86 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



Taxonomic study of Zelleriella is difficult, for the several forms 

 found in 49 species of Anura are very hard to distinguish. A few 

 forms stand out as clearly distinct species. As to some of the others 

 a definite opinion can hardly be formed. It is probable that the 

 seA^eral species of Opalinids have divergent races, as is so usual 

 among Euciliata. One group of forms, which might be called a 

 species, may at both extremes of its series of races overlap other simi- 

 larly divergent groups of forms. If clear-cut classification were the \ 

 goal in taxonomic study, I would give up the task for the forms of 

 Zelleriella^ but, of course, the task is to describe conditions as they 

 are, and this we must attempt in the present chapter upon the 

 taxonomy. The results, however, will not be as satisfactory for 

 Zelleriella as are those obtained from the study of the species of 

 Protoojmlina^ for there will be much of doubt and much danger of 

 confusion. This very fact of intergradation and overlapping of 

 species is itself one of the interesting phenomena in the genus Zel- 

 leriella and deserves emphasis. It is one of the indications of the 

 comparatively recent origin of this genus. 



The Zelleriellas, with the exception of Z. hinucleata (Raff) and 

 Z. macronucleata (Bezzenberger) are, so far as known, confined to 

 the Western Hemisphere — South America, Central America, the 

 West Indies, southern and southwestern North America. Geo- 

 graphical segregation, therefore, gives us little clue to the present 

 demarcation between species. As already mentioned, there is less 

 diversity between species in the mitotic condition of the resting 

 nuclei than there is in the genus Protoopalina. Size of body varies 

 with the race and with the time of the life cycle observed. Form 

 of body is of some help, though there is considerable diversity be- 

 tween the individuals of the same species. The size of the nucleus 

 relative to that of the body differs, first, in accordance with the 

 physiological state of the nucleus and, second, if the nuclear phe- 

 nomena described by Neresheimer are normal, the nuclei vary in 

 size according as they are primary nuclei (before the chromidia 

 have been extruded previous to the sexual phases of the life cycle), 

 or are secondary nuclei such as are found during the sexual period 

 of the life history, the former being much the larger. Even if the 

 nuclear degeneration is abnormal, as seems to be the case, it occurs, 

 and when the conditions so established are found they are likely to 

 prove confusing. One must, therefore, in his comparisons of species, 

 be sure that he is comparing corresponding phases of life cycle 

 and physiological condition in the species under consideration. The 

 interval between the lines of insertion of the cilia is not only differ- 

 ent in different regions of the body, as already noted for Proto- 

 opalina, but is different in the corresponding parts of the body in 



