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the organs that perform the exhange of gases between an organism and its external atmosphere. Respiratory organs are found only in aerobic animals, which obtain free oxygen directly from atmospheric air (aerial respiration) or from air dissolved in water (aquatic respiration). Unicellular and multicellular anaerobic organisms have no respiratory organs. In addition, many groups of lower aerobic organisms have no respiratory organs (protozoans, sponges, coelenterates, and many worms). In animals with thin, delicate body coverings oxygen may penetrate the body by diffusion through the entire body surface (cutaneous respiration). This type of respiration is sometimes combined with intestinal respiration, in which the oxygen enters the body through the wall of the intestinal canal (for example, in coelenterates). Among vertebrates, cutaneous respiration is important in many fish and amphibians. The origin and structure of the respiratory organs of animals differ. Aerial respiratory organs are called lungs and tracheae, and aquatic respiratory organs are called gills.
Of special interest are the supplementary respiratory organs found in fish that live in slime (certain Clupeidae, Characinidae, and Siluridae). These supplementary organs are racemose processes that grow out of the gill arches and are used for aerial respiration. In labyrinthic fish, bony plates develop on the first gill arch which are covered with a mucous membrane through which oxygen is absorbed. This device makes it possible for the climbing fish (Anabas) to remain on dry land for long periods.
In many fish, such as Amphipnous and Saccobranchus, air bladders similar to true lungs may grow out of the upper part of the gill pouches. In the individual development of many organisms, there is a succession of various respiratory organs. Thus, in the order of insects Ephemeroptera, which develop in the water, cutaneous diffusion respiration takes place at first. It is replaced by respiration through tracheal gills, which gives way to the tracheal aerial respiration characteristic of the adult insect. In fish larvae before the final development of gills, respiration is performed first by the yolk sac, which is supplied with a rich network of blood vessels; and later by the vessels of unpaired fins and sometimes by temporary (larval) external gills. In amphibians in the larval stage the network of blood vessels of the yolk sac is used for respiration. Later, external gills usually develop, and often, vascular networks develop in the unpaired (caudal) fin. All embryos of amniotes breathe initially by means of the network of blood vessels of the yolk sac. Subsequently, so-called allantoid respiration supplements respiration by the yolk sac and replaces it when a dense network of vessels has developed in the walls of the allantois. Only after hatching (reptiles and birds) or after birth (mammals and humans) do the final respiratory organs—the lungs—begin to function.
A. N. DRUZHININ