182 ANIMAL LIFE 



ing 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 

 all belong to the groups of primitive simple animals. 

 Parasitism is found in the whole range of animal life, 

 from primitive to highest. 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 parasite. Many 

 parasites in their young stages are free, active animals 

 with a better or more complex body than they possess in 

 their fully developed or adult stage. The simplicity of 

 parasites is the result of degeneration a degeneration 

 that has been brought about by their adoption of a seden- 

 tary, non-competitive parasitic life. And this simplicity of 

 degeneration, and the simplicity of primitiveness should be 

 sharply distinguished. Animals that are primitively simple 

 have had only simple ancestors; animals that are simple 

 by degeneration often have had highly organized, complex 

 ancestors. And while in the life history or development of 

 a primitively simple animal all the young stages are simpler 

 than the adult, in a degenerate animal the young stages 

 may be, and usually are, more complex and more highly 

 organized than the adult stage. 



In the examples of parasitism that are described in 

 the following pages all these general statements are illus- 

 trated. 



96. Gregarina. In the intestines of cray-fishes, centi- 

 peds, and several kinds of insects may often be found 

 certain one-celled animals (Protozoa) which are living as 

 parasites. Their food, which they take into their minute 

 body by absorption, is the intestinal fluids in which they 

 lie. These parasitic Protozoa belong to the genus Grega- 

 rina (Fig. 9) (see Chapter I). Because the body of any 



