306 W. J. CROZIER AND LESLIE B. AREY 



papilla when isolated by itself. In stagnant (i.e., non-circulating) 

 water copulation is interfered with (Crozier, '18 d), although the 

 nudibranchs may be active and healthy. These facts indicate a 

 chemopositive response to low concentrations of some secretion, 

 which is inhibited by higher concentrations. Mating behavior 

 remains the same after the 'rhinophores' have been amputated. 

 Therefore the 'rhinophores' are not of special importance for this 

 response. 



Not only is the genital papilla protruded, but the pharynx 

 as well is everted, even before two conjugating individuals come 

 into contact. It is very difficult to experiment with the sensi- 

 tivity of the phai-ynx, because it is seldom extruded in a position 



Fig. 8 The 'mouthing' behavior of Chromodoris preparatory to copulation 



favorable for observations, but it is undoubtedly very sensitive 

 both to touch and to chemical activation. To KCl, M/20 in 

 sea-water, the lips of the fully protruded pharynx were found more 

 sensitive than its outer surface, whereas the walls of the organ 

 (section I) were more reactive than the lips to touch. It usually 

 happens that before becoming mutually adjusted for copulation 

 (Crozier, '18 d), the nudibranchs pass the end of the pharynx 

 over each other's surface, moving closer together the while (fig. 8). 

 It seems hardly doubtful that the lips are the chief chemore- 

 ceptive regions in this case, because they may be the only parts 

 in contact with the other animal. This form of chemical attrac- 

 tion is curious, because 'sexual' secretions can hardly be involved 

 in a functioning hermaphrodite when reciprocal fertilization 



