By Martin Wheeldon and Wesley Wright (1999)
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Necturus rarely breathes air, but the structures needed for pulmonary respiration are present. Air enters the oral cavity and pharynx by way of the nares and the choanae and leaves the pharynx through the glottis. Amphibials that ventilate their lungs do so by moving the floor of their mouth and pharynx, which, rather than pumping water across the gills, pumps air into the lungs. After the lungs are filled, the glottis is closed, and the air is held in the lungs at greater than atmospheric pressure. When the glottis is opened, pressure in the lungs and recoil of the lungs drives out the air. This mechanism used by amphibians for breathing is called a positive pressure pump. Necturus lacks a nasopharynx, which in later organisms allows them to eat and breath simultaneously.
Through evolutionary time amphibians that are neotenic or retain larval characteristics, such as Necturus, have gill slits and two types of gills. External gills, which are also present in some fish larvae, develop as outgrowths from the neck near the gill slits. Some amphibians also have internal gills that replace external gills as the organism passes through metamorphosis. Internal gills develop closer to the gill arch and may be homologous to the internal gills of fishes. Necturus also illustrates larval retention of a vertical transverse septum and poorly formed tongue. Amphibians also complement pulmonary respiration with cutaneous respiration and eliminate most of their carbon dioxide through the skin. In some circumstances, such as in Necturus, when the organism is under water the skin is also the site of oxygen uptake. The small size of modern amphibians and the resulting large surface area to volume ratio assists cutaneous respiration.