596 L. R- Cary, 



Introdiictiou. 



Ou examining- specimens of the fresh water snail, Goniobasis 

 virginica, brought into the laboratory in September 1908, it was 

 found that practically all of them were heavily infected with the 

 sporocysts or rediae of a trematode. This species of snail is very 

 abundant in the canals and rivers throughout the region about 

 Princeton. So, at the suggestion of Prof. Conklin, I undertook a 

 study of the nature of the development of the germ cells in the 

 nurse generations of this form, the wealth of material available 

 offering an exceptional opportunity for the solution of this problem. 



Since only the generations — Sporocyst and Rediae — from 

 the secondary host were known, experiments were undertaken to 

 determine the life history. AVhen the adults were finally secured, 

 and their identity established, it was found that only this stage in 

 the life cycle had been described, so it was thought best to describe 

 and figure all of the stages; the more especially since the rediae 

 and cercariae differ very markedly in many characteristics from the 

 corresponding stages of the European form, AmpMsfomum {Diplodiscus) 

 stibclavatum, with which the species under consideration has been 

 confused by all authors previous to Staffoed (1905). 



The work of which this paper is an account, has been carried 

 on in the Zoological Laboratory of Princeton University during the 

 session of 1908—1909. 



I take pleasure in expressing my sincere thanks to Prof. E. G. 

 Conklin for his unfailing interest, and for many helpful suggestions 

 which have been of great value in carrying out this research. 



Material and methods. 



The greater mumber of infected snails collected were secured 

 from two localities: first, the Deleware and Raritan canal, near 

 Princeton; and secondly from the Deleware River at Trenton. 

 Practically all of those from the first mentioned body of water con- 

 tained rediae, in the most of which were found well developed 

 cercariae. Those from the Deleware River, on the other hand, with 

 few exceptions, contained sporocysts, and inside these unsegmented 

 eggs and young embryos (Germ Balls) were found in abundance. It 

 should be pointed out that in the snails from the last mentioned 

 locality the germ balls within the sporocysts were giving rise to 

 cercariae and not to rediae. The greater number of infected snails 



