SENSORY REACTIONS OF CHROMODORIS ZEBRA 303 



parts. CaClo and MgS04 (0.625 N) did not activate; MgCl2 gave 

 fairly good responses over the whole surface of the nudibranch. 



The limiting dilutions of different substances effective for acti- 

 vation are Hkewise characteristic. For any one substance the 

 distribution of limiting effective concentrations for the different 

 parts of the body affords a measure of their respective recep- 

 tivities. In the following summary it will be seen that the 

 several concentrations, especially for the oral region, are of the 

 orders of magnitude found in the stimulation of other animals : 



Picric Acid, dissolved in sea-water. 



M/150 Good, strong reactions everywhere. 



M/200 Gill plumes respond weakly, or merely flatten out against the 

 body wall. 



M/500 

 M/1,000 



Good responses everywhere, except from the gills. 



M/4,000 Oral tentacles still very sensitive 'Rhinophores' less sensi- 

 tive: they may merely bend before the stream of acid, and 

 not distinctly contract. The dorsal skin is also slightly 

 sensitive 

 M/8,000 Same as M/4,000, but weaker. 



M/10,000 The mantle edge fails to respond; the edge of the foot, espe- 

 cially its anterior part, is still sensitive. Response from the 

 oral tentacles is more constant than from the 'rhinophores.' 

 This solution is distinctly bitter to human taste. 

 KCl, in rain-water. 



M/700 Gill-plume reaction weak. 

 M/1,000 No response from the gills. 



No responses distinguishable from those to an equal volume 

 of rain-water were obtainable with weaker concentrations. 

 In sea-water solution the reactions from the different regions of the body, at 

 M/400 concentration, indicated the following regional order of decreasing irrita- 

 bility (Crozier, '16 a, p. 272) : 



anterior tentacles, 'rhinophores' > base of the gill > crown > buccal 

 mantle > posterior mantle veil > lateral mantle edge (ventral surface) 

 > edge of foot (at the sides) > dorsal integument. 

 KOH, in rain-water. 



M/300 Gill plumes commonly fail to react. 

 KCl, in rain-water. 



M/3 Responses from all parts. 



M/16 No responses from gill plumes. 



M/50 Doubtful if the responses are distinct from those to rain-water 



in sea-water. 

 M/15 Gill plume reactions fail. 



M/20 No reactions from any part. 



