Chief 

 Classes 



CHAPTER X. 



UNSEGMENTEB "WORMS." 



A. Turbellaria, Trematoda, and Cestoda 



(the Platy helminth or Flat-worm Series). 



B. Nemertea or Nemertini. 



C Nematoda, Gordiacea, and Acantho- 

 cephala (the Nematohelminth or Round- 

 l worm Series). 



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

 venient name for a shape. The animals to which the name 

 is applied form a heterogeneous mob, including about a 

 dozen classes whose relationships are imperfectly known. 



But the zoological interest of the diverse types of 

 " worms " is great. For amid the diversity we discern 

 affinities with Ccelentera, Echinoderms, Arthropods, 

 Molluscs, and Vertebrates. 



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

 certain " worms " were the first animals 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, head region. But a brain arises from the insink- 

 ing of ectodermic cells, and its beginning in the cerebral 

 ganglion of the simplest " worms " is thus in part 

 explained. 



Again, it may be noted that worm types begin the series 

 of triploblastic cxlotnate animals, i.e. of those which have a 



