196 LESLIE B. AREY AND W. J. CROZIER 



tion is fairly complete, during which touching locally does not 

 evoke any response. The duration of this 'refractory period' de- 

 pends upon the intensity of the original stimulation. The foot 

 recovers very quickly and is not easily fatigued. 



As in the stimulation of the dorsal surface of the girdle, the 

 periphery of the ventral aspect is more sensitive than its more 

 medial regions. 



The most striking feature of these reactions is the strictly 

 homolateral character of the response on the part of the ctenidia. 

 In no case did the reaction spread to the gills of the unstimulated 

 side* Homolaterality in response is not so clearly shown in the 

 behavior of the foot. 



e. Tactile stimulation of the mantle lining of the shell in the 

 region of the ctenidia induces movements of the foot and gill 

 filaments similar to those which follow touching the ventral 

 surface of the girdle at a corresponding level. The responses 

 obtained from the stimulation of the mantle lining of the ante- 

 rior and posterior extremities are also similar to those obtained 

 from the ventral surface of the girdle in the same regions, but 

 the surface of the girdle is decidedly less sensitive. As with the 

 girdle, no movements of the ctenidia are produced by tactile 

 excitation of the mantle except along the anteroposterior extent 

 of the ctenidia themselves. 



/. The region of the anus is not excitable in any special way. 

 To a light touch it may appear quite insensitive; to a stronger 

 touch the anal papilla responds by local contraction on the side 

 touched, and it may be somewhat retracted. When the mantle 

 in this region is activated the behavior of the girdle and of the 

 foot is essentially as already noted for other regions. 



g. The ctenidia, when directly touched, respond as has already 

 been described in the cases where the girdle and ventral mantle 

 in the gill region were touched. Touching either dorsal or ven- 

 tral surface of a filament results in the same response, although 

 in one case it is directed toward, in the other case away from, 

 the source of stimulation. Contraction in the manner pre- 

 viously described is the single mode of response of the gills. The 

 base of each filament is more sensitive than the free tip. The 



