FLATWORMS VANGUARD OF THE HIGHER ANIMALS 



Eye 



gestlve 

 tract 



Pharynx 

 withdrawn 

 in sheath 



Opening of 

 pharynx 



Mouth 



501 



Cut edge of 

 body wall 



Mouth 



Extended 

 pharynx 



Opening of 

 pharynx 



Ventrol 

 nerve 

 cord 



Nerves 



Fig. 25.3. Digestive and nervous systems of a planarian. A, the digestive 

 system, so distributed through the body that digestion and absorption take place 

 in every locality. B, view showing the pharynx extended to capture food. C, the 

 nervous system. Compared to hydra and the sea anemones, the nervous system is 

 distinctly centralized. 



by far the most sensitive part. Planarians strive to keep their undersides in 

 contact with a supporting surface and in their attempts to do so, the head 

 takes the lead just as it does when a turtle turned wrong side up flops over to 

 gain a foothold. Turned onto its dorsal side, a planarian twists into a spiral 

 so that the ventral surface of its head comes in contact with the substratum. 

 The head then glides forward and the body unwinding the spiral follows after. 



In the eyes of Dugesia and other planarians the pigmented cells form a cup 

 into which one to many neurosensory cells project. These are retinal cells com- 

 parable to the rod and cone cells of the human eye. Turbellarians, in general, 

 avoid the light; fresh-water planarians seek the darker sides of stones, the under- 

 sides of submerged leaves. When placed on contrasting backgrounds, such as 

 an experimental one of black and white circles, planarians (Dugesia lugiibris) 

 followed the black circles. After they were blinded, they made no distinction 

 between white and black. 



Locomotion. Planarians glide about by means of the assembled help of 

 millions of cilia located on their ventral sides and by muscular contractions, 

 the latter more important than the cilia. The roles of the cilia and muscles have 

 been separated by treating planarians with lithium chloride which paralyzes 

 the cilia, but not the muscles, and with magnesium chloride which paralyzes 

 the muscles but not the cilia. The slime trail secreted by mucous cells is an 

 important asset for gliding. The cilia are whipped into the slime, strike against 



