258 PROTOZOA 



prove to be selfers. Gruchy (1955) reports that about 5% of the 

 wild strains collected were selfers. If some varieties regularly 

 self, like variety 13 of P. aurelia, these would be overlooked."'' At 

 present there seems to be no satisfactory way to get around this, 

 for it has proved difficult or impossible to discover the variety 

 to which such strains belong. One wonders whether the tech- 

 nique of using the animals left over after a culture selfs and 

 before it is fed again would work, as it does with variety 13 of 

 P. aurelia. Nanney (1953) tried another and ingenious approach 

 to the problem in wild selfers of another species of Tetrahymena, 

 but without success. These selfers gave 100% death of exconju- 

 gants. On the assumption that exconjugant survival might be im- 

 proved by outcrosses, he mixed selfers derived from different 

 sources and examined the viability of the conjugants isolated 

 from the mixtures. No difference was observed from the results 

 obtained with selfmg conjugants from a single source. This should 

 not discourage further use of the method since it is the result 

 expected if the two strains of selfers belonged to different va- 

 rieties. Perhaps other combinations of selfers would give a differ- 

 ent result, indicating that they belong to the same variety. The 

 method could even be profitably extended to mixtures between 

 a selfer and a pure type. 



Varietal Differences. The nine varieties appear to be alike in 

 many respects, even in some that distinguish varieties of other 

 species of Ciliates. No diurnal mating periodicities have been 

 found, and differences in temperature requirements for mating 

 seem not to exist (Elliott and Hayes, 1953; Gruchy, 1955). The 

 haploid chromosome set consists of one large, one small, and 

 three intermediate chromosomes, all with median or submedian 

 centromeres; and this seems to be the same in all varieties that 

 have been examined, i.e., in all but varieties 3, 7, and 8 (Ray, 

 1954, 1956; Ray and Elliott, 1954; Gruchy, 1955). There arc, 

 however, some differences among the varieties. Differences in 

 distribution and in preference for running or still water were 

 mentioned earlier in this section; others follow. 



1. Size, morphology, and cytology. According to Gruchy 

 (1955), animals of varieties 3 and 7 are unusually large, and 



