PARASITIC FLATWORMS 373 



such larvae are produced in great abundance and infected mollusks 

 are also abundant and widely distributed, there are few records of 

 cercariae in reports on aquatic life. 



Leidy found cercariae free in the Delaware River and in a Wy- 

 oming pool. Wright discovered the remarkable anchor- tailed cer- 

 caria among weeds, and I have taken several forms including the 

 striking Cercaria gorgonocephala in the tow with a plankton net. 

 None the less among the fresh-water organisms that are least 

 known one may well list the free-swimming stages of parasitic 

 worms. 



From this survey of the life history it is evident that the degree 

 of trematode infection depends: first, on the presence of water at 

 the time when the cercariae or miracidia swarm out; and second, 

 on the occurrence of mollusks in the region to act as intermediate 

 hosts. Hence flukes are rare in arid areas and also in regions 

 lacking in lime where mollusks are all but wanting. 



In general, infection is seasonal and may be traced to the climatic 

 conditions because periods of excessive moisture permit the swarm- 

 ing of the larvae, whereas during dry months the egg-shells remain 

 unbroken. The study of the adult parasites has shown that in 

 most cases observed the flukes produce eggs continually and seem 

 to display equal reproductive activity in all parts of the year. 

 The number of flukes found in a given host does not appear to vary 

 seasonally although it does vary widely in individual hosts. 



The fishes, amphibians, reptiles, birds, and mammals that occur 

 in and around various fresh-water bodies shelter a multitude of 

 species of trematodes. The group has never been studied care- 

 fully on this continent and data available include mostly casual or 

 fragmentary observations on a few of its members. Pratt made 

 the first general list of these species. Since then a number of 

 students of individual genera or groups of flukes have added to 

 the count. Even this has only made a start at recording the 

 North American species in the region which has been studied and 

 one can hardly venture to predict the number of species in parts 

 of the country where no collections at all have been made. The 

 total trematode fauna of North America is greatly beyond any 

 present records and cannot be estimated from the data at hand. 



