The Probert Encyclopaedia of Nature

SLUG

Slugs are a group of gastropods, similar to snails, but in which the shell is absent or rudimentary. Because they lack a shell, slugs can only be active during damp weather, otherwise they would dry out and die. As a protection, slugs secrete a foul tasting mucous which they cover themselves with as a defence against being eaten by birds.
Research Slug