THE SENSOEY RESPONSES OF CHITON 251 



and that probably the relative richness of general sensory inner- 

 vation is not the sole factor determining the chemical sensitivity 

 of any one region. Further evidence for the distinctness of the 

 chemoreceptors will be considered in the following section. 



Through the comparison of the effectiveness in stimulation for 

 different materials in Chiton and in other animals (table 6), it 

 will be seen that the integumentary sensitivity of invertebrates 

 corresponds in its general features (limiting effective dilutions) 

 with that of taste in man rather than with the common chemical 

 sense; there are, however, noteworthy differences from the 

 physiology of taste excitation (Parker and Metcalf, '07; Crozier, 

 '15 b). The fact that the isolated substances considered are, as 

 such, foreign to the daily experience of Chiton, has nothing to do 

 with the information they give concerning the process of excita- 

 tion. That we are not concerned with general 'pain' reactions 

 in the Chiton experiments can be shown in this way : The ventral 

 surface of the girdle of Chiton, although relatively the most 

 insensitive region to chemical excitation, is nevertheless decidedly 

 reactive to touch. This region is excitable by pure anaesthetics, 

 saturated sea-water solutions of essential oils, rain-water, by 

 10/8 M sea-water, and by 5 M glycerin, but is inexcitable (un- 

 der the conditions of our experiments) by alkaline chlorides 

 (other than KCl at 5/8 M concentration in rain-water), by KCl 

 (in sea-water) more dilute than M/16, by HCl or KOH more 

 dilute than M/10, by picric acid more dilute than M/700, or by 

 ethyl alcohol even in 10 M concentration. Hence it would 

 appear that only under conditions of an excessively heterologous 

 quality may the ventral surface of the girdle be excited by these 

 chemical agents, under such conditions in fact that 'pain,' tac- 

 tile, or any other form of sense organ might be activated. 



This result adds to the conviction that the general chemical 

 sensitivity of Chiton's soft tissues is distinct from any form of 

 tactile irritability, and is not consistent with the view that here 

 — as there may be in Balanoglossus (Crozier, '15 a), or in Synap- 

 tula (Olmsted, '17 b) — there are 'generahzed sense organs' 

 (Nagel). Some indications are afforded that a distinct general 

 chemical sense is adequately represented in Chiton. This con- 



