FEEDING PAKAMECIUM KNOWN BACTERIA 139 



days when the initial rapidity of bacterial growth had somewhat 

 subsided. In order to secure bacteria from an old culture sup- 

 posedly unfavorable to Paramecium, plates were made from a 

 laboratory culture from which the pure lines of Paramecium used 

 in this work were taken, but in which, for some reason, the ani- 

 mals were no longer abundant. The usual technique for making 

 bacterial plates was followed, and only such colonies selected as 

 were predominant on the plates after they had remained at labo- 

 ratory temperature for forty-eight hours. It is believed that the 

 bacteria secured in this way were fairly representative of those 

 which furnish the bacterial food of Protozoa introduced into such 

 an infusion. Pure cultures of the bacteria from the predominat- 

 ing colonies were grown upon agar slants in the usual way and 

 served as the source of the material used in testing the behavior 

 of Paramecium with reference to its food. That these bacteria 

 were representative of those occurring in fresh, middle-aged and 

 old infusions may be safely assumed. 



d. Characteristics of the bacteria used for feeding Paramecium 



When Miss Watt attempted to classify the bacteria used in 

 these experiments, she found, as did Hargitt and Fray, that these 

 saprophj^tic forms are very difficult to identify positively. The 

 classification of Chester ('14) proved more satisfactory for this 

 investigation than the more recent work of the Society of Ameri- 

 can Bacteriologists (Winslow, '20). Chester's classification is 

 based upon reactions with media as well as morphological charac- 

 teristics, and the classification of the American Society of Bac- 

 teriologists lacks completeness in this respect. Neither work is 

 satisfactory in classifying saprophytic bacteria beyond genera; 

 species, therefore, have not been given except tentatively in one 

 or two cases. The organisms have been placed in groups accord- 

 ing to their reactions with media, and each organism has been 

 given the letter used in the experimental work. 



With the exception of one group of bacteria, the organisms 

 used in the feeding experiments were isolated from infusions 

 which had been inoculated from hay or actually used for growing 

 Paramecium. One set of experiments was undertaken in which 



