Parsons, Thomas S. Ramsay Wright Zoological Laboratories, University of Toronto, Ontario, Canada.
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Either of the paired, air-filled respiratory sacs, usually located in the anterior or anteroventral part of the trunk of most tetrapods. Lungs function as specialized organs of respiration. In general, there are two paired lungs, and they are particularly adapted to the respiratory exchange of gases between air and the blood of most tetrapods, including humans (see illustration). However, many elongated tetrapods, such as snakes, possess a simple lung, and one group of salamanders have lost their lungs completely, with gas exchange occurring through the skin of the back. The shape and volume of the lung, because of its pliability, conform almost completely to the size of its cavity. Lungs originated early in fish evolution, presumably in the placoderms, as an adaptation to obtain atmospheric oxygen required for life in stagnant, oxygen-depleted waters. Lungs are still found in a few living species of freshwater fishes [such as lungfish (subclass Dipnoi)]; in these cases, the lungs may fuse to form a single organ and may come to lie well dorsally in the body. In many fishes, the lungs have evolved into the swim bladder, which is a dorsal hydrostatic organ. See also: Adaptation (biology); Animal evolution; Dipnoi; Oxygen; Placodermi; Respiration; Respiratory system; Swim bladder; Tetrapoda
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