72 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



chance of many of the thousands of eggs that are produced 

 by the adult fluke ever meeting successfully all the danger 

 that beset their path. The eggs may fail to reach the water; or 

 if hatched the larvae may not be able to find a snail; or if the 

 snail is found it may be destroyed before they have completed 

 their transformations. The cercariae are always in danger of 

 being eaten by aquatic animals while in the water, or if not 

 eaten after they form their cysts the particular blade of grass 

 to which they are attached may never be eaten by a sheep. 

 Yet the number of eggs that are produced is so great that 

 hosts of the young flukes do successfully overcome all their 

 difficulties and infect so many sheep that they become an 

 important economic factor in sheep-raising in many regions. 



With a knowledge of the life history of the parasite, however, 

 it is a comparatively easy matter to prevent the infection of the 

 sheep. This is accomplished by keeping them away from land 

 subject to overflow, or where the snails occur in numbers. 

 Springs and other watering places are particularly dangerous 

 when there are many snails about them. 



This same species of liver-fluke, Fasciola hepatica, some- 

 times occurs in cattle, horses and other domestic animals 

 and even, although very rarely, in man. 



Other Flukes. The large American fluke, Fasciola magna, 

 which is often a serious pest of cattle in the Southern states, 

 also occurs in the deer, which was probably its original host. 

 The life history has not yet been worked out, but it is probably 

 somewhat similar to that of the liver-fluke of sheep. Another 

 species of fluke is common in ducks, and many other animals 

 may be more or less seriously affected by still other kinds. 

 One species occurring in Egypt is a dangerous parasite of man, 

 infesting the urinary and abdominal blood-vessels where it 

 causes serious inflammation. The infection is probably from 

 bad water. 



Some species are external parasites and attach themselves to 

 their hosts by means of a series of suckers. One kind that 

 attaches itself to the gills and fins of fresh-water fishes is 

 viviparous, that is, it brings forth its young alive, and the 

 embryo, before it is extruded, itself contains another embryo 



