466 L. R. CLKYl I.AXD. 



of these and many other free-living ciliates will throw light on the 

 problem. 



After having oxygenated the ciliates, the results with the plant- 

 like flagellates Englena and Ileteronenni are not surprising. Some 

 animal-like free-living flagellates would probably yield toxicity 

 results quite similar to those obtained with parasitic flagellates. 



SUMMARY AND CONCLUSIONS. 



The toxicity of oxygen at various pressures for four genera of 

 termites has been determined. At a pressure of 3.5 atmospheres 

 the protozoa are all killed in two genera in 30 minutes, in one in 

 35 minutes, and in another in 40 minutes, while the termites 

 themselves are not killed until 45 hours. Thus, oxygen is more 

 than forty times as toxic for the protozoa as it is for the termites. 

 This makes it possible to remove all protozoa from termites very 

 easily and without injury to the host. 



The protozoa of two termite genera were not killed at one 

 atmosphere of oxygen even in ten days, while in two other genera 

 they were killed in one and three days respectively. This gave 

 an excellent opportunity to work out what effect, if any, partial 

 pressures of other gases of the air, particularly nitrogen, had on 

 oxygen toxicity. All four genera when confined in five atmos- 

 pheres of air (partial O* pressure of 5 atms. of air approximates the 

 total O 2 pressure of I atm. of O 2 ) gave exactly the same result as 

 when confined in one atmosphere of oxygen for the same time. 

 Thus, the toxicity of oxygen is in no way connected with or af- 

 fected by the partial pressures of other gases of the air. It is the 

 partial pressure of oxygen, and not mere mechanical pressure, 

 that matters. 



Cockroaches harbor many kinds of protozoa, all of which were 

 removed by oxygenation at 3.5 atmospheres in 3-3 hours; the 

 flagellates, Lophomonas and Polymastix, were killed in 40 minutes, 

 and the ciliates, Nyctotherus and Balantidium, in 3^ hours. The 

 cockroaches themselves were not killed until 90 hours. Thus, 

 oxygen at this pressure is 135 times as toxic for the flagellates ami 

 26 times as toxic for the ciliates living in cockroaches as it is for 

 the insects themselves. 



It is highly probable that all insect-inhabiting protozoa may be 

 removed by oxygenation without injury to their hosts. If so, the 



