450 ESSENTIALS OF ZOOLOGY 



possessing a pharynx and a rudimentary gut). Each redia gives 

 birth to six or eight cercariae, which emerge from the snail and swim 

 around in the water by means of a very large, undivided tail. When 

 a cerearia comes in contact with a fish, it enters the skin and encysts 

 either in the skin or in the muscles just below the skin. It is now 

 called a metaceixaria, or agamodistoynum. Man becomes infected by 

 eating these metacercariae in poorly cooked fish. When swallowed, 

 the cysts are dissolved by digestive juices of the host, the larva 

 escapes into the duodenum, migrates up the bile duct to the liver, 

 and there develops into an adult. There is evidence that the adult 

 Clonorchis may live as long as twenty years in the liver of man. 



Treatment of clonorchiasis is not very satisfactory. Prevention is 

 simple : avoid eating fish which are not thoroughly cooked. 



It will be noted that the Clonorchis life cycle involves three hosts : 

 a mammal as the final host, a snail as the first intermediate host, 

 and a fish as the second intermediate host. Infection of the fish is 

 by active invasion of the cercariae, and infection of the final host 

 is passive. 



Other Trematodes. — One of the best known parasites of domestic 

 animals is the sheep liver fluke, Fasciola hepatica, which occurs in 

 all sheep-raising countries in which wet pastures are common. It is 

 also a common parasite of goats and cattle. Like Clonorchis, Fasciola 

 lives in the bile passages, and its eggs pass out with the feces of 

 the host, but unlike Clonorchis, the eggs hatch in water and the 

 free-swimming miracidium actively seeks and penetrates the snail 

 host. Sporocysts in the snail give rise to rediae which produce 

 cercariae, but the cercariae encyst on any surface, including grass 

 blades and even the surface film of the water. Sheep become infected 

 by eating grass bearing encysted larvae or by swallowing floating 

 cysts while drinking water. Fascioloides magna, the large liver fluke 

 of cattle and sheep in Louisiana, Arkansas, and Texas, is very similar 

 in structure and life history. 



Other important flukes are the human intestinal fluke, Fasciolopsis 

 huskii, which is common among the Chinese, who become infected by 

 eating the cysts on various aquatic food plants; the human lung 

 fluke, Paragonimus westermanii of eastern Asia, where the natives 

 become infected by eating the encysted larvae in fresh-water crabs 

 and crayfishes; C otyloplioron cotylophorum, a stomach parasite oi 

 cattle in Louisiana, and Dicrocoelium lanceatum, a common liver fluke 

 of herbivorous mammals in Europe and Asia. 



