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 

 insinking 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 coelomate 

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

 and a coelom or body cavity lined with mesoderm and 

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

 appearance of a well-developed ccelom and mesoderm is 

 very gradual ; thus there is practically no coelom in the 

 Platyhelminthes, and the mesoderm is sometimes not more 

 definite than in Ctenophora. 



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