THE OPALINID CILIATE INFUSORIANS, 247 



those species which are circular in cross section, or at least not greatly 

 flattened. They have two large nuclei, 0.01 mm. or more in the 

 longest diameter, except that one species P. frimordialis^ is described 

 as having a single nucleus characteristically in mitosis. In just 

 what stage of mitosis Awerinzew, the discoverer, does not say. There 

 are also two species with more than two nuclei; one, P. quadH- 

 nucleata regularly has four nuclei, and the other, P. axonucleta, 

 may have as many as eight or more nuclei, usually in pairs. The 

 nuclei in this genus have few (from 4 to 10) macrochromosomes ; 

 the number of microchromosomes has not been determined except for 

 two species, but they are but few, apparently no more numerous than 

 are the macrochromosomes. In mitosis both the macrochromosomes 

 and the microchromosomes are equally divided,, one of each pair 

 going to each daughter nucleus. 



Zelleriella includes the much flattened binucleated species. These 

 also have large nuclei, about 0.01 mm. or more in greatest diameter, 

 and they have few macrochromosomes, 4 to 10, so far as observed. 

 The microchromosomes, in the one species studied as to this point, 

 are equal in number to the macrochromosomes, and in some other 

 species the numbers of the two sets of chromosomes are about equal, 

 if not exactly so. In mitosis both the macrochromosomes and the 

 microchromosomes are equally divided, one of each pair going to each 

 daughter nucleus. These two genera together constitute the subfam- 

 ily Protoo'palininae^ which includes all the binucleated species both 

 cylindrical and flat. 



The subfamily Opalininae includes the multinucleated species, 

 with usually small nuclei. The number of macrochromosomes, in 

 those species in which it has been determined, is 12 or more. The 

 microchromosomes are present in about the same number, but have 

 not been well studied. In the mitosis, at least of some species, some 

 of the macrochromosomes may not divide equally, or even may not 

 divide at all. Some of the nuclei of these multinucleated forms may 

 thus become distorted in their chromatin balance and this may 

 account for the presence of occasional degenerating nuclei. No 

 similar irregularity of division of the macrochromosomes has been 

 observed in any Protoopalina or Zelleriella. The subfamily Opal- 

 ininae includes two genera : Cepedea^ including the species which in 

 cross section are circular, or nearly so, or at least are not uniformly 

 flattened throughout the body; some species are considerably flat- 

 tened, especially anteriorly; and Opalina, including the uniformly 

 flattened species, among which is Opalina ranarum, the type species 

 of the family. A division of the genus Opalina into Opaline an- 

 gustae, more or less slender species with the posterior end narrowed 

 or pointed {ohtrigona-like or virgula-like forms), and Opali/nae 

 latae, broader species with the posterior end usually broadly rounded 



