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Nudibranchia (sea slugs; class Gastropoda, subclass Opisthobranchia) An order of molluscs in which the shell and mantle cavity are absent and the body is secondarily (see SECONDARY) bilaterally symmetrical. Many nudibranchs have projections from the body surface (cerata). These occur in a variety of shapes and are often brightly and beautifully coloured. Lacking shells for defence, most nudibranchs swim well and many possess skin glands that secrete sulphuric acid. Some utilize nematocysts from prey they have eaten; the ingested nematocysts are transported along ciliary tracts to cnidosacs at the distal tips of the cerata, where they are used for defence, being replaced every few days.