Anyone who cares to look at slugs for more than a few minutes, will notice how useful they are at cleaning up. They eat almost anything, including carrion, vegetation and fungi. they will also eat the 'waste products' of other animals - which immediately makes everyone screw their face up and say "yuk", but imagine how filthy the world would be without slugs and their like.
By the way, the 'hole' in the slug's right hand side near it's head is called a pneumostome and it isthrough this that the slug breathes. The pneumostome is situated in the slug's 'mantle' - the smoother part of the body at the front.
At the front of the slug there are four tentacles. The two large tentacles on top are the optical tentacles, while the smaller ones below are sensory, olfactory tentacles and provide the slug with a sense of smell. Slugs are prone to drying out (being made mostly of water) so produce a thick, sticky mucus to help prevent this. For that reason, they are more active in wet weather and at night. The slime trail which is so abhorrent to us, is there to help prevent damage to the soft underside - or foot - of the slug.
See! Fascinating things, slugs!