ANAEROBIC RABITATS 37 



are not absolutely fresh are not reliable. Von Brand 

 and Weise (1932) carried out a series of analyses on the 

 bile of freshly killed cattle, sheep and dogs. The values 

 were always very low and ranged from to 0.084 cc. in 

 100 cc. bile. In the case of cattle and sheep, Vo hour 

 elapsed between the securing and the analyzing of the 

 samples. The possibility that some oxygen might have 

 disappeared spontaneously cannot be excluded with cer- 

 tainty. The bile of two dogs, however, was analyzed 

 immediately after death and both samples were found 

 to be oxygen-free. It can therefore be concluded that 

 the bile is, at best, a habitat extremely poor in oxygen. 



Occasionally invertebrates are found inside the tis- 

 sues of living plants. In most cases they probably have 

 access to a sufficient amount of oxygen, but exceptions 

 may occur. Ege (1916) found in the roots and rhizomes 

 of aquatic plants oxygen tensions as low as 15 mm. Hg 

 during the summer and 4 mm. during the winter. 



CONCLUDING REMARKS 



Only when animals are found in environments abso- 

 lutely devoid of oxygen and only as long as they remain 

 there can one be reasonably certain that they lead a truly 

 anaerobic life. But even when the environment contains 

 no oxygen aerobic life is sometimes possible. Thus pro- 

 tozoa harboring symbiotic algae may be found in anaero- 

 bic surroundings. If sufficient light is present there is 

 no reason to suppose that these protozoa would not be 

 able to use the oxygen produced by their symbionts dur- 

 ing photosynthesis. 



Most of the habitats mentioned in the preceding par- 

 agraphs are not absolutely oxygen-free. They are only 

 poor in oxygen to a more or less marked degree. Animals 

 found in habitats of this type should not a priori be as- 

 sumed to lead a truly anaerobic life. Neither can one 

 assume that fermentations would necessarily have a pre- 

 ponderance over aerobic oxidations. Each case must re- 

 ceive individual attention. Onlv then can errors and con- 



