T. M. SONNEBORN 161 



whether they should be given species names. Meanwhile, I shall 

 continue to refer to them as varieties and will reserve the term 

 species for the species now recognized by taxonomists. 



In the next eight sections, I shall deal with six genera of Cili- 

 ates that have been examined with respect to mating types and 

 varieties: Paramecium, Tetrahymena, Colpidiam, Euplotes, Sty- 

 lonychia, and Oxytricha. We begin with Paramecium, the first 

 genus to be studied in this way and the one about which most 

 is known. Pertinent information is available on six species of 

 Paramecium — a good deal on P. aurelia, P. multimicronucleatum, 

 P. bursaria and P. caudatum; less on P. calkinsi and P. trichium. 

 The first four will be dealt with in extenso and in order in the 

 immediately following sections; the other two will be only briefly 

 mentioned in the last section on Ciliates, along with the other 

 lesser studied Ciliates ( Stylonychia, Oxytricha, and Colpidium), 

 after sections that take up work on the more fully known Ciliates, 

 Tetrahymena, and Euplotes. The sections on Ciliates will be fol- 

 lowed by a section on obligatory inbreeding and asexually repro- 

 ducing Protozoa, chiefly Flagellates and Rhizopods. The last two 

 sections set forth the major conclusions and give a summary of 

 the whole paper. 



Before proceeding to the four intensively studied species of 

 Paramecium, it should be emphasized that the six species of 

 Paramecium to be discussed are, with one possible exception 

 presently to be mentioned, readily recognizable and sharply set 

 off from one another by combinations of a few morphological 

 features: mainly, body shape and the number and structure of 

 the micronuclei. Three species have the familiar cigar-shaped 

 body: P. caudatum, P. aurelia, and P. multimicronucleatum. 

 P. caudatum has a single, compact, relatively large micronucleus. 

 P. aurelia has two smaller micronuclei which are vesicular in 

 structure. P. multimicronucleatum has the same kind 2 of micro- 

 nuclei as P. aurelia, but their number varies from 2 to 7. As will 

 appear, I doubt whether a clear distinction 2 can be made between 

 P. aurelia and P. multimicronucleatum. Pending clarification of 

 this difficulty, I shall treat the organisms of both "species" as if 

 they were all P. aurelia. The other three species have bodies 



