544 H. S. Jennings and George T. Hargitt. 



However, from the varying descriptions as to trichocysts, etc., 

 it is probable that there exist a number of diverse races of P. 

 bursaria, just as there do of P. caudatum and P. aurelia; some of 

 these then might be called P. putrinum. An outline copy of 

 Schewiakoff's figure of the latter is given in our fig. 24, b, to show 

 the form. A figure of P. putrinum, copied from Roux ('99) is 

 given in Lang's Lehrbuch ('01). Roux represents P. putrinum 

 as having a tuft of longer cilia at the posterior end, while according 

 to Schewiakoff ('96) it is without such a tuft. 



It is clear from the form and other characteristics that P. bur- 

 saria and P. putrinum are not the animals we are dealing with in 

 the present paper.^^ All in all, it is evident that none of the 

 named species of Paramecium are based upon the races described 

 in this paper, save P. aurelia and P. caudatum. When any of the 

 races were observed, they have been referred to one or the other 

 of these two species. 



7. Indications of the existence of diverse races of Protozoa 

 in the reports of other observers 



It appears probable that many of the diversities in the descrip- 

 tions of the various species of Paramecium, given by different 

 observers, are due to the fact that different races were under 

 observation. The reported differences in size are doubtless 

 partly due to different environmental conditions^^^ but it is clear 

 that we must not go too far in attributing all size differences to 

 this. The differences due to diversity of race are much more 

 extensive, besides being permanent and common. 



" Only when individuals of caudatum or aurelia are excessively fed do they take 

 on an oval form in any way similar to that of bursaria or putrinum (See fig. 22). 

 But under such conditions the peristomal groove practically disappears, whereas 

 it is represented as evident in the species last named. Furthermore, caudatum 

 and aurelia are almost never seen in this condition in the wild state, and the slen- 

 der form reappears as soon as food becomes scarce. The species bursaria and 

 putrinum have been observed for long periods by many observers, so that it is 

 clear that they are not mere temporary forms of caudatum or aurelia. 



^ See Jennings ('08) for an exhaustive study of the effects of varied environ- 

 ment in causing differences in size in P. caudatum and P. aurelia. 



