THK OPALINID CILIATK IXFUSOKIAXS. 201 



nuiiiorous instances this does not seem true. (ir()ii[)s of related 

 species within the genus Protooiml'ma are described in section 7. in 

 connection with problems of geo<rraphical distribution. 



Among the Zelleriellas there is less diversity. Tlie genus is a 

 very compact one. Specific distinctions are difficult to make. Diver- 

 sifying evolution has not gone far in this genus and no attempt will 

 be made to distingui.sh subgeneric group of species. AVe will see 

 after considering the geographical distribution of the Opalinidae 

 that there is reason to believe that 7.d]eriella is prol)al)ly the most 

 recent of the four genera to arise, and it has had less time to evolve 

 into diverse forms than have the other three genera. But ap- 

 parently Zelleriella shows less tendency toward divergent specia- 

 ticm than do the other genera. In the discussion of the taxonomy of 

 the species of ZeUeriella attention must be given to minute detail. 

 Form and size of the body, length of cilia, denseness of ciliation, 

 position of nuclei, number of chromosomes, relative thickness of 

 endosarc and ectosarc. size and shape of endospherules, and even the 

 general ai)pearance of the cytoplasm, in different cases prove of assist- 

 ance in distinguishing forms which at first glance seem very simi- 

 lar. Some of these features have to be used with great caution, for 

 they vary with the individual and with the stage in the life cycle 

 observed, if not indeed with the physiological condition. I do not 

 doubt that in some instances in the present studies I have been misled. 

 As already emphasized, it is unsafe to treat taxonomically a good 

 many of the species of the Ojjalinids without having in mind data 

 from the whole life history. But in the absence of more complete 

 data all that we can do is to scrutinize the phenomena of Avhich we 

 do have knowledge, and exercise the best judgment we can in view 

 of these phenomena. It has seemed surely worth while to under- 

 take this general revicAv of the family upon the l)asis of the data 

 at hand. Many questions remain unsettled, but the data show us 

 many things of interest, not only about the Opalinidae themselves, 

 but about their hosts and about the geologic history of the regions 

 they inhabit, and we find the data as to the Opalinidae telling us 

 important things about the Ciliata in general. 



In the Opalinhuu' there are many forms closely similar to one 

 another, especially in the genus Opalhui. Nuclear characters are 

 not so diverse or so readily observed, ^^'e must rely more on other 

 features, some of which, as form and size, vary within the sf^ecies 

 and with the stage of the life-cycle observed. 



The Opalinidae. like very many other groups of organisms. i)re- 

 sent conditions which make it extremely difficult to say just what 

 may best be considered species, what sul)species. what merely fonnne. 

 It is largely a matter of judgment. It seems very probalde that 

 most of the so-called species present diverse races, and the more di- 



