gills, external respiratory organs of most aquatic animals. In fishes the gills are located in gill chambers at the rear of the mouth (pharynx). Water is taken in through the mouth, is forced through openings called gill slits, and then passes through the gill clefts, spaces between the ranks of delicate gills, bathing them continuously. Each gill is composed of numerous threadlike gill filaments containing capillaries enclosed in a thin membrane; oxygen is absorbed from the passing water and carbon dioxide is discharged. The gills, which may be platelike or tufted, are attached to the outer edges of a series of paired cartilaginous or bony gill (or branchial) arches. Gill rakers, bony comblike projections on the inner edge of the arches, strain solid material from the water, preventing it from passing out through the gill slits and directing it down the esophagus. Gill rakers are present in all fishes except those that feed on large organisms. In primitive fishes (e.g., the shark) the gill slits are exposed; in the bony fishes they are protected by an operculum, or gill cover. In the higher aquatic invertebrates the gills protrude from the body surface and contain extensions of the vascular system. In the crustaceans these external gills are covered by a protective carapace, part of the shell; in the echinoderms they are branched appendages extending from various parts of the body. In the mollusks the gills (called ctenidia) are internal and are located inside the mantle cavity. Horseshoe crabs have gill books, which are membranous flaps like the pages of a book. Amphibians breathe by means of external gills in their aquatic larval stage; a few forms retain the gills after metamorphosing into terrestrial adults. Aquatic insect larvae accomplish the oxygen–carbon-dioxide exchange by means of tracheal gills, projections from the walls of the air tubes (tracheae); these gills disappear when the insect leaves the water. The embryos of all higher vertebrates pass through a stage in which rudimentary gill slits occur, but these never become functional and disappear as the embryo continues to develop.
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