Gastropods are the least changed from their ancestors. There are almost 40,000 described species in the world and they occupy nearly every habitat on earth. Equally diverse is their feeding habits, their shapes, their colors, and their sizes.
Agriolimacidae (Terrestrial Slugs)
Meadow Slug — Deroceras leave
AKA Marsh slug, Brown slug
A very small terrestrial slug with no shell is common around the world and has adapted to a variety of habitats but does require wetter areas. This family is mostly herbivorous; some have reputations for being garden and ag pests. Most slugs in Wisconsin on non-native. I wonder why this is?
Terrestrial Mollusk Tool has some good info on slugs and other mollusks