200 SECRETS OF ANIMAL LIFE 



host. There are partial parasites which spend a 

 chapter or two of their life in freedom, and there 

 are complete parasites which pass from host to host 

 in a never-broken vicious circle. In proportion to 

 the intimacy of the dependence is the degeneration 

 of the parasite, which affects especially the sensory, 

 nervous, muscular, and alimentary systems. The 

 reproductive system, on the other hand, is often 

 highly developed and the multiplication very pro- 

 lific. This may be correlated primarily with the 

 abundance of stimulating food available without 

 exertion, and secondarily with the enormous chances 

 of death in the life-history. For most of the 

 parasites owe their survival to being many, not to 

 being strong. The intricacies in the life-histories 

 are often extraordinary, and are due in part to the 

 fact that the parasite has to share in the knots in 

 which their hosts are involved in the web of life, 

 for it is natural enough that the bladderworm of 

 the mouse should become the tapeworm of the cat. 

 Ugly parasites are common, but many are conspicu- 

 ously well adapted. Thus the tapeworm absorbs 

 food by the whole surface of its body; it is fixed 

 to its host by muscular adhesive suckers and often 

 by attaching hooks as well; it can thrive with a 

 minimum of oxygen; it has a mysterious "anti- 

 body " which saves it from being digested in its 

 host's intestine; it produces millions of eggs which 

 it is able to fertilize of itself. It may be repulsive, 

 but in the technical biological sense, relative to 

 given conditions, it is " fit." 



