Flatworms and Threadworms 



291 



redia. These rediae then break through the sporocyst and enter the 

 host's liver. The rediae have germ-cells within them, and these germ- 

 cells give rise to little cercaria which look something like tiny tadpoles. 

 These tadpole-like cercaria leave the snail and swim to the shore to form 

 cysts on surrounding vegetation. 



As the sheep pass along and eat the vegetation bearing these cysts, 



-Eggs. 

 -Parasite. 



-Changes 

 Due to 

 Inflammation. 



Fig. 180. Schistosomum Haematobium. 



(Distoma Haematobium.) 



From the submucosa of the large intestine of man. 



(From a photograph lent the author by 



Dr. E. L. Miloslavich.) 



the life-cycle is again begun. It will be noted from the account just 

 given that the larval stages breed in cold-blooded animals, while the 

 adult stages must have warm-blooded animals for their hosts. 



The liver fluke is by no means unknown to affect the human liver, 

 and where this is known to be the case, great care must be exercised 

 in eating uncooked vegetables. 



From the complicated life-cycle the liver fluke displays, it can 

 readily be understood that many thousands of eggs must be produced 

 by a single animal if liver flukes are not to die out; for, it is not at all 

 likely that many of the miracidia wall find a snail host; and then again, 

 it is not very likely that many of the cysts on the shore vegetation will 

 be eaten by sheep. 



One liver fluke will produce as high as five hundred thousand eggs, 

 and a single sheep may contain over two hundred adult flukes. This 

 means that over a hundred million of eggs may develop in a single 

 sheep. 

 Trematode Infections. 



Schistosomum haematobium (Fig. 180), (also called Bilharzia 

 haematobia), which causes the disease known as bilharziosis, is by no 



