290 



GENERAL BIOLOGY 



into the snail where a complete change in the parasite takes place. It 

 takes about two weeks for the fluke larvae to form a sac-like sporocyst. 

 Each germ-cell in this sporocyst passes through a blastula and gastrula 

 stage and then becomes a second kind of larva which is now called a 

 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 to 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 singrle animal if liver flukes are not to die out; for, it is not at all likely 

 that many of the miracidia will 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 millions of eggs may develop in a single 

 sheep. 



