TREMATODE PARASITES IN THE SKIN AND 

 FLESH OF FISH AND THE AGENCY OF BIRDS 

 IN THEIR OCCURRENCE 



By Edwin Linton 



In this paper it is my purpose to discuss briefly a few- 

 cases of parasitism due to trematodes, confining my remarks 

 to one form which occurs encysted in the flesh of certain 

 fresh-water fishes, and to a few cases of cysts in the skin of 

 fresh-water and of marine fishes. 



LIFE-HISTORY OF VERMIAN PARASITES 



It is a fact well known to students of zoology, though not 

 so well known to those unlearned in that science, that the 

 group of vermian parasites belonging to the flatworms 

 makes use of two or more animals in completing the round 

 of life. An animal that acts as a place of lodgment of a 

 parasite is known as a host. In one of these animals the 

 parasite is in the larval stage, in which case the host is said 

 to be intermediate; in the other animal the parasite is ma- 

 ture and produces eggs, its entertainer being then called the 

 final host. There are cases known in which one and the 

 same animal may act as both an intermediate and a final host 

 for the same parasite. In such cases the acquisition of the 

 larval stage of the parasite is accidental and exceptional. 

 Somewhere between the chain of events which links to- 

 gether the various stages in the life-history of any parasitic 

 flatworm an intermediate host, together with its larval para- 

 sites enclosed in cysts in its tissues, has been eaten by an- 

 other animal, usually the final host. And, invariably, as the 

 last link to the chain, the final host has swallowed the larval 

 or immature form which has inhabited one or more inter- 

 mediate hosts before it has attained its final resting place. 



In the order of flatworms known as cestodes or tape- 

 worms, the relation between the intermediate host or hosts 

 and the final host is direct, and may be characterized by the 



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