PARASITES OP ANIMALS. 105 



their tails, but eventually they, in all probability, again enter 

 the bodies of other small snails, and losing their tails, become 

 encysted in little capsules. Finally these snails, with their 

 parasites, are swallowed by sheep and cattle, while adhering 

 to the herbage growing on moist land or near the water, 

 or when adhering to water-cresses they might be swallowed 

 by mankind. In the stomach and intestine of these higher 

 animals they are again liberated from their cysts. They then 

 gain access to the liver, where they rapidly develop their 

 sexual organs and become mature. Eventually, after they 

 have matured and perhaps discharged a large part of their 

 eggs, they themselves, or at least a part of them, enter the 

 intestine and are discharged, while still living, in company 

 with innumerable eggs. The eggs that they still contain may 

 be laid after they reach the earth or water. These eggs are 

 exceedingly minute and may be diffused in various ways, as 

 by rains, winds, insects, and the feet of animals, and getting 

 into water, or moist situations, they are ready to hatch and 

 commence another series of transformations. 



The history of most of the flukes, which have hitherto been 

 fully investigated, agrees in the main with that above given, 

 and there can be little doubt but that this will prove to be the 

 case with the common fluke, when its full history shall have 

 been ascertained. At any rate it is certain that the eggs can 

 hatch only in water, or in moist places. Therefore it is evi- 

 dent that sheep and cattle will be much more liable to the 

 disease when kept in low wet pastures, than in dryer and 

 more elevated situations ; and also that wet seasons will be 

 favorable for the parasites and dry ones unfavorable. These 

 conclusions have been fully sustained by the experience of 

 sheep-owners in all countries. 



Modes of Occurrence. 

 The liver-fluke is one of the few internal parasites that is 

 capable of living in several very different animals. It is most 

 common in sheep ; less so in cattle, goats, horses, and the 

 ass ; and quite rare in man. It has also been found in the 

 hog, elephant, camel, beaver, squirrels, rabbit, hare, deer, 

 and antelopes of several kinds, and in the great kangaroo. 

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