202 ASSOCIATION OF ORGANISMS 



greater number of species the effects of parasitism are not 

 so marked, yet in practically every case there are special 

 clinging organs — hooks, spines, claws, and suckers — 

 and an unusual development of eggs, as there are great 

 difficulties in the way of young reaching a new host. 

 One species of tapeworm, for example, lays 80,000,000 

 eggs a year, and it is probable that not more than 

 one or two offspring will gain another host. 



Degeneration. — The term degeneration often implies 

 in the popular mind that animals which have lost one or 

 more organs as a result of parasitism are unfit; that an 

 active organism with delicate senses, relatively great 

 strength, and powerful organs of offense and defense is 

 better fitted for life's battle than a parasite. According 

 to such a belief a hornet or a lion is more successful in 

 the struggle for existence than a tapeworm or a hook- 

 w^orm. Such, however, is not the case. The very fact 

 that these species exist is sufficient evidence that each is 

 well adapted for a particular mode of life. In one case 

 stings and claws help to win the battle; in the other, 

 suckers and a suitable absorbing system play an import- 

 ant part. In both cases the individuals leave offspring 

 and the species persists. Biologically speaking, each is 

 equally successful. 



Life Histories. — The life histories of parasites are im- 

 portant from a scientific standpoint, and practically the 

 successful treatment of many diseases depends upon a 

 thorough knowledge of the development of the causative 

 organism. In the case of most external parasites the prob- 

 lem is comparatively simple, but with many of the inter- 

 nal parasites, one or two immediate hosts must be 

 inhabited before the adult stage is reached, and so the 

 subject becomes complex. Two examples will serve to 

 illustrate this fact: A tapeworm inhabits the dog. One 

 of its eggs, that has passed from the host, and become 

 attached to grass, is eaten by a rabbit. The tiny worm 

 hatches in the intestines, bores its way into a blood vessel, 



