Sensory Discrimination : the Chemical Sense 75 



to stimulate the polar plates. Nagel apparently made no 

 experiments on the behavior of Beroe with reference to food 

 stimuli; for chemical stimulation he used picric acid, dilute 

 hydrochloric acid, quinine, strychnine, saccharine, coumarin, 

 vanillin, and naphthalin. To all these unwonted stimuli the 

 animal responded by some form of negative reaction, indicat- 

 ing possible unpleasant feeling. The edges of the mouth, 

 where the nerves end in bulblike structures, reacted to 

 quinine, vanillin, and coumarin by stretching the mouth into 

 a circular form instead of its usual slitlike shape ; suggesting 

 an effort to get rid of the stimulus. Precisely similar re- 

 actions were produced by stimulation with lukewarm water. 

 Nagel concludes that the organs for chemical and ther-y^ 

 mal stimulation are identical; whether the sensation quali-' 

 ties are different is, he thinks, an open question. There is 

 at least no evidence that they are different (289, 291). 



19. The Chemical Sense in Flatworms 

 Next to the coelenterates zoologists place the phylum of the 

 Platyhelminthes or flatworms, which possess a bilaterally 

 instead of a radially symmetrical structure. Many repre- 

 sentatives of the group are parasitic, and so far as the writer 

 is aware, no extended study of the reactions of these forms to 

 stimulation has been made. Most of our knowledge in re- 

 gard to the sensory life of the flatworms is confined to the class 

 Turbellaria, including the common freshwater and marine 

 planarians. These are small slow-moving creatures which 

 crawl about on solid objects under water or on films covering 

 the surface. The mouth is situated on the ventral side of 

 the body, sometimes quite far removed from the head end 

 (Fig. 9). One chief interest of planarians to physiologists 

 has lain in their remarkable power to regenerate parts lost 

 by mutilation. 



