CHAPTER 11 



The Phylum Platyhelminthes 



The flatworms or Platyhelminthes are wormlike animals with a single 

 major opening to the gut, which tunctions as both mouth and anus. Be- 

 tween the gastrodermis and epidermis the body is filled with tissues, 

 including layers of muscle, connective tissues and reproductive organs. 

 Neither a body cavity (such as will be described in later chapters) nor a 

 circulatory system is present. 



Included in the flatworms are two major groups of animal parasites, 

 the flukes (class Trematoda) and the tapeworms (class Cestoda), which 

 will be discussed further in Chapter 39. The free-living forms (class Tur- 

 bellaria) range in size from 0.1 to 600 mm. and are found in fresh water, 

 in salt water, and on land. 



The phylum is best approached by a study of its free-living mem- 

 bers. We will begin with an example of a turbellarian which is inter- 

 mediate in size and complexity. 



72. Dugesia: Habitat and Appearance 



The most familiar free-living flatworms are the planarians, abun- 

 dant in ponds and streams all over the world. This common name is 

 used for an entire order, but only one genus has the scientific name 

 Planaria. In the United States the more common planarians belong to 

 the related genus, Dugesia. The species D. dorotocephala occurs in ponds 

 and streams and is available from biological supply houses. 



Dugesia is about half an inch long, with a distinct head having what 

 appear to be crossed eyes and pointed ears (Fig. 11.1). The surface of 

 the body is a single layer of cuboidal cells (Fig. 11.4), the epidermis. 

 Planarians glide about on the ciliated ventral side of this surface. Slime 

 glands among the ventral epidermis cells (Fig. 11.4) secrete a lubricating 

 slime that smooths the path. 



73. Dugesia: Feeding and Digestion 



As the animal glides along hunting for food, the anterior end is 

 usually slightly elevated (Fig. 11.2). Should a small organism come close, 

 the head turns quickly toward it. Adhesive glands along the edges of 

 the body, and especially prominent in the head region, secrete a glue 

 to which the passing organism adheres tightly, and the head of the 

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