A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist. The microscopic structure of a gill presents a large surface area to the external environment.

Many microscopic aquatic animals, and some that are larger but inactive, can absorb adequate oxygen through the entire surface of their bodies, and so can respire adequately without a gill. However, more complex or more active aquatic organisms usually require a gill or gills.

Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area. A high surface area is crucial to the gas exchange of aquatic organisms as water contains only a small fraction of the dissolved oxygen that air does. A cubic meter of air contains about 250 grams of oxygen at STP. For a typical freshwater oxygen concentration of 5 parts per millon by mass, a cubic meter of water will contain 5 grams of oxygen. This is about 1/50th of the oxygen of the same volume of air.

With the exception of some aquatic insects, the filaments and lamellae (folds) contain blood or coelomic fluid, from which gases are exchanged through the thin walls. The blood carries oxygen to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water. Gills or gill-like organs, located in different parts of the body, are found in various groups of aquatic animals, including mollusks, crustaceans, insects, fish, and amphibians.

Read more about Gill:  Vertebrate Gills, Invertebrate Gills

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