54 SURVEY OF INVERTEBRATES 



appreciably lowered, at least in the deeper strata, even if 

 the surface of the fluid is exposed to air. This view is 

 substantiated by Jacobs' (1941) observation that in tubes 

 in which Endamocha histolytica is grown together with 

 bacteria, low oxidation reduction potentials are estab- 

 lished. Analyses of the gases accumulating in flask cul- 

 tures of the same parasite likewise support this idea (von 

 Brand, Rees, Jacobs and Reardon, 1943). 



A few observations have been reported indicating that 

 at least some cf the protozoa dealt w^ith in this section 

 prefer low to high oxygen tensions. Snyder and Meleney 

 (1941) produced the excystment of Endamoeha histoly- 

 tica in bacteriologically sterile surroundings only when 

 the conditions were anaerobic. The same investigators 

 recently pointed out (1943) that culture forms of this 

 amoeba which had been grown anaerobically were quite 

 sensitive to molecular oxygen. Birch-Hirschfeld (1936), 

 on the other hand, seems to have cultured Endamoeha 

 histolytica under clearly aerobic conditions. A ditferent 

 behavior of various strains or a gradual adaptation to 

 various conditions may explain these differences. 



Cleveland's (1928) experiments are very suggestive 

 in this connection. He showed that Trichomonas fecalis 

 could be grown with dead bacteria as food only if the 

 oxygen tension of the medium was maintained low by a 

 vaseline seal. This precaution was not necessary if liv- 

 ing, oxygen-consuming bacteria were present in the 

 culture. Chatton (1918 and 1918a), reported that a Tri- 

 chomastix species from a gecko and grown in a bacterio- 

 logically sterile medium was injured by an excess of ox- 

 ygen. The necessity of small amounts of oxygen seems 

 to be well established for this species. 



B. Type of life in normal habitat. It would be prema- 

 ture to draw general conclusions as to the mode of life of 

 the intestinal protozoa w^ithin the intestine, from the scat- 

 tered observations available. The oxygen relationships 

 are probably different in various species. 



