THE SENSORY RESPONSES OF CHITON 



249 



numerous protoplasmic processes. The effects of salts, acids, 

 alkalies, anaesthetics, and other substances in evoking reactions 

 are best understood upon the assumption that cells of the external 

 epithelium are acted upon by these substances after the manner 

 of cells in general. Thus, with anaesthetics: chloretone (M/200) 

 gave well-defined responses, whereas urethane (M/200) did not; 

 the anaesthetic effect of these materials follows the same order 

 (Crozier, '16 a). Some form of union with the surface of the 

 epithelial cells is undoubtedly involved in the process of excita- 

 tion, but these experiments cannot be understood on the assump- 

 tion that excitation is determined by an increase in cell perme- 

 ability. The decreased permeability produced by bivalent 

 cations (e.g., Ca) is a specific function of the cation; nevertheless, 



The minimal concentrations of various substances effective in the stimulation of 



Chiton 



while CaCl2 apparently does not serve as a sensory activator 

 under the conditions of these tests, good reactions are elicited by 

 Ca(N03)2 (compare also the case in Balanoglossus, Crozier, '15 a). 

 There is no specific parallelism between efficiency in sensory 

 activation and permeability-increasing properties. (Compare, 

 for acids and alkalies, Crozier, '18 a.) 



3. The chemoreceptors 



a. The ventral parts of Chiton exhibit a chemical-sensitivity 

 which is essentially similar to that found for the general integu- 

 ment of other marine invertebrates (table 6). The limiting 

 dilutions of various substances effective in activation of various 

 parts of the animal (table 6) follow an order which is, for each 



