Definition: "The primary respiration organ in some species of tadpoles, whose function is the extraction of oxygen from water and the excretion of carbon dioxide. The microscopic structure of a gill is such that it presents a very large surface area to the external environment. Gills usually consist of thin filaments of tissue, branches, or slender tufted processes which have a highly folded surface to increase surface area. A high surface area is crucial for effective gas exchange."