PARASITISM IN I'LANTS AND ANIMALS. 285 



maturity, passes an intermediate stage of imperfect development 

 in the body of some entirely different animal, which, is therefore 

 called an "intermediate host," the creature in whose body the 

 mature stage is reached being the "final host." The parasites 

 in which these changes have been most carefully studied are the 

 common tape-worms and flukes. Let us take as an illustration 

 the liver-fluke of the sheep, an animal not only zoologically in- 

 teresting, but of the greatest economic importance as the cause 

 of that wide-spread and fatal disease called "rot" in sheep. 

 From the ravages of this pest, it is said that there has been, in 

 some years, a loss to British farmers of as much as a million 

 pounds sterling. The fluke, which in its adult condition inhab- 

 its the liver and bile-ducts of the sheep, is a flattened leaf-like 

 animal, almost an inch and a half in length and somewhat oval 

 or pear-shaped in outline : it has a couple of suckers near the 

 front of the body, through one of which comes the gullet, open- 

 ing externally by a simple mouth and leading backwards into a 

 much ramified ailimentary canal, the branches of which are im- 

 perforate. The animal produces an immense number of very 

 minute eggs, which, being thrown into the ducts, emerge into 

 the intestine of the host and so into the external world. Unless, 

 however, they find their way into water or on to very moist 

 ground the eggs are incapable of development, and so, for want of 

 a suitable environment, the vast majority of them must doubtless 

 perish. But supposing the ovum to reach a pool of water it im- 

 mediately sets free its contained embryo in the form of a minute 

 ciliated, free-swimming creature, which moves about actively 

 for the space of about four hoars, but if before the end of that 

 period it does not find the means of further development, it 

 straightway dies. On the other hand, should chance bring it in 

 the way of a particular little fresh-water snail, Limncea truncat- 

 ula, it at once makes its way into the breathing-cavity of that 

 animal and there undergoes its next transformation, developing 

 in its interior germs of two different kinds. Some of these 

 germs have tails and suckers, and are able to work their way 

 into the liver or the muscular tissue of the snail, ultimately 

 emerging from the animal into the outer world. They then take 



