453] LARVAL TREMATODES—CORT 



INTRODUCTION 



Practically nothing is known of the life-histories of the trematodes 

 of North America. Even in Europe where many new adults are being 

 described each year only a few developmental cycles are completely 

 known. One reason for this is to be found in the difficulties involved. 



Two methods of attacking trematode life-history problems have been 

 employed. One is to attempt to prove specific identity between cercariae 

 and adults by structural comparisons, and the other is to attempt to 

 find the relationship experimentally. Positive results from the first 

 method have been few. The structure of certain types of cercariae gives 

 no suggestion of the family to which the adult belongs, and in the more 

 differentiated forms generic identification is very rarely possible. Even 

 when the comparison is narrowed to very similar cercariae and adults 

 from limited localities it is safest to consider the results as merely sug- 

 gestive. Many errors have crept into the literature from too much reli- 

 ance on this method. In some cases the mere suggestion of probable 

 identity by one author has been taken by another as if it were an estab- 

 lished fact. It is only when the adult has been experimentally raised 

 from the cercariae or the larvae from the eggs of the adult that the 

 establishment of specific identity can be made sure. The experimental 

 method of study also has its dangers. The factors involved are so com- 

 plex and so little understood that only the most carefully controlled 

 experiments can be considered as conclusive. In some very recent work 

 as well as in older papers larvae and adults, shown later to be entirely 

 unrelated, have been joined experimentally. 



In connection with efforts to solve developmental problems it is 

 often argued that given the structure of a cercaria, it is possible to 

 draw conclusions as to the environment where it encysts, the life it 

 leads, etc. Further, that if the course of development of one member 

 of a group is known, it can be concluded that the others follow 

 the same lines. Dollfus (1914) finds evidence against both generaliza- 

 tions. He states that after a comparative examination of cercariae and 

 the environment in which they live, he can assert that cercariae very 

 similar in structure dwell in different hosts and have very different kinds 

 of development, and that cercariae very different morphologically live 



