218 Comparative Animal Physiology 



consist essentially of respiratory cavities filled with water, rhythmically drawn 

 into and expelled from the body. Respiratory trees of holothurians are alter- 

 nately filled and emptied by means of muscular movements of the body wall, 

 and gas exchange occurs by simple diffusion between the lung water and body 

 fluid.^^ The entire hind-gut of the gephyrean worm, JJrechis caupo, consti- 

 tutes a respiratory organ, muscular contractions of the cloacal region serving 

 as the pumping mechanism.^^' This thin-walled respiratory surface, adjacent 

 to the coelomic fluid which is constantly agitated by antiperistaltic waves in 

 the hind-gut, provides adequate gas exchange; water expelled from the hind- 

 gut contains less oxygen by about 40 per cent and more carbon dioxide than 

 the surrounding medium.-^'* The pulmonate snails, Livinaea and Planorhis, 

 are able to live under water for considerable periods of time, the lungs filling 

 with water and aiding the skin in respiration. Water can be rapidly sucked 

 into the "Endblase" or hindmost gut in the dragonfly larva (Aeschna) and 

 brought in contact with the respiratory surfaces.-"*^^ 



Alimentary Mucosa. Some vertebrates possess modified gastrointestinal 

 epithelium which permits uptake of oxygen from swallowed air. Gastric 

 respiration is known to occur in such tropical forms as Plecostomiis and Ancis- 

 trus,^^' ^^ and intestinal respiration has been demonstrated in a great many 

 other varieties of fish. In the loach, Cohitiis, gas exchange is indicated not only 

 by the histologic nature of the mucosa but also by direct determination of the 

 gas of the intestinal lumen, which shows less oxygen (15.7 per cent) and more 

 carbon dioxide (3.0 per cent) than does air."^** Some of the South American 

 tropical fish which inhabit waters low in oxygen are also regarded as intestinal 

 breathers (Doras, Loricaria, Callichthys, and Hoplosternum) .^"^ 



Gas Bladder. The gas bladder, considered by many to be the forerunner of 

 the vertebrate lung, functions in respiratory exchange in many of the physo- 

 stome (open-duct) teleosts, ganoids, and dipnoans. To be efficient as a respira- 

 tory organ, the gas bladder must have some renewal mechanism. Many 

 gas bladders have a respiratory type of epithelium, partitions forming "alveoli," 

 and their own blood supply derived from the pulmonary arch (Fig. 40). The 

 gas bladder can serve merely as an accessory organ when the oxygen tension 

 falls, as in the actinopterygian, Polypterus, or as the main respiratory mechan- 

 ism in the true lungfishes, Protopterus, Lepidosiren, and N eoceratodus.^^-' ^'^'■^• 

 287, 289 (^Pqj q stimulating discussion of the adaptability of the African lung- 

 fish, see Homer W. Smith's Kamongo.''^'-^'^') As a respiratory organ the gas 

 bladder must give up oxygen and take on carbon dioxide so that analyses 

 should indicate less oxygen and more carbon dioxide than in the inspired 

 atmospheric air, a situation shown to exist in many physostome fish.^^^- ^^^ 

 TTie occurrence of a very high oxygen concentration, up to 87 per cent in some 

 cases, in the gas bladder of physoclyst (closed duct) fish from great depths is 

 of interest in connection with possible gas secretion and hydrostatic function 

 but apparently is of little respiratory consequence. If the bladder is punctured 

 or the fish put under considerable pressure so as to increase the specific gravity 

 of the animal, the oxygen content of the swim bladder increases, indicating gas 

 secretion. This gas exchange is under nervous control.--" The physoclyst 

 perch, Perca flavescens, taken from surface water contains oxygen in approxi- 

 mate equilibrium with atmospheric air, but if the fish is subjected suddenly 

 to an oxygen deficiency, as by rapid submergence to a considerable depth, the 



