PARASITISM AND DEGENERATION 351 



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 sedentary, non- 

 competitive parasitic life. And this simplicity of degenera- 

 tion, 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 few examples of parasitism 

 (selected from various animal groups) 

 that are described in the following pages 

 all these general statements are illus- 

 trated. 



In the intestines of crayfishes, centi- 

 pedes, and several kinds of insects may 

 often be found certain one-celled animals FIG. 210. --The wingless 

 (Protozoa) which are living as parasites. !?f, t ; tic a k l ; Nycier i bia - 



^ . , . (After Sharp; much en- 



Their food, which they take into their larged.) 

 minute body by absorption, is the intes- 

 tinal fluid in which they lie. These parasitic Protozoa belong 

 to the genus Gregarina. Because the body of any protozoan 

 is as simple as an animal's body can well be, being com- 

 posed of but a single cell, degeneration cannot occur in the 

 cases of these parasites. There are, besides Gregarina, many 

 other parasitic one-celled animals, several kinds living inside 

 the cells of their host's body. Several kinds of these have 

 been proved to be the causal agents of serious human diseases. 

 Conspicuous among these are the minute parasitic Sporozoa 

 which are the actual cause of the malarial and similar fevers 

 that rack the human body in nearly all parts of the world. 



In the class of Sporozoa (of the great branch Protozoa or 

 one-celled animals) is an order called Hemosporidia (or Hemo- 

 cytozoa) comprising numerous kinds of unicellular parasites 

 which live in the blood of vertebrates (with certain inverte- 



