Races of Paramecium. 511 



The table gives measurements of five lines (each derived from 

 a single individual or pair) in each of the two groups.^ But some 

 of these lines evidently belong to nearly or quite the same race, 

 so far as size is concerned. Three races clearly distinguished by 

 permanent differentiations in size are shown in both the caudatum 

 and aurelia groups. In the caudatum group the largest race is 

 represented by La and 20, the next by A, the smallest by D. 

 In the aurelia group the largest race is represented bj'' d, k, and 

 g; the next in size by c, the smallest by i. In the paper of 1908 

 a race iV/2, intermediate in size between the caudatum and aurelia 

 groups, was described. Unfortunately this race was lost before 

 its other characteristics could be studied. Fig. 12 shows the 

 characteristic relative mean sizes of a number of different 

 races. 



The measurements given in table 1 are the means for cultures 

 as a whole, including young and old; they doubtless give nearly 

 the dimensions of adults. These data were supplemented by 

 measurements of the various races under identical conditions taken 

 at a given age. The age selected was ten minutes. The measure- 

 ments are thus for very young individuals, and are therefore less 

 than those of table 1, but they should of course show equally the 

 diversities of the races. 



The measurements were made in the following way. The ani- 

 mals examined had (save in case of the line c) been living in 

 ''i standard" hay infusion, regularly changed, for more than two 

 months just prior to the taking of the measurements, so that they 

 were in similar nutritive conditions. The race c was kept under 

 the same conditions as the others for a week before the measure- 

 ments, so that it was doubtless in similar condition. The day 

 before making the measurements all were transferred to fresh 

 "^ standard" hay infusion. The next day many were found to be 

 dividing. Dividing specimens were isolated, and the moment of 

 separation observed. The animals were then kept to the age of 



• The races Gi, Ai and g are included in the table, although they were lost before 

 the number and structure of the micro-nuclei was determined. But from their 

 close resemblance to known races, there can be little doubt that Gi and A, belong 

 with caudatum, g with aurelia. 



