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LESLIE B. AREY AND W. J. CROZIER 



tissue, consistent among different substances. This suggests 

 that — since the 'thickness/ 'toughness,' or density of these parts 

 is in the same general order (hps > edge of foot > ctenidia > 

 sole of foot > girdle), with the exception, perhaps, of the 

 ctenidia — we are in reality dealing with a generalized form of 

 sensitivity, the effectiveness in arousing reactions depending 

 upon, 1) the ease with which the surface of the cells locally con- 

 cerned may be actively penetrated by the excitant, and, 2) upon 

 the relative richness with which these parts are respectively 



Miniryial concentrations of various substances effective in the sensory activation of 

 various animals 



1 "Uncorrected" concentrations; Hecht's paper ('18). 



innervated. In a previous chapter we have shown that the 

 tactile reactivity of these regions of chiton's surface follows 

 approximately the order: 



head (palp, lips), ctenidia > edge of foot > girdle > sole of 

 foot. 



There is, thus, a distinct inconsistency in the relative sensitivity 

 of these parts to touch and to chemical excitation, which shows 

 that differences in the method of excitation undoubtedly exist 



