532 H. S. Jeunings and George T. Hargitt. 



p. 336; p. 343). It should be mentiond therefore that in all the 

 races here described two contractile vacuoles are present, in the 

 usual position (shown in fig. 17, a and d). 



Summary. — Thus it is clear that the races of caudatum do 

 differ from those of aurelia in certain other features besides the 

 size and the number and structure of the micro-nuclei. The differ- 

 ences are mainly these : under the same conditions (1) the individ- 

 uals of caudatum have a breadth slightly less in proportion to 

 the length ; (2) the posterior half of the body tapers more rapidly 

 in caudatum; (3) the posterior point persists in caudatum even 

 when the animals become v^ery plump, while in aurelia under these 

 conditions the posterior point disappears. Among the different 

 races of either group (caudatum or aurelia) no characteristic differ- 

 ences of form were noted. 



3. Differences among the diverse races with respect to conjugation 



As I have set forth in a paper dealing with conjugation (Jen- 

 nings, '10) the different races differ greatly in their readinessto con- 

 jugate and in regard to the conditions inducing conjugation. 

 Referring to the paper just mentioned for details, we state here 

 merely the essential facts. In the aurelia group, the race k is distin- 

 guished for its remarkable readiness to conjugate. More than 

 twenty epidemics of conjugation have been observed in it during 

 the period of two years and four months in which it has been in 

 the laboratory. If it is allowed to multiply slowly for two or three 

 weeks, then a considerable quantity of nutrition is added, so that 

 multiplication becomes rapid, then the nutrition is allowed to fall 

 again, the animals almost invariably conjugate. The races i and 

 c, on the other hand, when subjected to the same treatment usually 

 do not conjugate; epidemics of conjugation are extremely rare in 

 these races. Although c has been in the laboratory for three years 

 and one month, but two epidemics of conjugation have been 

 observed in it, in spite of the fact that extreme efforts have been 

 made to induce it to conjugate more frequently, by the use of the 

 means so efficacious with k. In the race i but two partial epi- 

 demics have been observed, though the same treatment was em- 

 ployed as for k. Only a small proportion of the individuals con- 



