361] NORTH AMERICAN PARAMPHISTOMIDAE—STUKKARD 81 



from the other two suborders which thru the presence of similar young 

 forms appear to be more closely related. 



The morphological structure and direct development of the Polys- 

 tomidae at once places them with the Heterocotylea. In the adoption 

 of an endoparasitic mode of life, however, thy show a distinct departure 

 from the other members of the suborder. The present study of the 

 Polystomidae has emphasized the unusual morphological variation and 

 wide geographic distribution which exists in the family. This may mean 

 either that the family is very old and has been subjected to conditions 

 producing wide variation, or that the group really lacks family entity 

 and consists of various heterocotylean forms which have specialized in 

 the direction of an endoparasitic habit and that the morphological re- 

 semblance is eenogenetic. 



Pratt (1908) reviews the literature and arguments for convergent 

 development which are based on trematode' morphology. Johnston 

 (1914) argues for divergence as the true explanation of the variation of 

 the species of Pneumoeneces, Gorgoderinae, Brachycoelinae, etc., and 

 believes that the elucidation of trematode phylogeny may be sought in 

 the study of the relationships between the distribution of trematode 

 parasites and the distribution of their hosts. No doubt the likenesses 

 and differences in the structure of present species are the result of both 

 convergence and divergence; yet it seems that the distributional factor 

 emphasized by Johnston is not of major importance. Parasitic distri- 

 bution could precede the distribution of the primary and secondary hosts 

 only in ease the parasites changed to new primary or secondary hosts. 

 Bdt today more than one species may serve as primary or secondary 

 host; the parasite is probably in a restricted degree able to adapt its 

 life history physiologically so other species may serve as hosts, and 

 primitively this adaptability may have been greater than now. The 

 distribution of the parasites certainly depends to a large extent on the 

 distribution of the primary host, and to a less extent on the distribution 

 of the secondary host, but the presence of two similar parasites in the 

 same region does not prove that their hosts had primitively the same 

 or different parasites. The life history of the trematodes is so imper- 

 fectly known that at present no final decision can be made on this basis. 



The wide variation in structure of the members of the genus Poly- 

 stoma can not be adequately explained thru migration, or thru differ- 

 ences in the age of the parasite, type of host, or location in the host. In 

 the genus so far as is known, the long uterus containing many eggs is 

 confined to species infesting the urinary bladder of amphibian hosts of 

 the Old World. However in respect to other characters, e. g., the shape 

 of the caudal disc and absence of great hooks, these amphibian forms of 



