PROTOZOA 



as to whether there was a third group of E. patella. This vague- 

 ness is apparently due to the fact that the only group extensively 

 studied as to breeding relations was one of the groups of E. 

 patella. 



The groups of strains of E. patella, although they are sexually 

 isolated, were not assigned specific names by Pierson because 

 she agreed with the views of Sonneborn (1938) and Kimball 

 ( 1943 ) that this should not be done unless the species could be 

 readily identified. They correspond exactly to the varieties of 

 Paramecium.. Kimball (1943) and Pierson (1943) agree that 

 sexual isolation probably preceded morphological divergence in 

 the speciation of Euplotes (and other Ciliates as well). This is 

 based on the plausible assumption that the varieties of E. patella, 

 which are morphologically identical so far as known, are more 

 closely related than the previously mentioned species, which 

 nevertheless are much alike in morphology and well marked off 

 from other species of Euplotes. The very similar species are there- 

 fore considered' to be intermediate in evolutionary divergence 

 and relationship between morphologically very different species 

 and morphologically indistinguishable varieties. 



The preceding work established the existence of mating sys- 

 tems based on mating types in E. patella, E. eunjstomus, and E. 

 aediculatus. Katashima ( 1952 ) reported mating types in E. harpa, 

 which is apparently not a member of the group of species that is 

 most closely related to E. patella. Thus far, no survey has been 

 made which would indicate the number of varieties in any species 

 of Euplotes. I suspect that it may be small. The full number of 

 mating types in a variety is also at present unknown. Katashima 

 (1952) reports upon only two cultures collected in Hiroshima 

 City; these were of different mating types. Kimball (1939, 1942) 

 also confined his studies to the progeny of two individuals of 

 complementary mating type obtained from neighboring ponds, 

 but he included clones derived sexually from the two original 

 cultures. In this way, he discovered four more mating types. It 

 might be supposed that this procedure would reveal all the 

 mating types of the species, as it appears to do in P. bursaria. 

 Kimball ( L942 points out, however, that his discoveries of the 



