6 THE INTEENAL PAEASITES OP 



in particular, is this erroneous conception of cause and 

 effect still maintained, but it is also stoutly contended for 

 by those who ought to know better. 



It is desirable that the student of parasitism should 

 clearly understand what sort of creatures ought or ought 

 not to be called parasites. The ideas usually entertained 

 on this subject are altogether vague ; consequently, many 

 of the lower forms of animal life are improperly regarded 

 as parasites, when they have no parasitic habits, whilst 

 others again are not looked upon as parasites, though 

 they should unquestionably enjoy this distinction. The 

 fellow-boarders, notwithstanding that they adhere to their 

 bearers, occupy this peculiar position, not for the purpose 

 of " sucking advantage" for themselves at the expense 

 of the larger animal, but for the mutual welfare of either 

 individual. In like manner, a kind of partial parasitism 

 is seen in the case of certain insects, which, merely pay 

 the host a passing visit. No one thinks of speaking of 

 these winged tormentors as parasites ; nevertheless, as their 

 attacks are made for the express purpose of acquiring 

 nutriment, they fairly come under the all-embracing title 

 of parasites. I think, therefore, that the term should be 

 made to include all those forms of creatures which in 

 some direct manner derive sustenance by dwelling in or 

 upon — or it might be by merely visiting or instantane- 

 ously alighting on the surface of — other living animals. 



These few preliminary remarks have appeared to me 

 necessary in view of conveying accurate ideas respecting 

 the essential nature of parasitism. Let me now direct 

 attention to individual examples of bovine parasites, 

 commencing with the common liver fluke. 



This parasite (Fasciola hepatica) is a rare guest of the 

 ox, being far more abundant in the sheep. It varies in 



