THE SENSORY RESPONSES OF CHITON 223 



3. Summary 



The evidence we have presented relative to the existence of a 

 temperature sense in Chiton shows that if specific thermoceptors 

 of some sort do indeed occur upon the surface of the animal, they 

 are of a very poorly developed kind. Responses to high tem- 

 perature, under the various conditions of these tests, cannot be 

 adequately distinguished from tactile effects or even from direct 

 influences upon muscle. The minimal temperature (37° to 40°) 

 eliciting a 'heat response' is very close to the maximal temperature 

 which the chitons successfully withstand, and is even higher than 

 that which induces a distinct effect upon the muscular 'sphincter' 

 about the oscula of Stylotella (Parker, '10), where no receptor 

 organs are involved. Although this temperature is identical 

 with that producing heat responses in Amphioxus (Parker, '08), 

 it cannot be clearly shown by exhaustion tests — as apparently it 

 can in Amphioxus — that 'heat' and tactile receptivity are in any 

 way organically distinct. Only in the case of the intertegmental 

 mantle is there a suggestion of special thermal sensitivity, and 

 here the response elicited is not of a character favorable for 

 analysis. With low temperatures, as with high, the limiting 

 temperature producing perceptible responses in Chiton is prob- 

 ably just outside the range of its normal thermal experience. 

 The 'cold' responses, however, elicited at 12° to 15°, are of a 

 definite character and may apparently be separated, through 

 differential exhaustion, from purely tactile responses; that they 

 are mediated by special sensory structures remains uncertain, but 

 is possible. 



This matter of sensory differentiation is an exceedingly com- 

 plex one. The fact that isolated cells of the metazoan body 

 (e.g., chromatophores) are capable of excitation by heat, as well 

 as by chemical agents, local pressure, and light (Spaeth, '13), has 

 of course no decisive weight as an argument for 'generalized 

 receptors'; yet the degree of heat (high temperature) effective as 

 a stimulus is in such cases of an order of magnitude comparable 

 to that found effective in the sensory activation of many inver- 

 tebrates. In comparing the relative sensitivity of different 



