CHAPTER XXI 

 THE FLATWORMS 



THE flatworms are, like the roundworms, chiefly parasitic, 

 and hence of the utmost importance to man. The fresh-water 

 flatworm, Planaria, will be chosen first for study, since it is very 

 abundant, can easily be studied in the laboratory, and is com- 

 paratively simple. Planarians and leeches are often confused, 

 since both are frequently found clinging to the underside of 

 logs or stones in ponds and streams. They can be distin- 

 guished quite readily, however, since the leech is segmented and 

 has suckers, whereas the flatworm is unsegmented and devoid 

 of suckers. 



4 

 FIG. 104. A fresh-water flatworm, planaria. 



1, eye ; 2, side of head ; 3, proboscis ; 4, pharynx sheath ; 5, genital pore. 

 (From Shipley and MacBride.) 



Planaria. The shape of Planaria is indicated in Figure 104. 

 The animal moves along over stones or other objects by means of 

 muscular contractions or swims through the water with the aid 

 of the cilia which cover it. At the anterior end are two eye- 

 spots (Fig. 104, i), sensitive to light, and two sensory pits, one 

 on either side of the head (2), situated in earlike projections. 

 On the ventral surface a trifle back of the middle is the mouth 

 opening (3). 



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