244 ZOOLOGY. 



these, also asexually, are developed cercarice, which 

 leave the snail, pass into water, attach themselves to 

 plants, and reach the intestine of the sheep later on. 

 The cercarise become flukes by a process of meta- 

 morphosis, and are to be regarded as fluke larvae. 



It is easily explained why the " fluke disease," or 

 " liver rot," caused by the fluke should appear much 

 oftener in some regions than others. Low-lying 

 pastures which are now and then flooded are best 

 suited for the development of the disease, since the 

 fluke eggs have a better chance of developing upon 

 them. This is because at certain times of the year 

 there are in such spots numerous submerged plants 

 on which cercarice encyst, and which, later on, when 

 the pools dry up, are nibbled by sheep. It seems that 

 the cercarice mostly attach themselves to certain kinds 

 of plants, hence the local nature of the disease, which 

 of course also depends on the presence of Lymnceus 

 trv/ncatuJ/us (Fig. 142, i). 



Fluke disease first makes itself apparent in the 

 sheep one or two months after infection. If the sheep 

 does not die the parasites often remain in its liver 

 till the next spring. The liver of a single sheep may 

 contain from 200 to 250 individuals. They live in 

 the various branches of the bile-duct, and the irrita- 

 tion they set up first causes an unusual quantity of 

 blood to flow to the liver. Inflammation and internal 

 bleeding may result, and gall stones are deposited in 

 the thickened walls of the biliary passages. The flow 

 of bile from the liver is made difficult or impossible. 

 Later on, the parasites draw so much blood from the 

 liver that this organ is very insufficiently nourished, 

 so that no more bile is secreted, and the liver substance 

 shrivels up. In consequence of these changes the 

 digestion is incomplete, and the whole body of the 

 sheep is very insufficiently nourished. The diseased 

 sheep are therefore sluggish and enfeebled ; they eat 

 little, but drink a great deal, and suffer from poverty 



