CHAPTER XVI 



PHYLUM PLATYHELMIA 



Flatworms are bilaterally symmetrical and usually have 

 a broad leaf-like form. Some have a digestive cavity into 

 which there is only one opening, the mouth ; in tapeworms, 

 however, no digestive system is present. There are three 

 classes: 



Class 1. Turbellaria; free living Platyhelmia with cilia on the out- 

 side of the body and a well-developed digestive system. 



Class 2. Trematoda, or flukes; parasitic flatworms, with a tough 

 cuticle on the outside of the body and no cilia. 



Class 3. Cestoidea, or tapeworms, which are parasitic, have a cuticu- 

 lar covering, and lack any digestive system. 



CLASS I. TURBELLARIA 



Turbellarians are the only flatworms that are not para- 

 sitic. They live for the most part in quiet places among 

 vegetation, both in the ocean and in fresh- water, and seek 

 actively for small animals which are swallowed whole. 

 The fresh-water planarians are probably the most familiar 

 representatives, and the activities of one of these worms will 

 be considered. 



Planaria maculata Leidy 



Self -maintenance. This turbellarian (Fig. 73) frequents 

 weedy ponds, springs, and the shores of lakes. It crawls 

 ' about among aquatic plants looking for food. Locomotion 

 is accomplished by the movement of cilia covering the out- 

 side of the body and is also greatly facilitated by the slime, 

 which gives the worm a firm enough hold on solid objects so 



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