7 8 The Animal Mind 



usual slit-like shape, suggesting an effort to get rid of the 

 stimulus. Precisely similar reactions were produced by 

 stimulation with lukewarm water. Nagel concludes that 

 the organs for chemical and thermal stimulation are iden- 

 tical; whether the sensation qualities are different is, he 

 thinks, an open question. There is at least no evidence 

 that they are different (519, 521). 



17. The Chemical Sense in Flatworms 



Next to the ccelenterates 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 

 regard to the sensory life of the flatworms in 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 re- 

 moved 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. 



Planaria maculata, a common freshwater planarian, re- 

 sponds to stimulation by two forms of negative reaction, 

 a positive reaction, and a feeding reaction. The negative 

 and positive responses are given either to mechanical or 

 to chemical stimuli, the former being produced by strong, 

 the latter by weak stimulation. Hence they do not sug- 

 gest correlation with qualitatively different sensation con- 

 tents, but rather with unpleasantness and pleasantness. 



