CHAPTER VIII 



FREE-LIVING FLATWORMS 



Planaria 



Most flatworms are parasites. All of the tapeworms which 

 abound in the food canals of fishes, and the host of flukes like 

 those which live on the skin and gills of mud puppies, on the 

 mussel Anodonta and within the bodies of turtles, belong to 

 this group. The turbellarians are the only flatworms that 

 are not parasitic. They are small animals whose steady glid- 

 ing motion is due to thousands of cilia whose turbulent lash- 

 ings have given the group its name Turhellaria. Although 

 turbellarians are generally abundant in fresh water, only the 

 planarians, the largest of them, are familiar; the others being 

 so minute that they are not often discovered. The small ones 

 are mentioned later but nearly all of this discussion refers 

 to the large planarians. 



Form and habits of planarians. — Planarians glide over sub- 

 merged stems and leaves and forage on alga-covered stones. 

 They are soft, gray or velvety black flatworms commonly 

 measuring a half inch or less. They are flattened and look 

 like very small leeches. The shape of the head and body 

 varies but little, earlike flaps, called auricles, being character- 

 istic of many of the family. The mouth is on the end of a 

 short muscular tube, the pharynx, located in the middle of 

 the body. When a planarian is feeding it thrusts its pharynx 

 well out of the body, exploring with it like a kind of feeler, 



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