178 RUTH L. PHILLIPS 



sary to maintain this rate. The question arises, does this 

 generahzation hold true for the Protozoa? Will Paramecium 

 thrive upon a single type of food, as illustrated by a pure culture 

 of a bacterium, or is a mixed food better for this animal? The 

 fact that Paramecia were able to live upon the streptothrix C for 

 a period of nine months, when the line was discontinued, would 

 seem to indicate that it is possible for them to live upon a single 

 article of diet. Moreover, in the case described, the metabolic 

 rate was more uniform than with any food save the chance mix- 

 ture during the first month. The division rate was not so high 

 among animals fed with C as with the unknown mixture before 

 it failed, or the mixtures A'C and A'B'C. That such organisms 

 as C, which are capable of supporting the life of Paramecium 

 when fed in pure culture, are not numerous is also indicated. Of 

 the nine cultures of bacteria isolated from infusions and tested, 

 only C could be so used. It is also true that satisfactory mix- 

 tures of known bacteria are hard to find. Two only were dis- 

 covered out of twelve tested. There is no question but that the 

 method used in this work, and by Hargitt and Fray, is not suit- 

 able for providing a thoroughly efficient food, but on the other 

 hand it is the only one yet devised whereby the food can be 

 adequately controlled and known types of bacteria included. 

 These bacteria can be determined only by long and tedious tests, 

 and they must first be obtained in pure culture if the food is to 

 be adequately controlled. 



Pure cultures of bacteria usually fail to support the life of 

 Paramecium. This failure may be due to one of two reasons. 

 Paramecium may not be able to digest them and so utilize their 

 contained energy, or else the bacteria may contain or excrete 

 substances which are toxic to this animal. In the first instance 

 the bacteria are ingested, but are not digested. The animal 

 starves as it would in a sterile medium. In the second case death 

 comes more quickly. 



Khainsky ('10) undertook to study the relationship between 

 the structure and the physiological state in Paramecium cauda- 

 tum. He studied the course of the food vacuole and the changes 

 occurring within it by the use of vital stains. It is possible that 



