CHAPTER 22 



THE PHYLA PLATYHELMINTHES 

 AND NEMERTINA 



The term "worm" is applied to the animals of several different 

 phyla. The only characteristic which these animals have in common 

 is that all possess elongated, slender bodies. The most primitive of 

 these "worm" phyla are the members of the phylum Platyhelminthes, the 

 flatworms. In general, they can be distinguished from the other worms 

 by the fact that they are distinctly dorsoventrally flattened. The free- 

 living planarians, the flukes, and the tapeworms are all members of this 

 phylum. 



Compared to the other worm groups (the annelids, for example), 

 the flatworms seem to be relatively simple, unspecialized forms ; how- 

 ever, compared to the coelenterates, they demonstrate certain important 

 advances. 



They represent the first of the bilateral phyla. Associated with 

 this bilaterality is the first appearance of a definite head region, that is, 

 cephalization occurs. A true mesoderm is present in these forms as in 

 the higher coelenterates and ctenophores, but here it is differentiated 

 into definite organs. These organs thus represent the first steps toward 

 the utilization of the mesoderm for the development of organ systems. 

 This again represents a tremendous advance in organization inasmuch 

 as these animals are of an organ grade rather than simply a tissue level 

 of construction. 



The members of this phylum show many diverse structures and 

 modes of living. They range in habit from those that are free living 

 to those that are completely parasitic. Many have complex life cycles 

 involving several hosts. Some species have been of importance to 

 biologists in investigating basic generalizations ; others are of tremendous 

 economic importance as parasites of man and his domestic animals. 



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