8 ILLINOIS BIOLOGICAL MONOGRAPHS [454 



in identical environments and have similar courses of development. Cer- 

 tainly in the study of the development of digenetic trematodes it will 

 be necessary to increase very greatly the number of particular instances 

 known before the induction of general principles can safely go very 

 far. 



In Europe, where more is known of trematode development than 

 elsewhere, the foundations of this study were laid in the middle of the 

 last century by the work of such men as Leuckart, Wagener, Pagen- 

 stecker, La Valette St. George, de Filippi, and Moulinie. Altho they 

 made but little progress in the actual working out of developmental 

 cycles, their descriptions of large numbers of cercariae from molluscs 

 have formed the basis for later work. In England Nicoll and Marie 

 Lebour have very recently made some headway in the study of larval 

 trematodes altho as yet little experimental work has been done. In North 

 America only a beginning has been made in the study of adult trema- 

 todes and as yet there are only a very few scattered observations of larval 

 stages. The present work was undertaken by the writer at the sug- 

 gestion of Professor Henry B. Ward as an attempt to open up this almost 

 untouched field in North America. 



Most of the descriptions of larval trematodes from North American 

 molluscs are very inadequate and in many cases it is impossible to tell 

 to which general group of cercariae the forms belong. In only a very 

 few instances are either drawings or measurements given. The follow- 

 ing list is an attempt to bring together all references to date on larval 

 trematodes from North American molluscs. 



1. Cercaria hyalocauda Haldeman (date?) also reported by Evarts 

 (1880). Host Physa heterostropha Say. Locality (?) . 



2. Cercaria bilineata Haldeman (1840). Host Limnea catascopium, 

 Camden, Delaware. 



3. Cercariaeum vagens (Leidy) (1847: 220-221) 

 Syn. Distoma helicis (Leidy 1847:220-221) 



pericardium Creplin (1849) 

 vagens Leidy (1850: 304-310) 

 Cercariaeum helicis alternatae Diesing (1855:398) 

 398) 

 " vagens Diesing (1858:42) 

 Host Helix alternata and Helix alboldbris, Philadelphia, Pa. 



4. Cercaria agilis Leidy (1858: 110). Found free in the Delaware 

 river. 



5. Monostoma (Glenocercaria) lucania Leidy (1877:200-201) 

 Host Planorbis parvus. Philadelphia, Pa. 



