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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.
organs of aquatic respiration in animals; body processes through whose thin walls CO2 from the blood or chamber fluid circulating in them is discharged into the surrounding water and O2 from the water is absorbed.
In the process of animal evolution, gills first appear in annelid worms, in the form of simple, pectinate, or bipinnate plates and simple or branched filaments. They are located on the lateral motor appendages (in freely moving forms) or at the head end of the body (in those living in tubes). The gills of mollusks are ctenidia, bipinnate processes in the mantle cavity. In lamellibranchiate mollusks they have been modified to reticular plates. In gastropod mollusks, owing to the asymmetry of their bodies, the right gill often disappears; in many of them, the gills are gradually reduced and part of the mantle cavity is converted into an organ of air respiration as a result of the transition from aquatic to terrestrial life. The gills of crustaceans develop as appendages of the extremities; in the majority of lower crustaceans (copepods and ostracods) the gills are absent. In the majority of higher crustaceans, gills are presented as bipinnate branched or lamellar appendages of the thoracic legs. Owing to the transfer of certain crustaceans to dry land (tropical species of crabs and hermit crabs), their gills were gradually reduced and the gill cavity was transformed into the lung cavity, adapted for air respiration.
Aquatic larvae of insects (such as mayflies, stone flies, and certain dragonflies) have so-called tracheal gills—thin-walled processes on various parts of the body, through whose walls an exchange of gases takes place. Among the echinoderms, only sea urchins and starfish have gills.
All primarily aquatic chordate animals have gill openings, or slits, in the pharynx, which open either directly to the exterior or into a special peribranchial cavity that is connected to the external environment. In enteropneustal, tunicates, and acraniates (lancelet), gas exchange is accomplished when water passes through the gill slits, through whose partitions the blood vessels go. Cyclostomi have six to 14 pairs of gill pouches bearing branchial lobes of entodermal origin. Unsegmented gill arches are located outward from the gill pouches. In fish the gill lamellae are formed of ectoderm. The semigills of two neighboring gill slits, located anteriorly and posteriorly on the gill partition, form a complete gill. On the interior pharyngeal edge of the gill partitions there are cartilaginous or bony articulated gill arches. The outer edges of the gill partitions in sharks (only in sharks) cover each gill slit in the form of cutaneous folds. In sturgeons the gill partitions are shorter than in sharks; in bony fish they almost completely disappear; the gill lamellae are attached directly to the gill arch and are suspended freely in the gill chamber, which is covered from the outside with a movable gill cover (or operculum) and is protected inside by processes of the gill arches called gill rakers. The larvae of Polypterus, Dipnoi, and certain bony fish (such as groundlings) have external gills in the form of feathery processes on the gill arches.
In amphibians, gills develop as larval organs of aquatic respiration, or in the form of external gills (caudate amphibians) or external and internal gills (acaudal amphibians). In some caudate amphibians, the external gills remain throughout life. A number of salamanders, having converted to a terrestrial, air-breathing life and become viviparous, lack the larval stage with the external gills (for example, the California salamander). In a number of acaudal amphibians that have lost contact with water even during the spawning period (such as the tropical frog of the tree-frog family), this stage of development is also absent. In some frogs with underdeveloped lungs, numerous cutaneous processes that function as gills may develop temporarily.
REFERENCESDogel’, V. A. SravniteVnaia anatomiia bespozvonochnykh, part 1. Leningrad, 1938.
Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Balabai, P. P. Morfologiia i filogeneticheskoe razvitie gruppy bescheliustnykh. Kiev, 1956.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vols. 1–2. Moscow, 1964.
V. N. NIKITIN