Chemical stimulation. Equal volumes of various chemical solu- 

 tions were applied from a constant distance with a pipet. Solutions 

 of 1 M of maltose, or sucrose, or M/2 of lactose were without effect 

 upon all parts of the body, although 3 M glycerine did evoke general 

 responses. Several alkaloids had very weak effects or none at all. 

 Alcohols and organic acids in concentrations of M/10 called forth 

 strong general responses. The chlorides of the alkali metals Na, K, 

 NH 4 and Li likewise stimulated the body in general, the rhinophores 

 and oral tentacles, however, showing the greatest sensitivity. Solu- 

 tions of substances w r hich produce in man the taste sensations recog- 

 nized as acid, bitter, salty and alkaline were applied in various con- 

 centrations. The gills fail usually before other parts, although all 

 responses gradually weaken with increasing dilution. 



From the foregoing tests it becomes evident that the rhinophore is 

 not only extremely sensitive to chemical stimulation of diverse sorts, 

 but that this sensitivity is only second to, if indeed it does not equal, 

 that of the oral tentacles, which from their position might be suspected 

 a priori of a specialized gustatory or common chemical function. 



Olfactory stimulation. Saturated solutions in sea water of numerous 

 essential oils and decoctions of decaying marine invertebrates were 

 prepared and applied by the pipet method. The rhinophores react 

 strongly to these solutions, but other parts of the body appear to be, 

 so far as one can judge from the dissimilarity of the responses, equally 

 sensitive. When a drop of oil is held for some time midway between 

 the rhinophores no response ensues. If the rhinophore, or body, be 

 touched gently with a drop of pure oil, the reaction is weaker than to 

 a saturated solution. Here the number of sense organs stimulated 

 is undoubtedly a complicating factor, yet there is suggested further 

 that the response is one to an olfactory, rather than to an irritative 

 or 'smarting' stimulus. 



An essentially similar behavior to odorous substances was found in 

 Chromadoris roseapicta, Elysia crispa and Fiona marina. Besides 

 the rhinophores, Facelina goslingi possesses long, more or less dorsal 

 tentacles, and short oral ones. The longer pair reacts more vigor- 

 ously to solutions of the oils than do the rhinophores. 



Summary. The entire body of Chromodoris zebra is sensitive to 

 mechanical, common chemical, gustatory and olfactory stimuli. The 

 head region is somewhat responsive to the application of increased 

 temperature. Several other nudibranchs exhibit general olfactory 

 sensitivity. Of the various parts of the body, the rhinophore is most 

 sensitive to touch; is second, if not equal to the oral tentacle, in its 

 response to chemical stimulation; and shares its sensitivity to odorous 

 substances with the oral tentacle. In at least one species, Facelina, 

 the long posterior tentacles react more vigorously to solutions of es- 

 sential oils than do the rhinophores. Only to thermal stimulation is 

 the rhinophore (of Chromodoris) clearly inferior in sensitivity. Hence 

 the so-called rhinophore, like the insect antenna, is a compound sense 

 organ, for which the misleading term 'rhinophore' is highly inapt. 



