232 GARY N. CALKINS 



1.18 for the last 20 days, falling finally to zero with encystment 

 of all the living material in approximately the 131st generation 

 (chart A). 



The percentage of encystment, computed from the total num- 

 ber of individuals to encyst in the 5-day periods and the total 

 number of individuals under observation, is shown at the bottom 

 of chart A. There is but little ground for comment here, the 

 fluctuations coinciding more or less with those of the division 

 rate. The percentage was low at the outset but increased later 

 until it finally rose to 100 per cent. 



The death rate, compute^ in the same way as the encystment 

 rate, was comparatively low throughout the cycle, never rising 

 aboVe 8 per cent (chart A, top). 



The records of the numbers of Paramecium eaten, determined 

 by the numbers found alive at the end of twenty-four hours, 

 were not begun until one month after the cultures were started. 

 These records furnish the basis for a study of the variations in 

 what may be termed the 'appetite' of Didinium, shown in the 

 dotted line of chart A. The data for this curve were obtained 

 as follows: In 5-day periods the five lines of culture material 

 are provided with 45 Paramecium daily, or 225 during the period. 

 Add to these 25 per cent for approximate increase by division 

 before being eaten, giving 280 Paramecium for the 5-day period 

 for all five lines. The daily records give the numbers of Para- 

 mecium alive at the end of twenty-four hours. These are aver- 

 aged for 5-day periods and the average, divided by 280, gives 

 the percentage of uneaten Paramecium, which subtracted from 

 100 per cent gives the approximate percentage of Paramecium 

 eaten. Rough as this method is in illustrating the variations 

 in appetite of Didinium the curve nevertheless follows that of 

 the division rate with remarkable fidehty. It might be argued 

 that the division rate should follow the eating rate and that the 

 5-day periods in the 'appetite' curve should be twenty-four hours 

 in advance of the periods indicating the division rate. But it is 

 equally true that the feeding rate depends on the vitality of 

 Didinium, and as the records for feeding brought the initial 

 dates of the periods forty-eight hours later than those for the 



