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.

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