560 PROCEEDINGS OF THE NATIONAL MUSEUM fOL. 87 



part of the body, and a posterior point, or spine, or hook, either curved 

 or straight, but not a tail, is seen (figs. 98, 99, 100, d). In some 

 Opahnas, as also in some Cepedeas, it is evident that the posterior 

 spine or hook is situated a little to one side of the posterior tip of the 

 body. While no adult Cepedea or Opalina has been found with a 

 slender, tapering tail, this is their regular condition in their early 

 larval history. 



One who is familiar with the family as a whole cannot but feel that 

 the same schematic form underlies them all, though what are adult 

 features in Protoopalina may appear only in the larvae of the multi- 

 nucleate genera. 



What has just been written shows that one must be cautious in 

 evaluating conditions of form of body for purposes of taxonomic 

 description. He must be cautious also in the use of other characters. 

 There is much diversity in actual size of nuclei at different stages of 

 the life history. In general the nuclei decrease in size as they increase 

 in number during development in the multinucleate genera, but, 

 perhaps, as Hegner and Wu (1921) have claimed, there is for each 

 species a rather constant ratio between size of nucleus and the bulk 

 of cytoplasm over which each nucleus presides. Whether within a 

 species there are races that differ in the nucleocytoplasmic bulk rela- 

 tions is not determined. 



Nuclear conditions, especially the chromosomes, their number, 

 sizes, and forms, when carefully studied, can be relied upon for specific 

 diagnosis. The mitotic phase of the nuclei at the time when division 

 of the body begins might be constant and is possibly a usable character 

 in the binucleate genera, especially in Zelleriella, whose species are, 

 in many cases, so similar that any diagnostic possibility is eagerly 

 seized upon. The interval between lines of cilia, if we remember the 

 habit of interpolating secondary lines between the primary in the 

 anterior part of the body, appears to be a more constant character 

 than one might have anticipated, and there are a few cases of species 

 with cilia unusually closely set in the lines. If one is careful to com- . 

 pare corresponding portions of the body, these apparently quite 

 constant characters are useful. 



All this emphasizes the fact that one must study much material 

 from different sources and study it with minute attention, and that 

 even after such extensive and minute study there will still be uncer- 

 tainty because breeding experiments cannot be made to enable one to 

 differentiate between species and intraspecific races. There is great 

 probability that extreme races in one species may overlap extreme 

 races in another species. After studying the Opalinidae for 25 years 

 I am increasingly hesitant about positive specific diagnosis in numerous 

 cases. In others, on the other hand, one has no such hesitation. 

 The Opalinidae are an exasperating group taxonomically to one 



