444 GEO. T. HARGITT AND WALTER W. FRAY 



tion in the rate of division is the early death of the paramecia. 

 In every case where either of these two kinds of bacteria was 

 used alone as food death of the paramecia occurred within two 

 weeks, in some cases within a week. Since these bacteria were 

 present in great abundance in normal hay infusions, and were 

 present in the control of mixed bacteria, and since the protozoa 

 lived longer in these cultures than in sterile water, it seems clear 

 that the lowering of the rate of fission and the early death of 

 Paramecium is not due primarily, if at all, to toxic excretions of 

 the bacteria. If this is true the only possible conclusion is that 

 neither of these bacteria is suitable as food for Paramecium if 

 used alone. 



Paramecium caudatum was rather hard to cultivate in slide 

 cultures, even of mixed bacteria, and because of this difficulty 

 and the uncertainty in interpretation brought about by having 

 to replace the cultures which died out this species was no longer 

 used. All records hereafter refer to Paramecium aurelia only. 



Paramecium was next grown in pure cultures of other bac- 

 teria. The bacteria are designated by numbers for convenience 

 but their names and characteristics will be found in the table 

 at the end of the paper. The bacteria chosen for this second 

 series of cultures were of two sorts: those from normal hay 

 infusions which were but slightly zymogenic, and others taken 

 from abnormal infusions. It was believed that in the slightly 

 zymogenic forms we might find some which would prove to be 

 satisfactory as food, even when used alone (Bacteria III, IV, 

 V, XI, were isolated from normal hay infusions). Those bac- 

 teria (VI, VII, VIII, IX, X) taken from abnormal infusions 

 were chosen to determine whether they were favorable or unfa- 

 vorable as food. Controls were carried on in mixed cultures in 

 every case. 



The results show that the bacteria isolated from abnormal 

 haj^ infusions were so unfavorable for food that practically no 

 growth or divisions occurred in any of them. Death took- place 

 rather quickly, in some cases more quickly than in sterile water. 

 Not only is the unsuitability of these forms as food demon- 



