Slugs are hermaphrodites, having both female and male reproductive organs. The penis of these species is curled like a cork-screw and often becomes entangled in their mate's genitalia in the process of exchanging sperm. When all else fails, apophallation allows the slugs to separate themselves by one or both of the slugs chewing off the other's penis. Once its penis has been removed, a slug is still able to mate subsequently, but using only the female parts of its reproductive system.
From Wikipedia, July 16, 2008.