slug, name for a terrestrial gastropod mollusk in which the characteristic molluscan shell is reduced to a thin plate embedded in the tissues. Like the terrestrial snails of the same order, slugs have a distinct head with a mouth, tentacles bearing eyes, and a lung for breathing air. They move on a muscular foot over a trail of slime which they secrete. Certain species, such as Limax maximus, have become serious pests in gardens and truck farms, particularly in the W United States. Gliding out to feed at night, they devour both the roots and aerial portions of plants with their rasplike radula. Terrestrial slugs are classified in the phylum Mollusca, class Gastropoda, order Stylommatophora.