96 S. O. MAST. 



smallest found in the cultures worked with measured 0.117 mm. 

 in length and 0.034 mm. in width and one of the largest was 

 0.339 mm - long and 0.095 mm - wide, and there were many others 

 of similar size, both small and large, in the cultures. A clear 

 conception of variation in size can be gained by referring to the 

 various figures presented in this article. 



The amount of food available is one of the chief factors in 

 regulating the size of Didiiiiiun. In the absence of food they do 

 not immediately encyst as Thon (p. 293) says, but continue to 

 divide by fission, becoming smaller and smaller after each 

 division. On May i at 10:00 A. M. five exceptionally large 

 specimens were taken from a culture and put into a solution with- 

 out food. At 4:00 P. M. there were six present, one of which 

 was a double monster. May 3 at 9 : oo A. M. eleven ; May 4, 

 9: oo A. AT., eighteen; May 5, 9: oo A. M., twenty-three; May 6, 

 9: oo A. M., twenty-six ; May 7, 9 : oo A. M., twenty. After this 

 the number decreased. I am not positive that they encysted. The 

 monster did not divide at all, consequently twenty-five specimens 

 were produced from four in the total absence of food. One 

 would therefore expect the individuals of the final generation to 

 be much smaller than those of the original. They were in fact 

 about one sixth as large. 



These experiments show that size has little if any influence on 

 fission and that absence of food does not primarily cause 

 Didimum to encyst. Neither does the presence of food cause 

 these organisms to come out of the cysts. This was clearly 

 shown by adding Parauiccia to solutions containing encysted 

 Didinia. If only a small amount of liquid was transferred with 

 the Para/nccia, the Didinia seldom came out of the cysts; on the 

 other hand, they were induced to come out on several occasions 

 by merely adding water. This seems to indicate that the chemical 

 contents of the water in which the Didinia live has much to do 

 with encystment. The relative importance of the various factors 

 involved in this process is, however, a subject for future investi- 

 gation. 



FEEDING HABITS. 



If a number of Didinia are studied under a lens, they appear 

 to be darting about in wildest confusion, sharply turning to the 



