442 GEO. T. HARGITT AND WALTER W. FRAY 



ants of a single Paramecium. Furthermore all had been grown 

 in sUde cultures of mixed bacteria previous to being transferred 

 to the pure cultures of bacteria. 



For the first cultures bacteria I and II were used as the food. 

 These are both organisms which are abundant in normal hay 

 infusions. The table of characteristics of bacteria will give the 

 description of these forms. Both Paramecium caudatum and 

 Paramecium aurelia, washed clean of all bacteria, were introduced 

 into pure cultures of these bacteria. Along with these, other 

 sister paramecia were grown in the same fluids but with a mixed 

 lot of bacteria as food. This mixture was composed of un- 

 known forms and was obtained by inoculation of the medium 

 from a large stock culture of Paramecium. This sort of con- 

 trol was used since it represents the usual or normal conditions 

 under which the infusoria live. 



Since there was a possibility that the bacteria might be unfa- 

 vorable as food, or might excrete substances toxic to the infu- 

 soria, it was thought well to run a second control of sterile 

 paramecia in sterile tap water. Other workers have shown that 

 distilled water causes the death of Paramecium in a short time, 

 but the tap-water did not have any unfavorable effect. Since 

 the sterile water contained no bacteria whatever, and since the 

 animals introduced into the water had been washed free from 

 bacteria, there would be no food for Paramecium in these cul- 

 tures, and they would starve to death. The length of time 

 before this happened would be a check of the behavior of the 

 animals in other cultures. If the experimental cultures died 

 as soon as the sterile water one, this would mean merely that 

 the bacteria present could nOt be used as food and therefore the 

 animals starved. If they hved longer and divided the bacteria 

 were serviceable as food and their favorableness as food would 

 be indicated by the rate of division and the length of life. If 

 the protozoa in the experimental cultures died sooner than in 

 the sterile water, obviously there were substances of some sort 

 produced by the bacteria which were harmful to Paramecium. 

 In tables 5 and 6 are recorded the number of divisions, under the 

 various conditions mentioned. 



