518 EVOLUTION OF ANIMALS Part V 



species of minute crustaceans (copepods) in which they develop into the inac- 

 tive phase. Even so, they travel far since about 22 species of fishes feed upon 

 copepods. In the fishes the larvae migrate into the muscle, the "clean white 

 meat." The human infection occurs and the progress of the parasite goes on 

 when the meat is eaten without thorough cooking. 



Consequences of Parasitism 



Parasitism is an unbalanced relationship between organisms that has de- 

 veloped from a balanced one. Parasites and their hosts are close intimates. A 

 parasite must get on or into one host and stay, or it must have first one and 

 then another host. At one time or another, or all the time, it must cling to its 

 host. Its whole success depends upon this. 



Parasites ride about on or in their hosts. Those that ride most can move 

 about least by themselves. In general, the more they depend on the possessions 

 of the host, the fewer they have of their own. Tapeworms do indeed travel 

 light, without locomotor organs, without mouth or digestive tract, without skin 

 cover, without eyes, almost without sense organs. 



