I think both Jeff and Ludwig are correct. They are referred to as both a slug and a snail.
"The marine snails (previously ORDER Prosobranchia)are no longer considered to be a single group with a common phylogenetic ancestry. Amongst these animals, slugs have evolved many times. The Australian Elephant Slug, Scutus, in fact retains a shield like shell (white streak in photo) which is hidden beneath flaps of skin."