A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete 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. Branchia (pl. branchiae) is the zoologists' name for gills (from Ancient Greek ). 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. Semiterrestrial marine animals such as crabs and mudskippers have gill chambers in which they store water, enabling them to use the dissolved oxygen when they are on land. History Galen observed that fish had multitudes of openings (foramina), big enough to admit gases, but too fine to give passage to water. Pliny the Elder held that fish respired by their gills, but observed that Aristotle was of another opinion. The word branchia comes from the Greek , "gills", plural of (in singular, meaning a fin). Function Many microscopic aquatic animals, and some larger but inactive ones, can absorb sufficient oxygen through the entire surface of their bodies, and so can respire adequately without gills. However, more complex or more active aquatic organisms usually require a gill or gills. Many invertebrates, and even amphibians, use both the body surface and gills for gaseous exchange. Gills usually consist of thin filaments of tissue, lamellae (plates), branches, or slender, tufted processes that have a highly folded surface to increase surface area. The delicate nature of the gills is possible because the surrounding water provides support. The blood or other body fluid must be in intimate contact with the respiratory surface for ease of diffusion. 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. The concentration of oxygen in water is lower than in air