162 PROTOZOA 



shaped like a Dutch wooden shoe viewed from the side: P. bur- 

 saria, P. calkinsi, and P. trichium. P. calkinsi has two vesicular 

 micronuclei. P. bursaria has a single, relatively large, compact 

 micronucleus and is unique in being the only species that is 

 normally colored; it carries a symbiotic, unicellular, green Alga. 

 P. trichium also has a single, compact micronucleus; the possibil- 

 ity that it is related to P. bursaria in the same way as P. aurelia 

 is related to P. multimicronucleatum needs to be investigated. 

 The sizes of the animals are also said to be characteristic for 

 each of these six species. The figures given by Wichterman 

 ( 1953 ) are 70 to 90 v. for P. trichium, 85 to 150 y. for P. bursaria, 

 120 to 170 p for P. aurelia, 120 p for P. calkinsi, 170 to 290 |& for 

 P. caudatum, and 180 to 310 [a for P. multimicronucleatum. 2 

 Although these figures are the best available, they are not entirely 

 satisfactory. Size varies greatly with nutritive conditions, stages 

 of the fission cycle, and stages of the life cycle. No comparative 

 study of the six species which properly takes such variables into 

 consideration is available. On the whole, however, it seems rea- 

 sonably certain that on the average P. trichium is the smallest 

 species, that P. caudatum and P. multimicronucleatum are the 

 largest, and that the others are intermediate. 



Paramecium aurelia: Specificity of Mating Types; 

 Isolation of Varieties 



In this section will be discussed not only the organisms that 

 ordinarily go by the name of P. aurelia, but also those which are 

 designated as P. multimicronucleatum and intermediate forms'-' 

 which I have recently found. It is difficult if not impossible to 

 draw a sharp line where one species ends and the other begins 

 (Sonneborn and Dippcll, 1956). 2 The older name, P. aurelia, will 

 therefore be tentatively used for all these organisms. 



Tin's section begins with an enumeration of the varieties of 

 P. aurelia and an account of the primary basis of their recogni- 

 tion, mating type specificity. It proceeds to a consideration of 

 sexual and genetic isolation of the varieties, and then to their 

 geographical distribution. Lastly is discussed the question of 



