FEEDING PARAMECIUM KNOWN BACTERIA 151 



to this particular line of Paramecium when fed in combination 

 with C/, with the exception of B'C During the course of the 

 work with these organisms, the chance mixture M failed to pro- 

 vide suitable conditions for the life of Paramecium. The records 

 for M will be given, but after careful consideration, it has seemed 

 best to take the culture C as a control for the entire series of 

 observations. C showed greater uniformity in metabolic rate 

 throughout the entire time than any other culture and is particu- 

 larly well fitted to serve as an index for comparing the effect 

 of feeding Paramecium with the other bacteria of this group. 

 The chance mixture M serves as an additional index during the 

 time when it was supplying an adequate food. 



The tables which follow show the averages for the entire time 

 from August 28, 1920, to March 7, 1921, and for the separate 

 periods included in this time. The observations have included 

 a variety of experimental conditions. These were, 1), the be- 

 havior of Paramecium when fed upon the bacteria in pure culture 

 and in mixtures in 0.1 per cent standard timothy hay infusion 

 during the entire length of time from August to March; 2) the 

 effect of change in food without changes in medium ; 3) the effect 

 of changes in medium without changes in food; 4) the effect of 

 changes in food and medium; 5) the effect of sterile media un- 

 combined with food. It would be interesting to add to these 

 another condition, namely, the feeding of dead bacteria of known 

 types in sterile infusions. These various experimental conditions 

 will be described in the order given. 



1. The behavior of Paramecium when fed upon pure cultures of 

 the bacteria A', B', C and the mixtures A' B' , A'C, B'C, andA'B'C 

 in 0.1 per cent standard timothy hay infusion from August 28, 1920, 

 to March 7, 1921. The results obtained during this time are 

 summarized in table 1. In addition to the averages of the di- 

 vision rates and percentages of high divisions, low divisions, and 

 deaths, the number of days each line was under observation is 

 given. Differences in duration of the observations of A'C and 

 A'B'C are due to the fact that, owing to the time necessary for 

 washing the animals, it was not possible to start all lines at once. 

 In the case of the chance mixture, M, the shorter time is due to 



