350 EVOLUTION AND ANIMAL LIFE 



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 organs 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. Be- 

 cause these important organs, which depend for their success- 

 ful 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 elabo- 

 rated 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 break- 

 ing 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 circulatory organs, so that most fixed parasites 

 have these systems of organs in simple condition. Altogether 

 the body of a fixed, permanent parasite is so simplified and so 

 wanting in all those special structures which characterize the 

 higher, active, complex animals, that it often presents a very 

 different appearance from those animals with which we know 

 it to be nearly related. 



The simplicity of parasites does not indicate that they 

 belong to the groups of primitive simple animals. Parasitism 

 is found in the whole range of animal life, from primitive to 

 highest, although the vertebrate animals include very few para- 

 sites and these of little specialization of habit. But their 

 simplicity is something that has resulted from their mode of 

 life. It is the result of a change in the body structure which 

 we can often trace in the development of the individual para- 

 site. Many parasites in their young stages are free, active 



