46 RESPIRATORY MECHANISMS 



content is reduced to 2.5 ml/1 (55 mm tension), while a corre- 

 sponding emergency respiration is not induced in the fish 

 Leuciscus erythrophthalmus until the oxygen is as low as 0.6 ml/1 

 (Winterstein, 1908). Respiring from the surface, many ani- 

 mals will take in air which will increase the saturation of 

 the water passing the gills, but is not enough to characterize 

 them as air-breathers (Dighstra, 1933). 1 



True air-breathing can be accomplished in a large number 

 of ways. 



Fig. 20. Vascular supply of inside of operculum in Pseudapocryptes. 

 10/1. (Das.) 



Carter and Beadle (1930-32) describe the case of Hypopomus, 

 a small fish common in swamps in tropical South America. 

 This fish can live in well-aerated water, even when denied 

 access to the surface, but it will take in air when opportunity 

 offers and pass it through the branchial cavities. No special 

 organ for respiration of air was found, and the gills appear to 

 be used both for water and for air. The secondary lamellae 

 seem to be unusually far apart, but they are not described as 

 rigid. Perhaps the air is used mainly to aerate the water 

 passing over the gills. 



1 Also animals living in burrows in the tidal zone may aerate the water in 

 their burrows during ebb, as observed by Lindroth (1938) on Nereis. 



