Paired, air-filled respiratory sacs, usually in the anterior or anteroventral part of the trunk of most tetrapods. 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. Lungs originated, however, 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 fresh-water fishes (such as lungfish) in which they 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, a dorsal hydrostatic organ.