THE ART OF PARASITISM 



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cycle. The adult tapeworm matures sexually in the spring of the 

 year, the mature eggs being shed into shallow water where the fishes 

 come inshore to spawn. The eggs of the parasites are soon eaten by 

 copepods and the developmental period necessary for the larval 

 parasite to reach its second infective stage is closely correlated with 

 the time interval between the laying of the bass eggs and the absorp- 

 tion of the yolk sac of the bass fry. At the time the young fishes 

 begin feeding upon plankton, the copepods in the vicinity of bass 

 nests are found to be much more heavily parasitized than at other 

 seasons of the year. It is adaptations such as these which enable 

 parasites to complete complex life cycles. 



Flukes, or trematodes, probably undergo more complicated cycles 

 than any other group of parasites. In considering the complex 

 cycle of a trematode one should keep in mind that there are usually 





Diagram explaining the life cycle of endoparasitic trematodes. 



two free-living stages, — the miracidium and the cercaria. The 

 variations that may be expected in such a cycle are apparent upon 

 inspecting the above diagram. 



The frequent presence of a second intermediate host suggests a 

 characteristic of most trematodes. For example, the great blue 



H. W. H. — 16 



