64 ILLINOIS BIOLOGICAL MONOGRAPHS [510 



convergence due to adaptations to similar environments. Such an adap- 

 tation as the development of a boring spine which occurs in cercariae 

 widely different in other characters, can hardly be considered as show- 

 ing close relationship. When, however, cercariae are very similar in a 

 number of larval characters such likeness can hardly be ascribed to 

 convergence, and even if adult characters are not sufficiently developed 

 for comparison, such forms can with reasonable certainty be placed 

 together in natural groups. 



Another question which must be considered in classifying certain 

 cercariae, is whether the larvae of closely related adults might not be 

 different on account of modifications in larval life. From consideration 

 of conditions in other groups this would seem very possible. The little 

 evidence that we have, however, seems to indicate that the cercariae of 

 closely related forms are more alike than the adults. Liihe (1909:175) 

 suggests this as one reason for the small numbers of cercariae known 

 in comparison with the adults. 



"Vielfach sint iibrigens die Cercarien verschiedener Trematoden- 

 Arten einander so ausserordentlich ahnlich, dass ihre sichere Bestim- 

 mung, wenigstens bei uriseren jetztigen Kenntnissen, nicht moglich ist, 

 and manche alte Art-namen haben dadurch die Bedeutung von Gruppen- 

 statt von Artbezeichnungen gewonnen." 



The statement of these problems shows how merely tentative at the 

 present state of our knowledge must be considered any classification of 

 cercariae. 



The most extensive classification of the cercariae is that of Liihe 

 (1909:173-210). His main subdivisions are for the most part based 

 on the recognition of the relationship of the cercariae to the larger 

 recognized adult groups. In the subdivision of the distome cercariae, 

 however, his classification is to a considerable extent purely artificial, 

 being based on the character of the tail. A summary of Liihe 's classifi- 

 cation follows. 



luhe's classification of the cercariae 



A. Lophocercariae 



Cercariae with longitudinal projections along the sides of the body. 

 Ex. Cercaria crista La Valette. 



B. Gasterostome cercariae 



Two long projections from the end of the body. Mouth opening 

 in the middle of the ventral surface. Intestine simple sac-shaped. 

 Ex. Bucephalus polymorphus Baer. 



C. Monostome cercariae 



Ventral sucker lacking. Ex. Cercaria urbanensis Cort. 



