150 RUTH L. PHILLIPS 



A third series of cultures was started on September 3, 1920, 

 with the bacteria J', K', U, J'K', J'L', K'L', and J'K'L' as food. 

 The bacteria J', K', and L' were B. coli, B. cereus, and B. pro- 

 teus respectively. These organisms were unsatisfactory as food. 

 One hundred per cent of deaths occurred during the first interval 

 with L' (B. proteus) as food, and the same result took place 

 when K' (B. cereus) was added to L'. The addition of J' to the 

 mixture seemed to prevent this total mortality. 



The results of feeding these three groups of bacteria in pure 

 culture seem to support the contention of Hargitt and Fray ('17) 

 that pure cultures of bacteria, as a rule, do not prove to be satis- 

 factory food for Paramecium. The ability of one type of bacter- 

 ium to neutralize the harmful effect of another, as illustrated by 

 the combination of 3' with L', and the stimulating effect of com- 

 bining bacteria are interesting problems raised in this work. 

 Reactions of this type will be pointed out as they occur, leaving 

 their analysis for future investigation. The outstanding result 

 of these preliminary experiments, in which nine pure cultures of 

 bacteria were used and twelve combinations of these cultures, is 

 that in no case were they able to maintain the life of Paramecium 

 more than a few days. Moreover, of those cultures w^hich did 

 sustain the life of the animals for a few days, none were able to 

 produce a normal rate of metabolism, and were, therefore, very 

 unsatisfactory food. 



b. Behavior of Paramecium aurelia when fed upon pure cultures, 



and mixtures, of the bacteria A', B' , C under varying 



experimental conditions 



On August 28, 1920, feeding was started with the bacteria 

 A', B', C and the mixtures A'B', K'C, B'C, A'B'C. Observa- 

 tions were continued upon certain of these lines until March 7, 

 1921, with onlj^ such interruptions as the experimental conditions 

 demanded. The original pure lines of Paramecia with which 

 this series of experiments was conducted was still in existence in 

 the mixed food A'B'C and in the pure culture C, on June 2, 

 1921. Although neither A' nor B' proved to be satisfactory 

 when fed in pure culture, they were exceptionally well adapted 



