CHAPTER XVII 



THE FLATWORM, PLANARIA, OF PHYLUM 



PLATYHELMINTHES 



The representatives of Phylum Platyhelminthes (plat i hel min' 

 thez, broad worm) are usually called flatworms and in many ways 

 show considerable advance over the coelenterates. Some of the 

 species are parasitic, and the remainder of them are free-living. 

 The common fresh-water Planaria is an example of the free-living 

 type; while the parasitic flatworms are known as flukes or trematodes 

 and tapeworms or cestodes. All of these worms are bilaterally sym- 

 metrical and triploblastic. The nervous system in the free-living 

 forms is of the "ladder-type," and centralization is developed. They 

 possess a fairly well-differentiated mesoderm, and along with it have 

 developed some systems of organs. The alimentary cavity functions 

 as a gastrovascular cavity and has only one opening (the mouth) to 

 the exterior. The excretory sj^stem is composed of a pair of longitudi- 

 nal tubules, branch tubules, and ''flame cells." The gonads are within 

 the body and are connected with the exterior by accessory organs. 

 There are definite muscle cells, and excretory, and reproductive sys- 

 tems composed of the new mesoderm layer. 



The representatives of the two parasitic classes have, for the most 

 part, quite complex life histories and special adaptations. They 

 are very important economically because of their injury to man and 

 the domesticated animals. 



Classification 



There are four recognized classes in the group. 



Class Turbellaria (ter be la' ri a— little stirring).— This class 

 consists of a group of soft-bodied, elongate and usually free-living 

 forms. There are both land and water forms. Four orders (may be 

 combined into three) are known: Acoela, Rhabdocoelida, Tricladida, 

 and Polycladida. Planaria and Stenostomtim are examples. 



Class Trematoda (tre ma to' da — having pores). — These animals, 

 commonly called flukes, have no epidermis but a thick nonciliated 

 cuticle. This entire class is parasitic, and the immature stages fre- 



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