CHAPTER II 

 PARASITES IN GENERAL 



Definition. — According to the Standard Dictionary, a parasite 

 is a living organism, either animal or plant, that lives on or irT 

 some other organism from which it .derives its nom'ishment for 

 the whole or part of its existence. In the following pages only 

 those parasites which belong to the animal kingdom are taken 

 into consideration. The vegetable parasites, chiefly bacteria and 

 fungi, are dealt with only incidentally. 



It is often difficult to draw a sharp line between parasites and 

 predatory animals; a i)anther is unquestionably a predatory 

 animal, and a tapeworm is unquestionabl}' a parasite, but a 

 mosquito or horsefly might well belong in either category. It is 

 usual to look upon an organism as a parasite when it habitually 

 preys upon other organisms which are superior to it in size and 

 strength. In accordance with this view all animals which 

 habitually prey upon man, other than a few which occasionally 

 attack and overcome him by superior physical prowess, may be 

 considered as parasites and are so treated here. 



The state of dependence of an inferior on a superior organism 

 probably arose very soon after life began to differentiate in the 

 world. It would be difficult, if not impossible, to explain step 

 by step the details of the process of evolution by which some of 

 the highly specialized parasites reached their present condition. 

 In some cases parasitism has probably grown out of a harmless 

 association of different kinds of organisms, one of the members 

 of the association, by virtue, perhaps, of characteristics already 

 possessed, developing the power of living at the expense of the 

 other, and ultimately becoming more and more dependent upon it. 



Kinds of Parasites. — There are all kinds and degrees of para- 

 sitism. There are facultative parasites which may be para- 

 sitic or free-living at will, and obligatory parasites which nmst 

 live on or in some other organism during all or part of their lives, 

 and which perish if prevented from doing so. There are inter- 



12 



