THE SENSORY RESPONSES OF CHITON 201 



sory structures in the shell plates of Chiton have no functional 

 significance as touch-receptors. The sensitivity of the girdle will 

 be considered separately. 



On the foot and other soft parts a moving point source of tac- 

 tile irritation is very effective in inducing responses, its effect 

 equaling that of several or many repeated single touches. On the 

 soft surfaces, it would appear, there are many scattered tactile 

 receptors. The amplitude of the reactions which they mediate 

 depends, in any given region, upon the intensity of activation 

 and upon the number of the receptors which may be involved. 



The sense of touch exhibits also a certain degree of discrimina- 

 tion. Thus, to contact with small areas the foot reacts by local 

 retraction, but to contact with larger areas it becomes promptly 

 affixed. Furthermore, chitons in the field have been observed 

 creeping in a horizontal direction along more or less vertical 

 rock surfaces, just about at the level of the water at that time, 

 so that wavelets of some force were hitting the animals roughly; 

 they continued creeping quietly, the girdle being freely lifted, 

 and made no response to the intermittent slaps of the water; 

 but at the lightest touch possible with the finger upon the lateral 

 or anterior edge of the girdle they instantly stopped moving and 

 adhered firmly to the rock. 



That the tactile sense is served by distinct receptors upon the 

 soft ventral parts of Chiton may be shown through the physio- 

 logical isolation of the tactile responses. Thus, when the ven- 

 tral parts have been exhausted for photic excitation, by repeated 

 shadings {vide infra), the mantle and other parts are still fully 

 reactive to touch. When the ventral surface of the girdle was 

 repeatedly touched, the animal responded 'by rolling up to the 

 maximal extent obtainable with tactile stimulation; an addi- 

 tional vigorous response can still be obtained from the middle 

 half of the girdle upon the application of n/10 to n/40 HCl, 

 much greater contraction resulting from the use of acid in this 

 way than can possibly be obtained through touch alone. When 

 immersed in sea-water at 43°C. there was a considerable tempo- 

 rary augmentation of tactile responsiveness, but after a few 

 minutes no responses to touch could be secured; a normal re- 



