462 L. R. CLEVELAND. 



than others, so it became necessary to determine the minimum 

 time required to kill all individuals of each protozoan genus, and 

 in working this out it was noticed that the protozoa of a certain 

 genus would be killed more quickly in one host than in another; 

 consequently, in most instances, a fairly large number of hosts was 

 used. The number of hosts used, the range in time and the mean 

 in time required to kill the protozoa, are given in Table II. It has 

 been impossible to think of a plausible explanation of why the 

 protozoa of one frog are affected more adversely by oxygen than 

 those of another. This w T as noticed when several frogs were oxy- 

 genated in the same flask at the same time. The same phe- 

 nomenon w r as met in the oxygenation of termites and cockroaches. 

 Perhaps more work will throw light on it. 



It is interesting to note that it takes 28 hours to kill the ciliate 

 Nyctotherus in the frog and 3^ hours to kill Nyctotherus in the 

 cockroach. The flagellate Polymastix is killed in 40 minutes in 

 the cockroach while the species of this genus that lives in the frog 

 is not killed until approximately 7 hours. It would be most inter- 

 esting, indeed, to cultivate Nyctotherus and Polymastix from both 

 hosts and then subject them to the same oxygen pressure. It is 

 very probable, though not certain, in view of the oxygenation 

 studies of the frog Trichomonas in vivo and in vitro, that oxygen 

 is actually more toxic for Polymastix and Nyctotherus in the 

 cockroach. 



Some frogs live as long as 65 hours in 3.5 atmospheres of oxygen , 

 more than twice as long as their ciliate and five to six times as 

 long as their flagellate protozoa. 



Twenty tadpoles, 1 2 with Nyctotherus, 3 with Trichomonas, 3 

 with Opalina, 4 with Hexamitus, and 8 with Euglenamorpha, were 

 oxygenated at 3.5 atmospheres with the result that their protozoa 

 were killed in approximately the same time as those of frogs. 

 Euglenamorpha is not present in adult frogs, and it was primarily 

 for this reason that tadpoles were oxygenated. This flagellate is 

 very similar morphologically to plant-like free-living protozoa of 

 the genus Euglena which, as will be seen later, must be oxygenated 

 sixty five hours at 3.5 atmospheres before being killed, while 



1 The method used to determine what protozoa each tadpole harbored was 

 simple: Each individual was placed to itself in a small vesel; very soon a consid- 

 erable quantity of fecal material was passed, which, when macerated and cxaniim-.l 

 under the microscope, revealed the protozoa harbored. 



