509] LARVAL TREMATODES—CORT 63 



is definitely known of the relation of the cercariae types to the adults. 

 Therefore for convenience it has seemed advisable to build up a tentative 

 classification of the cercariae, treating them almost as if they were an 

 independent class of the animal kingdom. Of necessity such a classifica- 

 tion must be based pretty largely on superficial characters. As our 

 knowledge increases wherever possible natural groups must be substituted 

 for the artificial, and as more and more larvae are connected with the 

 adults, the classification of the cercariae will gradually be merged with 

 that of the adults. 



In order to understand clearly the classification that has been made 

 for larval trematodes, a careful analysis of the characters used for com- 

 parison must be made. Cercarial characters can be roughly divided 

 into two main groups: (1) adult characters, and (2) larval characters. 

 By adult characters of a cercaria are meant those which foreshadow 

 adult structure. It is by the use of these characters as a basis that 

 the greatest progress in natural classification can be made, since the more 

 the adult characters are developed, the more will the cercariae resemble 

 the adults, as in amphistome and echinostome larvae. For example the 

 digestive and the excretory systems of the cercariae of these groups are 

 much like those of the adult. Sometimes in the larva definite specific 

 peculiarities of the adult can be distinguished in detail. Thus Looss 

 (1896 :192-197) in attempting to prove by morphological comparison that 

 Monostomum verrucosum Froel (Notocotyle triseriale Diesing) and 

 Cercaria imbricata Looss belong to the same species, advances as his 

 strongest argument, that in the mature cercaria are found around the 

 excretory pore plications arranged as the rays of a circle like those 

 found in the adult. A combination of adult characters will often give 

 a clue to the family or even in a few cases to the genus to which the 

 cercaria belongs. Allowance must be made however for the fact that 

 adult characters may be somewhat modified in the development of the 

 cercaria. For example the loss of the tail modifies the excretory system, 

 and changes in shape and proportion of the body change considerably 

 the relative lengths of the different parts of the digestive system. 



Larval characters of cercariae may be defined as those which are 

 .not carried over into adult life. Many structures are developed to meet 

 the exigencies of larval conditions, and are merely temporary. In many 

 cercariae much dependence must be placed on such characters in classi- 

 fication, for often as in the forked-tailed and stylet cercariae, adult 

 characters are very little differentiated and the whole structure is 

 very largely dominated by larval characteristics. This brings up the 

 question as to how far such characters can be considered as expressing 

 relationship. Like structures in cercariae either show relationship or 



