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.

sea slug, name for a marine gastropod mollusk that lacks a shell as an adult and is usually brightly colored. Sea slugs, or nudibranchs, are distributed throughout the world, with the greatest numbers and the largest kinds found in tropical waters. They creep along the bottom or cling to submerged vegetation, usually in water just below the low tide line. Members of a few species swim on the surface in open ocean. Most sea slugs are under 1 in. (2.5 cm) long, although the largest, found in the Great Barrier Reef of Australia, reaches 12 in. (30 cm). Regarded by many people as the most beautiful of marine animals, sea slugs display a great array of solid colors and patterns. Many have feathery structures (ceratia) on the back, often in a contrasting color. Most sea slugs have two pairs of tentacles on the head, used for tactile and chemosensory reception, with a small eye at the base of each tentacle. Sea slugs graze on small sessile animals such as coelenterates, sponges, and bryozoans. Certain sea slugs that feed on corals and sea anemones ingest the stinging cells of their prey without discharging them; these then pass from the slug's digestive tract to the ceratia, where they are used by the slug for its own defense. Sea slugs are classified in the phylum Mollusca, class Gastropoda, subclass Opisthobranchia, order Nudibranchiata.

Any species of gastropod that glides along on a broad tapered foot and has no shell or one that is merely an internal plate or a series of granules. Most slugs use the mantle cavity (see mollusk) as a lung. Slugs have a soft, slimy body and live in moist habitats on land (except for one freshwater species). All are hermaphroditic. In temperate regions, the common slugs eat fungi and decaying leaves. Some tropical species eat plants, and some European species eat other snails and earthworms. Seealso nudibranch.

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or sea slug

Any marine gastropod in the order Nudibranchia. Most nudibranchs lack a shell, mantle cavity (see mollusk), and gills, and breathe through the body surface. The delicately colored body, up to 16 in. (43 cm) long, has bizarre defensive outgrowths, called cerata, that discharge nematocysts ingested from cnidarian prey. Antennalike organs arise from the head. Nudibranchs occur in shallow waters of all oceans, where they feed chiefly on other invertebrates, particularly sea anemones. Some species can swim; others are bottom creepers. The term sea slug sometimes refers to all members of the subclass Opisthobranchia.

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