MICROSCOPE AND AGRICULTURE 



tail and wends its way to the liver of its host, then 

 the story begins again. 



Having examined the adult liver fluke under the 

 microscope, we shall probably wish to find both the 

 free swimming young forms, and if we search care- 

 fully in ponds to which sheep have access we are 

 likely to be rewarded. It is obvious that the life 

 of a parasite such as the liver fluke is, of necessity, 

 precarious. It is only chance or luck, or whatever 

 one's favourite term may be, that brings the egg to 

 water, the young fluke to a snail, and the last free 

 swimming form to herbage that will be eaten by a 

 suitable animal. As usual in such cases, nature 

 makes provision for emergencies by providing a 

 large number of young, in order to insure that some 

 at least may be able to complete their development. 

 Owing to a series of changes, which we have omitted 

 to describe for the sake of simplicity, each liver fluke 

 egg may give rise to no less than three hundred and 

 twenty of the final free swimming forms. 



As we have remarked, the study of parasites is 

 difficult but it is interesting. Very few of these 

 creatures can complete their lives without living at 

 the expense of two different animals. The liver fluke 

 needs the water snail and some herb-feeding animal ; 

 there is another parasite which spends part of its 

 life in the pig and another part in the grub of the 

 cockchafer ; a third parasite dwells for a time within 

 the thrush, and for the rest of its time within the 

 garden snail, and so on. Apart from the interest of 

 the subject in itself, it brings us face to face with 



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