Abstract
Many investigators have contributed evidence pointing to the probable origin of air-breathing vertebrates from certain physostomous fishes, which apparently have some organ capable of supplementing respiration in times of need. Certain Ganoids and Dipnoians have been credited with this transition of vertebrates from water to land.
From anatomical data alone, some authors have declared the swimbladder to be a respiratory organ in several genera of fish, including Lepidosteus. It has been observed that Lepidosteus and a few others apparently do inhale atmospheric air, 1 , 2 but so far as the writer knows, there exists no proof that this inhaled air is carried into the swimbladder, which would be absolutely necessary if this organ is to be considered an organ of respiration. The existing hypothesis that the swimbladder has such a function is based upon the following facts: (a) that it has a cellular, spongy appearance internally, 3 (b) that blood is distributed to a rich capillary network in its walls, then collected and carried directly to the heart, 4 (c) it has a connection with the pharynx by the ductus pneumaticus and glottis and (d) that apparently the animal gulps air into the mouth. 1
In this article it is shown experimentally that Lepidosteus actually inhales atmospheric air into the swimbladder, where it serves in supplementing the respiration, both normally and to a much greater extent when the oxygen content of the water is low.
The method of procedure has been: (1) to carry on experiments with normal animals in deoxygenated water, allowing part of them to come to the surface for air, and confining others below the surface; (2) to inject melted paraffin through the glottis into the swimbladder of live animals, and run them against controls in normal and deoxygenated water; (3) analysis of gas from the swimbladders of the fish under various conditions, immediately following inhalation of air and again in the interval between inhalations, and (4) analysis of air from above deoxygented water (covered with a film of paraffin oil) in an air tight chamber where an animal is living.
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