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 ¢7iploblastic celomate 
animals, z.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 ccelom and mesoderm is 
very gradual; thus there is practically no ccelom in the 
Platyhelminthes, and the mesoderm is sometimes not more 
definite than in Ctenophora. 
