Respiration and Metabolism 



245 



diffusion in supplying the tissues with oxygen. This proportionaHty between 

 consumption and tension may apply to pressures above normal atmospheric 

 oxygen tensions in a number of non-regulatory animals, for example, Limax 

 and Limidus. Such a phenomenon indicates either an increase in oxidative 

 processes as a result of the added oxygen partial pressure or a normal unsatura- 

 tion of oxygen in the tissues at atmospheric pressures. Critical oxygen tensions 

 (to), gas pressures above which oxygen consumption is constant despite varia- 

 tions in the partial pressure, are given by Maloeuf^"*- as follows, for four 

 animals, all of which he regards as suffering a state of oxygen unsaturation at 

 normal ambient tensions: 



Cambarus clarkii 

 Fundulus heteroclitus 

 Limulus polyphemus 

 Triturus pyrrhogaster 



186 mm. Hg pOo 

 190 " " " 

 220 " " " 

 760 " " " 



In contrast to this group of individuals which adjust metabolism according 

 to environmental oxygen tensions stand those organisms with great ability to 



2 4 



CONC. 



6 e 10 



OXYGEN - cc./l- 



Fig. 55. Oxygen consumption plotted against oxygen tension for mayfly nymphs, 

 showing dependence in Baetis, independence in Cloeon, and intermediate conditions in 

 Leptophlehia and Ephemera. From Fox et al."" 



regulate so as to maintain a constant consumption in the face of changing 

 oxygen pressure. Uptake is therefore independent of oxygen tension, at least 

 over a very wide range— e.g., the crustaceans Astaciis and Carciniis, and the 

 molluscs Aplysia and Eledone. In Figure 55 the oxygen consumption of the 

 ephemerid, Cloeon dipterum, is shown constant down to about 1.6 cc. 02/liter 

 of water, corresponding to 32 mm. Hg partial pressure of oxygen."'' Such a 

 degree of respiratory independence indicates respiratory control or a state of 

 more than adequate oxygen saturation prevailing in the tissues even at low 



