From Wikipedia,

another interesting snippet of info about slugs:

'A commonly seen practice among many slugs is apophallation, when one or both of the slugs chew off the other's penis.

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.

Apophallation allows the slugs to separate themselves.

Once the penis has been removed, the slug remains female for the rest of its life.

Various species of slug can also reproduce via tiny "darts" of sperm which they fling in the direction of their mate's genitalia.'