THE LIVER-FLUKES, TAPE- WORMS, ETC. 71 



of its development. The adult fluke, which occurs in the 

 sheep's liver, has a flattened leaf-like body, and is from three- 

 quarters of an inch to more in length. There are two suckers 

 on the ventral side, one surrounding the mouth, the other 

 nearer the anterior end. Their presence in the liver produces 

 a disease known as liver rot because the tissues of the liver 

 degenerate. Affected sheep often die. 



The flukes are hermaphroditic, and each individual is capable 

 of producing about five hundred thousand 

 very minute eggs. These pass through the 

 bile ducts of the host into the alimentary 

 canal and thence with the excrement to the 

 ground. If the eggs fall on dry ground they 

 usually perish, but if they fall on damp herb- 

 age or in water there hatch from them small 

 ciliated larvae. These swim about in the 

 water for ten or twelve hours, and, if they 

 do not happen to come in contact with a 

 certain kind of snail, they perish. Those 

 that do succeed in finding a snail bore their 

 way through any of the soft parts into the 

 body, where they undergo certain changes, 

 finally forming sporocysts within which are 

 developed small larvae called redice, which in 

 turn produce other rediae. The rediae finally 

 give rise to certain heart-shaped bodies each 

 with a long flexible tail. These are called cer- 

 carice, and in this shape the parasites issue from the snail 

 host. Soon after leaving the snail the cercariae become en- 

 cysted and within the cyst further changes take place, the 

 animal becoming more like a minute adult fluke. If these 

 cysts are swallowed by a sheep when it is eating grass or 

 drinking water where they occur, the cyst is dissolved by the 

 digestive juices in the sheep's stomach and the young flukes 

 are liberated. They soon work their way from the stomach 

 to the duodenum and through the bile duct into the liver, 

 where they develop into the adult flukes. 



It will be seen that under ordinary conditions there is little 



FIG. 19. Liver- 

 fluke, Fasciola 

 hepatica. (Nearly 

 twice natural 

 size.) 



