PARASITISM AND DEGENERATION 181 



like that of the external and internal parasites, is simply a 

 matter of convenience, and does not indicate at all any 

 blood relationship among the members of any one group. 



95. The simple structure of parasites. In all cases the 

 body of a parasite is simpler in structure than the body of 

 other animals which are closely related to the parasite 

 that is, animals that live parasitically have simpler bodies 

 than animals that live free active lives, competing for 

 food with the other animals about them. This simplicity 

 is not primitive, but results from the loss or atrophy of the 

 structures which the mode of life renders useless. Many 

 parasites are attached firmly to their host, and do not move 

 about. They have no need of the power of locomotion. 

 They are carried by their host. Such parasites are usually 

 without wings, legs, or other locomotory organs. Because 

 they have given up locomotion they have no need of or- 

 gans of orientation, those special sense organs like eyes 

 and ears and feelers which serve to guide and direct the 

 moving animal; and most non-locomotory parasites will 

 be found to have no eyes, nor any of the organs of special 

 sense which are accessory to locomotion and which serve 

 for the detection of food or of enemies. Because these im- 

 portant organs, which depend for their successful activity 

 on a highly organized nervous system, are lacking, the 

 nervous system of parasites is usually very simple and un- 

 developed. Again, because the parasite usually has for 

 its sustenance the already digested highly nutritious food 

 elaborated by its host, most parasites have a very simple 

 alimentary canal, or even no alimentary canal at all. 

 Finally, as the fixed parasite leads a wholly sedentary and 

 inactive life, the breaking down and rebuilding of tissue in 

 its body go on very slowly and in minimum degree, and 

 there is no need of highly developed respiratory and circu- 

 latory organs ; so that most fixed parasites have these sys- 

 tems of organs in simple condition. Altogether the body 

 of a fixed, permanent parasite is so simplified and so want- 

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