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Fishes “breath” via gills. Gills, or branchiae, are extremely vascularized organs specialized in the gas exchange beneath water and exist in aquatic animals (that is, crustaceans, marine annelids, fishes and tadpoles). Gills are a respiratory organ (that is, analogous, for illustration, to lungs) having very thin lamellae with numerous apparent blood vessels in direct contact with water. In osteichthyes, the gills are wrapped by a bony flap which protects them termed as operculum. In chondrichthyes, there are no opercula.