FEEDING PAEAMECIUM KNOWN BACTERIA 173 



Since the animals fed with C, A'C, and A'B'C showed no 

 tendency toward diminished vigor for a period of more than six 

 months in the standard hay infusion, it is possible that the di- 

 verse results obtained by Woodruff and Calkins may have been 

 due to fundamental differences in the bacterial content of their 

 infusions rather than to the nature of the media or to the old 

 age of the Paramecia. May it not be that the hay infusion is not 

 so well suited to the maintenance of a favorable mixture of bac- 

 teria as is beef extract, and that this accounts for Woodruff's 

 results with this medium? Granted that substances do occur in 

 media which are capable of stimulating Paramecia for a time, is 

 it not more likely that the most important factor in the environ- 

 ment is food? 



The full importance of food in the behavior of Paramecium 

 was first recognized by Jennings ('08). He not only recognized 

 that cultural conditions should be identical for different series 

 of animals, but he took special precautions to make them so. 

 He realized that the nature of the bacteria in a given culture 

 was very important, for he says: "It is not sufficient to attend 

 merely to the basic fluid, the bacteria in the culture must be the 

 same." His methods were not directed toward a determination 

 of the exact nature of this food, and he was probably correct in 

 maintaining that if precautions were taken as to the method of 

 making cultures, frequent changes of the animals, and the like, 

 a reasonably constant mixture of bacteria would be obtained, in 

 which enough favorable types would be present to effect a normal 

 metabolism in the animals observed. 



The first account of which we have record of feeding Parame- 

 cium with a particular bacterium is that of Popoff ('10). He 

 speaks of feeding Paramecium caudatum with cultures of B. 

 proteus mirabilis grown upon potato. Popoff's attention was 

 centered upon the effect of various media on cells. In this case 

 the medium used was ammonia-rich water. He supposed that 

 the food was uniform, but makes no mention of having freed the 

 animals of other bacteria. These Paramecia lived but a few days, 

 and their death was attributed by Popoff to the nature of the 

 medium. Since my work has shown that Paramecium aurelia 



