Gill

Gill, one of the paired respiratory organs, also called branchia, in many animals that breathe air dissolved in water; also found in the embryo stage of air breathers that evolved from a marine environment. In general, gills are outgrowths of the body wall and are, characteristically, thin-walled structures plentifully supplied with blood vessels. The gill structure varies widely but is typically arranged so that it is constantly bathed in water; the oxygen in the dissolved air passes through the thin membranes of the gill and into the bloodstream of the animal. At the same time the waste carbon dioxide in the blood of the animal passes out through the gill membranes and into the water. The annelid worm, brachiopod, mollusk, echinoderm, certain arthropods, fish, and the larvae of amphibia are equipped with gills.