in case you don't find that unusual enough!) is that sometimes, but not always, when they finish mating one slug will chew the penis completely off the other, a process called Apophallation. Sometimes it happens that both slugs engage in chewing so that at the end of the mating encounter, both slugs are penis-less.

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