CHAPTER XII 



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 to the exterior. 

 The excretory system is composed of a pair of longitudinal 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 systems 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 (turbela'ria — little stirring). — This class con- 

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

 forms. The surface layer or epidermis is ciliated in patches, and 

 there is a plentiful supply of secreting cells in this layer. The 

 digestive tube is single, three-branched, or many-branched. The 

 mouth is located ventrally. There are both land and water forms. 

 Four orders are known: Acoela, Rhabdocoelida, Tricladida, and 

 Polycladida. Planaria and Stenostomum are examples. 



160 



