SURVEY OF INVERTEBRATES 55 



Obviously, the protozoa inhabiting the intestinal lumen 

 and having no direct contact with the wall of the intes- 

 tine, will gain their energy purely from anaerobic proc- 

 esses when the intestinal contents are found to be en- 

 tirely devoid of molecular oxygen. But when small 

 amounts of oxygen are found in analyses, one is hardly 

 justified in concluding the existence of an aerobic life. 

 There will be a competition between the bacterial flora 

 and the intestinal fauna for this oxygen. One can sur- 

 mise that the bacterial activity might cause a complete 

 local depletion of oxygen. The ''microatmosphere" in 

 which the protozoa live might thus be frequently anaero- 

 bic, even if the ''macroatmosphere" of the intestine is 

 otherwise. In this respect the situation is quite different 

 for the small protozoa and the much larger helminths. 

 It is interesting in this connection to note that Hinshaw 

 (1927) regards Trichomonas buccalis as an obligate an- 

 aerobe, despite the fact that it lives in the mouth of ver- 

 tebrates, which is a large, open cavity. If this is correct, 

 this parasite could survive in its normal habitat only if 

 the oxygen consumption of the local bacterial flora is po- 

 tent enough to change the microatmosphere from aerobic 

 to anaerobic. If, on the other hand, oxygen is at times 

 present in the immediate surroundings, the protozoa will 

 be able to get a maximum amount of oxygen more easily 

 than can the worms since their relative surface is incom- 

 parably greater. 



Doubtless, most of the intestinal protozoa (and those 

 occurring in related habitats) actually use oxygen when 

 they can get it. Some, like Balantidium and Trichomonas 

 foetus consume rather large amounts of this gas, at least 

 when its tension is high, as Daniel (1931) and Eiedmuel- 

 ler (1936) have demonstrated. Protozoa living close to 

 the intestinal wall may get oxygen directly from the host 

 cells. This is illustrated in the case of Giardia which has 

 a habit of fixing itself to the epithelial cells of the intes- 

 tine. Yon Brand (1934) is of the opinion that this or- 



