SURVEY OF INVERTEBRATES 



79 



traordinarily complex life histories, involving alternation of gen- 

 erations, in becoming adapted to various parasitic modes of life. 

 The study of Flukes and Tapeworms constitutes a major part of 

 the science of parasitology because they inhabit Man and beast 



/Eye 



'Genital pore 



Proboscis 



Lateral nerve 



Intestine 

 Testis 



Uterus' 

 Genital pore 



Pharynx 



Intestine 

 Mouth 



Fig. 43. — A, Planaria polychora, a fresh-water Flatworm; B, diagram of 

 internal organs of a Flatworm. (From Hegner, after Shipley and MacBride, 

 and von Graff.) 



and produce various important diseases. Accordingly we can more 

 advantageously consider their life histories later when discussing 

 the relations of biology to human welfare. (Figs. 251-253.) 



D. Roundworms 



If either number of species or number of individuals were the 

 sole criterion of a phylum's importance, unquestionably the lowly 

 Nemathelminthes, or Roundworms, would rank high in the 



