CHAPTER V 



SENSORY DISCRIMINATION: THE CHEMICAL SENSE 



1 8. The Chemical Sense in Ccdenterates 



WE have already discussed the responses to mechanical, 

 chemical, and food stimulation in those members of the Pro- 

 tozoa whose behavior has been most carefully studied, and 

 may begin the present chapter with an account of the corre- 

 sponding reactions in the lowest of the Metazoa, or many- 

 celled animals, the coelenterates. Although externally the 

 forms of different families of coelenterates differ widely, yet 

 the general plan of structure is the same in all : the body of 

 the typical ccelenterate is a hollow sac, whose walls consist 

 of two layers of cells, food being taken into a mouth at one 

 end of the sac, and the arrangement of cells being on the plan 

 of circular symmetry. In the phylum of the coelenterates are 

 included sea-anemones, jellyfish, the little green or yellow 

 Hydra, sponges, corals, and ctenophores. 



Hydra (Fig. 6), one of the simplest coelenterates, shows 

 a food reaction distinct from the contact reaction. Me- 

 chanical stimulation is followed by withdrawal of the ten- 

 tacles, and by contraction of the stem. This behavior may , 

 be called a negative or avoiding reaction, and no positive^ 

 reaction to a mechanical stimulus has been observed. The 

 food-taking reaction, on the other hand, consists in the seiz- 

 ing of the food by the tentacles. It seems to be given in 

 response to a combination of chemical wjth mechanical 

 stimulation, such as is offered by contact with a solicTedible 



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