8 RESPIRATORY MECHANISMS 



definitely reduced. At high temperatures this reduction is 

 fatal, but in winter the operated animals live on and pupate 

 normally. Similarly Zoond (1931) has found that the bran- 

 chial filaments of the sabellid worm Bispira voluticornis could 

 be removed without injuring the animal's life, although the 

 oxygen consumption would be reduced from 56 to 20 ml/kg/ 

 hour. K. Berg (1 938) finds that the larvae of Chironomus liebeli- 

 bathophilus grow much more slowly at depths of 17-20 m in 

 Esrom Lake, where the oxygen concentration is very low, 

 than at 11-14 m where it is generally ample. 



As matters stand at present there would appear to be three 

 possible ways for animals to react to an inadequate supply of 

 oxygen. One is a simple reduction of requirements by slow- 

 ing down vital processes, another is represented by incom- 

 plete oxidations, corresponding to an oxygen debt which is 

 paid off when oxygen again becomes available, and the third 

 is the definite transition to anaerobic metabolism by which 

 comparatively small amounts of energy are acquired by the 

 breakdown of large amounts of nutritive material and the 

 products of this breakdown eliminated. Harnisch (1937) has 

 shown that both in true anaerobionts (Ascaris) and in free 

 living forms exposed to lack of oxygen (Tubifex, Planaria, 

 Chironomus) the products of anaerobiosis are oxidized within 

 the body when oxygen becomes available. This "secondary 

 oxybiosis" is governed by oxidative enzymes in the haemo- 

 lymph, and its rate is clearly dependent upon oxygen concen- 

 tration. 



