Races of Paramecium, 557 



1. Caudatwn Group (Paramecimn caudatum Ehr.). One micro- 

 nucleus, of the structure shown in fig. 8. Animals larger, slightly 

 more slender, posterior half of the body tapering more rapidly 

 and regularly than in the aurelia group. 



Lo. The largest race that was thoroughly studied; mean 

 length under different conditions, 180 to 230 microns. Two 

 years and four months in the laboratory. (See Estabrook, '10, 

 for an extensive study of growth in this race.) 



Ai. Similar to Lo, but a little smaller. 



Gi. Similar to ^i. 



20. Similar to L2; conditions inducing fission differing from 

 those for L^ (see p. 514). 



43. A large race, differing physiologically from others. Pre- 

 cise size relative to other races not determinable, because it does 

 not live in the pure hay infusion employed as a culture medium. 



D. Smallest caudatum race studied; mean length about 175 

 microns under conditions in which L^ has a length of 200 microns 

 or more. More than three years in the laboratory. (See Jen- 

 nings, '08, for extensive studies of growth and environmental 

 action in this race.) 



2. Aurelia Group (Paramecium aurelia Miiller). Two micro- 

 nuclei, of the structure shown in fig. 9. Animals smaller, slightly 

 broader in proportion to the length, and tapering less rapidly 

 from the middle backward, than in the caudatum group. 



k. One of the larger aurelia races; length about 125 microns, 

 under conditions where L^ has a length of about 200 microns. 

 Conjugates frequently and readily. Rate of fission same as in 

 the caudatum group. Two years and four months in the labo- 

 ratory. (See Jennings, '10, for a study of the conditions inducing 

 conjugation, based largely on this race.) 



C2. Size about the same as in A;; rate of fission slower than in k. 

 Two years and four months in the laboratory. 



g. Size about as in k and d. One year and six months in the 

 laboratory. (See Jennings, '08, for experiments on selection in 

 this race.) 



c. Smaller than the races thus far mentioned; length about 100 

 microns, under conditions in which the length of k is about 125 



