Maech 11, 1921] 



SCIENCE 



227 



4 mm. to 5 mm. it can be seen that tlie sexual 

 dimorphism can be noted at a very early stage. 

 Even in the smaller stages the males have a 

 distinctly larger oral sucker than the females 

 and the body is wider. Also early in develop- 

 ment the females show a larger space between 

 the intestinal ceca in front of their point of 

 union than do the males. As development 

 proceeds the differences in size between the 

 suckers of the sexes becomes more distinct. 

 The males become broad and flat and finally 

 the sides of the post-acetabular region curl 

 up to form the gynsecophoric canal. In the 

 females the body tends to become round in 

 cross section and the width is constantly 

 much less than that of males of the same age. 

 A detailed description of this development 

 will be made in a future publication. Fuji- 

 nami and Nakamura^ in a paper published 

 in Japanese antedate my findings on early sex 

 dimorphism in Schistosoma japonicum. They 

 were able to distinguish the sexes in speci- 

 mens 0.5 mm. to 0.7 mm. in length, which 

 developed in dogs. They laid especial em- 

 phasis on differences in the width of the body 

 and in the character of the intestinal ceca 

 as characters for distinguishing sex. 



The next question which arises in this con- 

 nection is whether sexual dimorphism is 

 present in schistosome cercarise. Although 

 many workers have made studies and meas- 

 urements of the cercarise of the human 

 schistosomes no one has reported such differ- 

 ences. I have myself examined a number of 

 cercarise of 8. japonicum with this point espe- 

 cially in mind without noting any dimorph- 

 ism. Dr. S. Tokogawa, of the Medical College 

 of Formosa, also informs me that he has 

 made an extensive series of examinations and 

 measurements of this cercaria in an attempt 

 to find sexual differences without success. 

 Since the cercarise of the human schistosomes 

 are very small and can extend and contract 

 their bodies to an unusual extent, slight size 

 differences might escape notice in the living 



zEujinami, A., and Nakamura, H., 1911, "A 

 demonstratioiL of some specimens showing the de- 

 velopment of Schistosoma japonicvm." (Japanese). 

 Bio ri Gaku KoAshi, Vol. 1. 



specimens and be difficult if not impossible 

 to detect in measurements of preserved 

 material. 



Recently in some studies on a species of 

 schistosome cercaria with eyespots from Plan- 

 orbis trivolvis from Douglas Lake, Michigan, 

 I have been able to demonstrate two distinct 

 size types. This difference in size came to my 

 attention first when I found that the curve 

 plotted from the measurements of cercarise 

 from a number of infected snails was dis- 

 tinctly bimodal. More extensive studies 

 showed that the cercarise of this species fell 

 into two distinct size groups. I further 

 found from measurements of the cercarise 

 from eleven infested snails that in the cercarise 

 coming from a single snail only one of the 

 size types was represented. The difference in 

 size was so great between these two types that 

 it could be recognized with the naked eye 

 when free-swimming cercarise of the two types 

 were placed in separate bottles. Measure- 

 ments of the length of the body of the larger 

 type showed a range of variation from 0.234 

 mm. to 0.28 mm. while in the smaller type 

 the range was from 0.207 mm. to 0.24 mm. 

 Other measurements of the body and tail, 

 which in this species is unusually large, 

 showed like differences. The adult into which 

 this cercaria develops is not known, although 

 unsuccessful attempts were made to introduce 

 it into ducks and rats. An analysis of its 

 structure, however, places it near to the 

 human schistosomes in the family Schisto- 

 somidse. This relationship means that in all 

 probability in the adult stage of this species 

 the sexes are separate. I therefore interpret 

 the size differences in this species of cercaria 

 as a sexual dimorphism. If this view is ac- 

 cepted the fact that in one infested snail only 

 one of the types of cercaria is represented 

 immediately becomes very significant. A more 

 detailed account of the dimorphism of this 

 species of cercaria will be published later in 

 connection with a study of its structure and 

 activities. 



In this connection must be cited the work 

 of Tanabe,^ on Schistosoma japonicum. 



3 Tana;be, K., 1919, "A contribution to the 



