David George Gordon, Western Society of Malacologists Field Guide to the Slug.

"During courtship, two slugs will circle each other, often for hours, with both partners engaged in ritualized bouts of lunging, nipping, and sideswiping with their tails. The two slugs may also display their disproportionally large sex organs. The great gray garden slug's penis is nearly half its total body length...[great gray garden slugs] are able to copulate in midair, suspended by stretchy strands of mucs up to 17 3/4 inches (45 cm) long. As courtship progresses, a banana slug pair intertwines, wrapping themselves in an 'S' position and stimulating each other for several more hours. Their genital areas (immediately in front of the pneumostome) swell as the pair move even closer together. Penetration takes place, then each slug alternately releases and receives sperm. But in the case of the banana slug, that's hardly the end of this amazing routine. Now the slugs must disengage--a challenge for two animals so amply endowed and thoroughly covered in sticky mucus. After long bouts of writhing and pulling, the pair may resort to what scientists call apophallation. Translated, this means that one slug gnaws off the penis of the other." p. 32