26 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



2. This is especially true because as yet we are not successful in 

 rearing these animals in cultures outside the host, and comparison 

 of pure line groups is very difficult, since it would involve infecting 

 uncontaminated tadpoles each with a single cyst. Such infections 

 can not be made at will, but are possible only during the period of 

 the breeding of the host, which lasts only a few weeks and occurs 

 but once a year. Such study seems impracticable. 



3. There is considerable diversity of structure under different 

 physiological conditions and at different stages in the life cycle. For 

 example, in a given species — 



a. The nucleus varies in size and in structural condition accord- 

 ing to — 



a. The physiological state of the animal ; and 

 ^. The stage of the life cycle observed. 

 h. The presence, degree of development, and the form of the 

 excretory vacuole varies with the physiological condition. , 



c. The size of the animals is very different in different stages of 

 the life cycle. 



d. The form of the animal is different under different conditions — 



a. Daughter cells recently come from transverse division have 

 a shape different from that of the products of longitudinal divi- 

 sion or that of the ordinary individuals. It is not improbable 

 that form varies according to the relation between growth and 

 fission and the relative frequency of the two sorts of fission, 

 longitudinal and transverse. 



^. Before and after and during sexual reproduction there are 

 individuals whose form departs considerably from that of the 

 ordinary individuals, being in general much more slender. 



•/. One finds occasionally swollen forms, as yet not under- 

 stood, which may easily be mistaken for true species. Gepedea 

 "seZZeH," for example, seems to be but a swollen form of G. 

 dimidiata. 



I. Eibbed, plicate, and twisted forms are often found. Some- 

 times all, or almost all, the individuals in an infection will show 

 one of these conditions, suggesting specific diversity, but suffi- 

 ciently wide comparisons seem to indicate that these are probably 

 conditions due to physiological state. 



e. Measurements of such characters as the intervals between the 

 lines of cilia, or the number of cilia to the micromillimeter in the cilia 

 lines, can be used only with caution, for at the anterior end of the 

 body there are accessory interpolated lines of cilia decreasing the 

 width of the interspaces, and the cilia in the lines may be more 

 crowded anteriorly. After transverse division, the posterior daugh- 

 ter will show special transient conditions at its anterior end, and 

 to a less degree the posterior end of the anterior daughter will show 



