136 RUTH L. PHILLIPS 



several cultures, when used as food by Paramecium. However, 

 these data were not extensive, and it has seemed worth while to 

 carry the investigation further, in order to test the conclusions 

 announced and to extend the experiments over a wider field. 



Since food is so important a factor in the growth of animals 

 and since the food of Paramecium is so variable, it would seem 

 strange that so few investigations of this kind have been made, 

 were it not that it is a difficult matter to treat adequately both 

 the bacteriological and the protozoological sides of the subject. 

 A complete analysis of the hay infusion would be very desirable, 

 but the present knowledge of the saprophytic bacteria is so in- 

 complete and the bacteriological work is so time-consuming, that 

 it is impracticable for one person to carry it on together with the 

 study of Paramecium in cultures of bacteria isolated from such 

 infusions. The work described in this paper has accordingly 

 been limited to a study of the continued growth of Paramecium 

 aurelia in pure cultures and mixtures of the bacteria isolated from 

 hay infusions or other infusions used for growing Paramecium. 



I wish to express my thanks to Prof. George T. Hargitt, of the 

 Department of Zoology, for his valuable suggestions and criticisms 

 during this work; to Prof. Henry N. Jones, of the Department of 

 Bacteriology, for suggestions and for the use of apparatus for 

 the bacteriological investigations; to Miss Lucy J. Watt for the 

 classification of the bacteria used in feeding experiments; to Mr. 

 Clifton E. Halstead for the determination of the hydrogen ion 

 concentrations of the media, and to Dr. Vasil Obreshkove for 

 suggesting certain biometric methods. 



METHODS 



a. Media 



Since the purpose with which this work was undertaken was to 

 study the effect of food upon Paramecium, it was desirable that 

 the media used be as uniform as possible. To secure such uni- 

 formity, a sufficient amount of each medium was made to last 

 throughout the course of the experiments. These stock solutions 

 were sterilized and set aside to be diluted as needed. Hargitt 



