CHAPTER X. 



UNSEGMENTED "WORMS." 

 Chief Classes. 



1. TURBELLARIA \ Plathelminthes 



2. TREMATODA or 



3. CESTODA J Flat-worms. 



4. NEMATODA. 



5. NEMERTEA. 



THE title " worms" is hardly justifiable except as a con- 

 venient name for a shape. For there is no class of worms, 

 the animals to which the name is applied forming a hetero- 

 geneous mob, a collection of classes whose relationships 

 are imperfectly discerned. 



But the zoological interest of the diverse types, some- 

 times called "worms," is great. For amid the diversity we 

 discern affinities with Coelentera, Echinoderms, Arthro- 

 pods, Molluscs, and Vertebrates. 



Moreover, it is likely, as has been already noted, that 

 certain " worms " were the first definitely to abandon the 

 more primitive radial symmetry, to begin moving with one 

 part of the body always in front, to acquire head and sides. 

 And if one end of the body constantly experienced the first 

 impressions of external objects, it seems plausible that 

 sensitive and nervous cells would be most developed in 

 that much stimulated, over-educated, region. But a brain 

 arises from the insinking of ectodermic cells, and its be- 

 ginning in the cerebral ganglion of the simplest " worms " 

 is thus in part explained. 



Again it may be noted that with worm types begins the 

 series of triploblastic coelomate animals, i.e., of those which 



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