52 SURVEY OF TXVERThJ URATES 



periments with the cysts of Colpoda were performed for 

 a shorter period by Taylor and Strickland (1936). 



Concerning the influence of low oxygen tensions on 

 encystment the results reported vary. According to Brown 

 (1939) Colpoda does not encyst if the oxygen pressure 

 is below 15 mm. of mercury. Cysts, however, survived 

 the treatment, since excystment proceeded normally after 

 oxygen was readmitted to the medium. Johnson and 

 Evans (1941), on the other hand, found that low oxygen 

 tensions induced the formation of unstable cysts in Wood- 

 ruffia, but no data on the survival of these are given. 



Concluding remarks. One fact is brought forcibly to 

 mind if one weighs the evidence presented so far, and 

 that is the striking difference in the resistance of nor- 

 mally aerobic protozoa to the lack of oxygen under ex- 

 perimental and under natural conditions. In the former 

 case the periods of survival are, in general, relatively 

 short and the organisms rapidly exhibit abnormal phe- 

 nomena {e.g., cessation of amoeboid and ciliary move- 

 ment, of vacuolar contraction and of cyclosis). In nature, 

 on the other hand, a fauna rich in ''aerobic" protozoa 

 which appear perfectly normal, is often found in habi- 

 tats that, like the stagnating strata of lakes, are devoid 

 of oxygen for weeks or even months at a time. Accord- 

 ing to Juday (1908) and von Brand (1934) the explana- 

 tion for this curious discrepancy may lie in the fact that 

 in a natural habitat a gradual decrease of the oxygen 

 tension occurs, while under experimental conditions an 

 abrupt change to anaerobiosis takes place. The slow 

 transition in nature may allow a gradual adaptation, 

 while the sudden changes in the laboratory may be in- 

 jurious. It is well known that protozoa withstand much 

 higher concentrations of poisons and abnormal osmotic 

 concentrations if the changes are brought about step by 

 step than if the extreme conditions are established at 

 once. An experimental analysis of the anaerobiosis 

 problem from this angle should prove fruitful. 



