94 ILLINOIS BIOLOGICAL MONOGRAPHS [94 



individuals are apparently less highly differentiated morphologically and 

 younger physiologically than those which develop into cercariae. 



Finally the present study causes the writer to support the view that the 

 hermaphroditic phase of the life cycle of the Digenea is more closely 

 related to the ancestral group than the parthenita, and that the simplicity of 

 the parthenita has been assumed secondarily. This conclusion is based on 

 the evidence that the original type was a highly complex Plat)^helminth with 

 ciliary integument and eye-spots, characters found only in the miracidium. 

 The modification of the parthenita has come about as the direct result of 

 parasitism. It has lost its mesenchymatous matrix, its excretory tract has 

 been extraordinarily modified, and its germ cells have become uniquely sim- 

 ple. The nervous system of the redia has been simplified while the sporocyst 

 lacks a nervous system entirely. In the sporocyst even the muscle cells have 

 remained undifferentiated. Thus complexity in the hermaphroditic genera- 

 tion of Digenea is an index of the unmodified condition of the group most 

 early related to the prototype. 



SUMMARY 



1. Trematode infection of mollusks of the Bitter Root Valley, Montana, 

 is heavy. 



2. The history of the germ cells of the sporocyst and redia show them to 

 arise parthenogenetically. 



3. Parthenitae and adult hermaphoditic trematodes are comparable in- 

 dividuals: likewise their germ cells can be referred to a common type of ger- 

 minal epithelium. 



4. The integument of trematodes is mesodermal in origin. 



5. The fundamental systems of the adult hermaphroditic trematode are 

 well developed in the cercaria. 



6. The excretory, genital and nervous systems of the cercaria may be used 

 to show the natural relationships of the larvae. 



7. Holostomes, like distomes, monostomes and amphistomes, have an al- 

 ternation of hermaphroditic and parthenogenetic generations. 



8. Holostomes are probably of distome origin. 



9. Parthenitae are well adapted to their parasitic life because their struc- 

 ture is simple, in consequence of which they have become physiologically 

 young. 



