208 ENTO-PAEASITES AND HYGIENE. 



non-parasites cannot be drawn. In general, it is safe to 

 say that parasites are such organisms as do not earn 

 their necessities of life, but live as guests with, on, or in 

 other living organisms and satisfy their wants from the 

 property of the latter. They proceed in such a way as 

 not to eat them up entirely, or even partially, but to 

 draw food from the vital fluids which fill the bodies of 

 their hosts, without killing them. The important point 

 about a parasite, therefore, is its kind of food and the way 

 and manner in which it is taken in. 



Organisms which live on the vital liquids of other liv- 

 ing organisms are found not only in the animal king- 

 dom, but also among plants, the former being called 

 zooparasites, the latter phytoparasites. Among both 

 classes we meet those which live in or on plants and others 

 which lives in or on animals, so that we actually have 

 four classes: 



A. Zooparasites. 



1. In or on animals (leech, tape-worm). 



2. In or on plants (phylloxera, anguillula tritici). 



B. Phytoparasites. 



1. In or on animals (bacilli, bacteria). 



2, In or on plants (fungi, etc). 



The following remarks will be restricted to "A. 1, Zoo- 

 parasites ; living in or on animals," for they show the 

 effects of parasitic life in the most evident manner, in 

 their development as well as in their bodily construction. 



Only a few decades ago there prevailed the most ex- 

 travagant and nonsensical ideas about the origin and the 

 development of parasites. There was scarcely any ques- 

 tion concerning temporary parasites, such as fleas and 

 leeches, because their development was well known to 

 scientists and even to many other enthusiastic lovers of 

 nature ; complete darkness, however, surrounded the 

 existence of those forms which, with their slender, by no 



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