Races of Paramecium. 



525 



mean ratio of 38.1 per cent. Both these determinations con- 

 firm the previous general result, and show that even when living 

 in the same culture the aurelia races are proportionately a little 

 broader than the caudatum races. 



As to the rate of taper from the middle backward, the data are 

 given in table 3. 



TABLE 3 



Ratio of breadths at certain distances behind the middle to breadth at middle, 

 in 25 specimens of each race living in the same culture. 



The table shows that the relative breadths at the regions meas- 

 ured in the posterior half of the body are throughout less in the 

 caudatum race than in the aurelia race, thus demonstrating that 

 caudatum does taper more rapidly backward than aurelia, and 

 confirming the descriptions of Ehrenberg and Schewiakoff on 

 this point. 



If we construct from these data the typical dimensions for 

 the two races when living under these conditions, we find them to 

 be as shown in table 4. Here the mean length and mean breadth 



TABLE 4 



Typical mean dimensions in microns of the races Li (caudatum) and k (aurelia) 

 living in the same culture. 



Length 



Breadth at middle 



Breadth \ behind middle. 

 Breadth | behind middle. 

 Breadth f behind middle . 

 Breadth \ behind middle . 



