I discovered that the animal I thought was a slug was actually a sea hare, named for the its hare-like "ears." The sea hare does have a shell inside, to protect its heart and gills. It has an input siphon and an output siphon for its gills, and I'm pretty sure now that the opening and closing of the parapodial flaps had to do with the sea hare breathing.
The sea hare is hermaphroditic, and it has a very bizarre way of mating. Here is a link to a National Geographic video explaining how the sea hare reproduces.