SENSORY REACTIONS OF CHROMODORIS ZEBRA 301 



tinct from tactile sense organs and from photoreceptors. The 

 thermal sensitivity of Chromodoris seems distinctly superior to 

 that of Chiton (Arey and Crozier, '19). 



V. CHEMICAL EXCITATION 



a. The general surface of the body of Chromodoris is capa- 

 ble of being sensorially activated by diverse chemical agents 

 (Arey, '17, '18). The dorsal and lateral skin reacts locally to 

 chemical irritation by forming deep pit-like depressions. The 

 'rhinophores,' oral tentacles, gill plumes, and mantle edge are all 

 open to chemical activation. The 'rhinophores' and oral ten- 

 tacles are in a general way the most sensitive parts. The reac- 

 tions elicited from these and other portions of the nudibranch's 

 surface are identical with those to mechanical excitation. 



In testing the distribution of chemical excitability, equal 

 amounts of various solutions were allowed to flow from a capil- 

 lary pipette with its tip at an approximately uniform distance 

 from the surface in each test. Care was used to avoid mechan- 

 ical stimulation from the stream. About 0.5 cc. of fluid was 

 employed, the nudibranchs being submerged in sea-water. 



Rain water or distilled water used in this way affords a weak 

 stimulation everywhere, save on the gill plumes, where it usually 

 fails. The responses are local, never involving movements of the 

 body as a whole. Sea-water diluted with three times its volume 

 of rain-water did not induce any reactions. Sea-water evapo- 

 rated to half its original volume was effective for excitation; the 

 'rhinophores' and the head region were noticeably reactive, the 

 rest of the body less so. 



Rain-water is, of course, fatal as a medium for the whole 

 animal, which lives about forty-five minutes when completely 

 immersed in it; within fifteen minutes the sensitivity of the 

 'rhinophores' to mechanical and to chemical excitation is oblit- 

 erated. The 'rhinophores,' gill plumes, and body musculature 

 contract greatly on first submerging the animal in rain-water, but 

 the 'rhinophores' and gills are subsequently extended. Under 

 the conditions of the following tests, however, osmotic relations 

 are not of primary importance. 



