304 W. J. CROZIER AND LESLIE B. AREY 



c. From the comparative reactivities of different parts of the 

 body and from the relative Umiting concentrations of each sub- 

 stance required for the activation of these parts, the distribution 

 of general chemical sensitivity can be made out over the body 

 surface of Chromodoris. These two criteria lead to mutually 

 concordant results, as an inspection of the preceding paragraphs 

 will disclose. 



The gill plumes are distinctly the least sensitive of the out- 

 growths from the body; the oral tentacles probably the most 

 sensitive; the 'rhinophores' almost as sensitive as the oral ten- 

 tacles, but occupying apparently an intermediate position. On 

 the ground of distribution, it would appear that chemoreception 

 is served by distinct receptors, for the gill plumes are reactive 

 to shading, touch, etc., as already described, in a way which 

 indicates their possession of dehcate receptive mechanisms for 

 these sources of activation, yet their chemical reactivity is slight. 

 Evidence of similar import is afforded by comparing the responses 

 of the oral tentacles and of the 'rhinophores.' At elevated 

 temperatures (38°C.), tactile responsiveness is quickly destroyed 

 on all parts, but sensitivity to KCl solution (0.625 M) is pre- 

 served. After complete exhaustion to shading, the gills are 

 fully responsive to KCl, M/1,000 picric acid, etc. 



The genus Chromodoris is characterized by the fact that 

 many, or most, of its members tend to develop a blue or purple 

 pigmentation of the skin. This pigment is a delicate indicator 

 of acidity (Crozier, '14, '16 a), turning pink with acids. This 

 color change is not indicative of an alkaline reaction in the cell 

 interior (Simroth, '14, p. 484), because, although the cell contents 

 are more acid than sea-water, the pigment is still blue under 

 faintly acid conditions (Crozier, '16 a). This natural indicator 

 offers an exceedingly favorable opportunity for studies on the 

 penetrability of cells for acids, leading to the possibility of in- 

 vestigating the nature of the reaction between acid and tissue 

 in the process of stimulation (Crozier '18 a). In the case of 

 neutral salts, we must also suppose that stimulation is due to 

 some chemical influence of the salt upon the surface of the 

 receptive elements, possibly owing to the fact that the applied 



