CHAPTER X. 



UNSEGMENTED "WORMS." 



PHYLUM PLATYHELMINTHES : 



Chief Classes Turbellaria, Trematoda, Cestoda. 

 PHYLUM NEMERTEA. 

 PHYLUM NEMATHELMINTHES : 



Chief Classes Nematoda, Nematomorpha, Acanthocephala. 



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. 



It is likely 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. 



Worm types begin the series of triploblastic ccelomate 

 animals, i.e. of those which have a well-defined mesoderm, 

 and a ccelom or body cavity lined with mesoderm and 

 distinct from the gut. It must be noted, however, that the 

 appearance of a well-developed coilom and mesoderm is 

 very gradual ; thus there is practically no cuelom in the 



