50 



RESPIRATORY MECHANISMS 



pn.d. 



Fig. 28. Air bladder of Erythrinus 

 unitaniatus. pn.d. pneumatic duct, i.r. 

 internal ridges enclosing alveolar spaces 

 1/2. (Carter and Beadle.) 



gas bladder or any other cavity as an air-breathing organ are 

 first the regular or irregular renewal of the contained air. In 

 a physoclyst fish the swim bladder can never be a respiratory 

 organ, even if the contained oxygen is in times of distress used 



for metabolic purposes. In 

 physostome fishes mechan- 

 isms must be present for ven- 

 tilating the bladder, and air 

 must actually be passed in 

 and out. In Polypterus bichir 

 the gas bladder functions as 

 an accessory respiratory or- 

 gan only when the oxygen 

 content of the water is low 

 (Budgett, 1900). 

 The gas mixture expired must contain less O2 and more C0 2 

 than the inspired atmospheric air. The difference in C0 2 is 

 generally much less than the 2 deficit, because CO2 is given 

 off also to the water through the gills and the skin. 



Anatomically the walls of 

 the cavity must show a re- 

 spiratory epithelium with a 

 dense network of capillaries 

 close to the surface (Carter 

 and Beadle, Fig. 29 a, b, c). 

 In many cases, and probably 

 always, when the gas bladder 

 is the main respiratory or- 

 gan, the internal surface is 

 enlarged by epithelial ridges 

 often forming distinct alveoli 

 (Fig. 28). 



The older anatomists placed a great deal of reliance on the 

 character of the blood supply as a criterion, an arterial supply 

 being taken to preclude a respiratory function. This has 

 turned out to be a mistake, and the air-breathing organs in 

 fishes are supplied with blood in the most diverse ways. 



Fig. 29. c. Capillaries in air blad- 

 der alveoli, p.ep. respiratory epithe- 

 lium. (Carter and Beadle.) 



