Gill
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A gill ( ) is a specialized respiratory organ that many aquatic animals use for aquatic gas exchange, i.e. to extract dissolved oxygen from water and to excrete carbon dioxide. Branchia (pl.: branchiae, from Ancient Greek βράγχια) is the zoologists' academic name for gills.
The gills of some semi-aquatic clades (e.g. crabs, terrestrial hermit crabs and amphibious fishes), have also adapted to allow air respiration (breathing) on land provided the gills are kept moist. In some terrestrial chelicerates species (tetrapulmonates such as spiders and scorpions), their ancestral gills have evolved into fully air-breathing book lungs. In some planktivorous bony fish species (e.g. silver and bighead carps), the gills are also used as a filter feeding organ via comb-like projections called gill rakers.
The microscopic structure of a gill presents a large surface area in contact with the external environment, which allows optimal diffusion. With the exception of some aquatic insects, the gills of aquatic animals have filaments and lamellae (folds) that contain blood vessels or coelomic fluid, from which gases are exchanged into and out of blood/hemolymph through the thin gill walls before being distributed by the circulatory system to other parts of the body. Gills or gill-like organs, located in different parts of the body, are found in various groups of aquatic animals, including molluscs, crustaceans, xiphosurans, aquatic insects, polychaetes and most aquatic vertebrates (fish and amphibian tadpoles). Semi-terrestrial marine animals such as crabs and mudskippers have gill chambers in which they store water, enabling them to temporarily survive on the reservoir of dissolved oxygen when they are on land.
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