Radula of snail/slug. Drawing from Wikipedia
Slugs breath through a special apparatus called a pneumostome. It is an opening on the right side of the slug where air enters into the slug's single lung. Slugs are placed in family groups based on the location of the pneumostome on the slug's body.
Slugs also have an amazing mating ritual. This could be right out of a sci-fi movie. It is fascinating and actually beautiful, in a weird sort of way. I really could not do it justice by describing it, so I will let the master, David Attenborough, describe it for you in this video clip from Life in the Undergrowth.