164 RUTH L. PHILLIPS 



increase in metabolism results and the animals fail to reach the 

 metabolic level of their respective controls. Although these 

 results indicate that it is desirable to maintain a constancy of 

 satisfactory food, one would not be justified in maintaining that 

 this is always the case. Further investigation of such phenomena 

 as have been described is needed. 



After testing the effect of such changes in food, it was decided 

 to make a more radical change and determine how this particular 

 line of Paramecium would react toward certain of the bacteria 

 which had previously been found to be unsatisfactory. The 

 bacterial cultures J, K, and L were used separately and in com- 

 bination with one another. They proved to be as unsatisfactory 

 as when previously used, the change from a favorable food like 

 A'C being immediately followed by a marked depression of the 

 division rate and a speedy death of the animals- 



Under the conditions of the above experiments, change in 

 food of Paramecium had one of two effects dependent on the 

 nature of the food. WTien the change was from one satisfactory 

 type to another, the evidence was in favor of a constancy of 

 food under the conditions obtaining in this experiment. If, 

 however, the change was from a markedly satisfactory to a de- 

 cidedly unsatisfactory type of bacteria, a sudden lowering of the 

 metabolic rate occurred, resulting in the speedy death of the 

 animals. 



3. Changing the medium without varying the food. Since so 

 much emphasis has been laid by workers with Paramecium upon 

 the nature of the medium in which the animals were cultivated, 

 it was considered desirable to try the effect of a change in medium 

 without varying the food, in order to throw more light, if possible, 

 on the relation of this animal to the medium when it was furnished 

 with a food which long trial had shown to be satisfactory. The 

 media used w^ere made according to the same formula as the 

 standard hay infusion, and used in the same way for growing the 

 bacteria used for feeding. The first infusion tested was made 

 from dry, uncured swamp hay, and the results of using this are 

 summarized in table 6. 



