272 W. J. CROZIER AND LESLIE B. AREY 



The anal brim was stimulated by touching it with a glass rod. 

 After twenty-four applications, at successive intervals, the gill plumes 

 still responded by contraction; although after the first twelve responses 

 it demanded longer and harder stimulation to accomphsh complete 

 retraction of the gill crown. 



In another animal the same form of stimulation was used, but the 

 stimuli were supplied in groups of three successive touches. The indi- 

 vidual touches were throughout of about the same force. 



1st application of three touches — complete retraction within collar, 



2nd application of three touches — incomplete retraction; not drawn 

 within collar. 



3rd appHcation of three touches — about half the 'normal' response. 



4th application of three touches — shght contraction only of the gills. 



5th application of three touches — gills moved, but did not contract 

 longitudinally. 



6th apphcation of three touches — gills moved slightly. 



The integument of the head region is very reactive to touch. 

 The anterior edge of the mantle fold (buccal veil) seems to act 

 as the chief or immediately receptive part when the nudibranch 

 during creeping meets obstacles raised above the general level 

 of the substratum (fig. 2). A single stimulation of this part 

 has a slight effect on the tentacles; repeated light touches cause 

 first the homolateral tentacle, then the opposite one, to be fully 

 retracted — or, if the median region of the mantle edge be 

 touched, both simultaneously. 



The reaction of the 'rhinophores' when the buccal veil is 

 touched is very marked. It tends to be homolateral, as in the 

 case of the tentacles, but is much more pronounced. The 

 'rhinophores' react as fully and as quickly as when they them- 

 selves are directly touched, although in the latter case the re- 

 sponse is less easily fatigued. In some individuals this response 

 of the ' rhinophores' is elicited by tactile activation of the dorsal 

 integument as far back as the anterior level of the 'eye-spots;' 

 in others, only as far back as the level of the ' rhinophores' them- 

 selves. In the region behind this level the effect on the 'rhino- 

 phore' becomes suddenly much weaker, and a response from the 

 gill crown comes in, increasing in amplitude as places nearer the 

 branchial collar are touched. 



The dorsum of Chromodoris is soft, flexible, and very easily 

 stretched and distorted. Therefore the delimitation of the re- 



