CHAPTER 



n 



THE FLATWORMS: 



Phyla Platyhelminthes 

 and Nemertinea 



The name flatworm, applied to members of the phyla Platyhelminthes and 

 Nemertinea is appropriate because these animals are worm-like and are 

 flattened dorsoventrally. The nemertineans are often considered as members 

 of a class Nemertinea included within the phylum Platyhelminthes, but the 

 diflferences between these groups justify giving the nemertines the rank of a 

 separate phylum. In contrast to the Mesozoa, the Porifera, and the Coelen- 

 terata, the flatworms are bilaterally symmetrical animals. They show, also, 

 anteroposterior differentiation; that is, they have "head" and "tail" ends, 

 and they normally progress in locomotion with the head end foremost. 

 Bilateral symmetry usually implies dorsoventral differentiation, characteristic 

 of these worms as of the vertebrates with their "back" and "belly" surfaces. 



The Phylum Platyhelminthes 



The members of the phylum Platyhelminthes may be characterized as 

 animals with bilateral symmetry; with a digestive cavity or enteron having 

 a single opening, the mouth-anus; with epidermis, gastrodermis, and 

 mesenchyme, but without a body cavity of any kind between enteron and 

 body wall; with an excretory system of flame bulbs and ducts, called pro- 

 tonephridia; and with complex reproductive organs of a distinctive type. 

 The phylum includes three classes: the cla.ss Turbellaria, planarians and other 

 typically free-living flatworms; the class Trematoda, flukeworms; and the class 



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