PLATYHELMINTHES. FLAT-WORMS 



T. solium is a more dangerous parasite from the possibility of the 

 occurrence of cysticerci in man, forming vesicles known as hydatids, 

 which may cause death. These hydatids are embryos that, in certain 

 cases, develop in the intestine where they were born and then migrate 

 into the muscles of the first host. 



The Trematodes, Fig. 21, are represented by the liver, lung, 

 intestinal and venal flukes of man and other animals, the disease 

 being known, in a general way as Distomiasis, from Distoma, the 

 generic name of the common flukes. According to Osier there are 

 six species of liver flukes known in man. One of these is extremely 

 common in some parts of the Orient and has occasionally been found 

 in the United States. The patient may die after many years of illness. 

 One of the commonest of the flukes is Distomum 

 hepaticum, living in the bile ducts of sheep and other 

 herbivorous animals. It is found the world over and 

 sometimes causes severe loss among flocks from liver 

 rot. 



Since a single fluke is said to produce 500,000 eggs 

 and since there may be 200 adult flukes in a single 

 sheep, it would seem that the disease should be ex- 

 tremely common, but the life-history, as will be 

 presently seen, is so complicated that the mortality 

 is enormous and but few eggs ever produce adult 

 flukes. 



The life-history of the sheep fluke is, briefly, 

 as follows: The fertilized egg is deposited by the * genit ^ ! pore; 



J m, mouth; s, ven- 



hermaphroditic adult in the bile ducts of the sheep, trai suckers, 

 whence it passes to the intestine and thence to the 

 exterior with the excrement. If the ovum chance to get into water at 

 the right temperature it develops into a ciliated larva known as a 

 miracidium. In swimming about, if this larva come in contact, within 

 a few hours, with a fresh water snail of a certain species it burrows 

 into the soft tissue of the mollusc where, in about two weeks, it develops 

 into the next stage, the sac-like sporocyst. The sporocyst gives rise 

 to several redia, of which there may be one or more generations, 

 the final generation becoming the tailed cercaria. The cercaria leaves 

 the body of the snail and after swimming about for a time forms a 

 cyst on a blade of grass. If this blade of grass be eaten by a sheep 



FIG. 21. Liver 

 fluke, Fasciola 

 hepatica. Xi. 



