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DescriptionOnchidoris bilamellata can grow to 4 cm in length and are usually white with a brown pattern on the mantle. However immature specimens and the occasional adult may be completely white in colour. It bears many (usually white) club shaped projections (tubercles) on its mantle and has two ringed sensory tentacles on its head. The numerous (up to 29) retractile gills are speckled brown around the edges and arranged in a horseshoe shape. The head extends to form an oral veil which lacks tentacles. The spawn is in the shape of a flattered coil attached to the substratum by one edge and contains many thousand eggs.This species produces acid secretions from the mantle when disturbed. It is gregarious and feeds exclusively on barnacles, especially Semibalanus balanoides.