A slug is an invertebrate. It looks like a snail without a shell. They are most active at night eating plants and leaves. Gardeners do not like them because they damage gardens. During the day they hide in damp places. Slugs leave slimly trails that dries to a shiny silver color. The slime or mucus helps them to move and hold on to things like glass and even spider webs. The slime is not poisonous but taste unpleasant to predators such as snakes and turtles.
Source: A Child’s First Library of Learning- Insect World