ENCYSTMENT OF DIDINIUM NASUTUM. 107 



ments on the effect of temperature, food, and the age of the food- 

 culture on the encystment of Didiniuui nasutuin. 



MATERIALS AND METHODS. 



The didinia used in these experiments were all derived from 

 cultures started from a stem-culture kept continuously in the Labo- 

 ratory. Battery-jars were used as containers for these cultures. 

 From time to time, as the supply of food in the culture-jars became 

 depleted, a portion of the liquid was replaced by a corresponding 

 quantity of a fresh, vigorous, paramecium culture. In this way 

 the didinia were kept continuously in a flourishing and active con- 

 dition. 



The effect of temperature and food on encystment was ascer- 

 tained as follows : Into each of five square watch-glasses, previously 

 sterilized, was placed 3 c.c. of culture fluid containing numerous 

 paramecia, and into each of five similar watch-glasses 3 c.c. of 

 culture fluid taken from the same jar, but filtered so as to remove 

 all paramecia and other organisms that might serve as food for 

 Didinium. To each of the watch-glasses thus prepared there were 

 added vigorous didinia, usually five, all taken from the same jar. 

 Two of the watch-glasses, one with and one without food, were 

 then placed into each of five thermostats maintained at different 

 temperatures as indicated in Table I. All of these cultures were 

 examined for cysts once every day until they died out. In this 

 experiment there were consequently under observation simulta- 

 neously ten didinia cultures which were precisely the same with 

 the exception of food and temperature. This experiment was re- 

 peated a number of times as indicated in the accompanying tables. 



In a number of additional experiments there were fewer cultures 

 and temperatures under simultaneous observation. In an extended 

 series of experiments made after the main part of the work was 

 completed there were only two cultures, one with and one without 

 food, both at 27 in each experiment. Cultures containing fluid 

 taken from the jar which had contained the didinia used in the 

 tests were added to some of the thermostats in a few experiments. 

 This, however, had no appreciable effect on encystment. 



The results obtained in all of these experiments are summarized 

 in Table I. These results show conclusively that at 25-3O en- 



