SEX IN PROTOZOA 



209 



Fig. Y. Life cvcle of Opalina ranarum, from Prenant (1935), after Zeller 

 (1877) and Konsuloff (1921), redrawn. 



1, encysted zygote in gut of tadpole; 2-6, development of zygote into multinucleate 

 adult; 7, binary fission of adult; 8, rapid fission leading to precystic stages; 9, cyst 

 eliminated into water to be ingested by tadpoles; 10, mononucleate gametes from 

 excysted individuals; 11, fusion of gametes to produce zygote. 



satisfactory accounts of chromosome numbers, but Chen (1936, 1948) 

 showed that adults have a diploid set of paired chromosomes. Meiosis 

 has not been described. 



Subclass Euciliata 



Kudo divides this subclass into the Orders Holotricha, Spiro- 

 tricha, Chonotricha, and Peritricha. In these groups, as well as in the 

 Suctoria, sexual reproduction, with rare exceptions, takes the form 

 of conjugation, or some modification of that process. While there are 

 many variations in conjugation, these are not generally associated 

 with any particular order, except for the Chonotricha and Peritricha. 

 Consequently it seems best to depart from the strictly taxonomic 



