SURVEY OF IXTERTEBRATES 47 



important since the formation of toxic products can easily 

 simulate a sensitivity to the lack of oxygen. Kalmus 

 (1928) explains the rapid death of Paramaecia which he 

 had described in a previous paper (Kalmus, 1927) by this 

 mechanism. The animals had been confined in the ab- 

 sence of oxygen in a minute quantity of fluid, in a capil- 

 lary tube. 



(d) C y s t s . It seems likely that cysts of protozoa 

 would be less sensitive to lack of oxygen than the active 

 forms because of their reduced rate of metabolism, but 

 this is again a point where but little information is avail- 

 able. Alexander's (1930) observation that anaerobically 

 kept Eiiglena gracilis encyst but nevertheless die quickly 

 cannot be considered a proof of the sensitivity of the 

 cysts. The organisms may have already been injured fa- 

 tally before the cysts w^ere fully formed. Lindeman 

 (1942) is of the opinion that protozoa are not able to 

 withstand anaerobic conditions in the cyst stage. There 

 seems, however, to be no certainty that he was actually 

 dealing with cysts in his experiments. His account might 

 just as well be interpreted as meaning that the species 

 used by him were not able to form cysts under anaerobic 

 conditions. 



Van Rooyen (1932), on the contrary, was probably 

 dealing with cysts when he kept plate cultures of Hart- 

 manella castellani under anaerobic conditions for a week. 

 During this time no multiplication took place, but mul- 

 tiplication was resumed after the restoration of aerobic 

 conditions (this is why one can assume that the amoebae 

 were encysted). 



Becquerel (1936) subjected dried earth containing what 

 must have been the resting stages of various amoebae, of 

 Paramaecium, of Euglena viridis and of Actinophrys, 

 for 3 months to a high vacuum (10"^ mm. of mercury at 

 35° C) and found — even after exposure to extremely low 

 temperatures — that the organisms survived. Similar ex- 



