198 LESLIE B. AREY AND W. J. CROZIER 



responds positively by local attachment. The negative response 

 of the foot to the touch of small surfaces is entirely local for a 

 single stimulation. When repeatedly stimulated in the same 

 spot, the primary local puckering spreads slowly across the sur- 

 face of the foot, producing a dorsally arched depression 4 to 8 

 mm. broad, which is similar to the pedal locomotor wave, but 

 generally deeper. Repeated stimulation also tends to produce 

 the rolling-up response. 



The anterior and posterior quarters of the foot are more reac- 

 tive than the middle area. In these regions a single touch ini- 

 tiates a tendency to roll up, which can be produced on the mid 

 portion of the foot only through repeated applications. 



C. Distribution of sensitivity. The responses of Chiton when 

 different parts of its surface are touched enable us to outline the 

 distribution of tactile sensitivity over its body. In a general 

 way the anterior and posterior ends — as is almost, if not quite, 

 universal among animals — are more sensitive than the middle 

 portions, and the peripheral parts than those more medially 

 situated. Employing as criteria the relative effect in causing 

 the animal to roll up, and, on the sides, the relative effect 

 upon the gills, the following orders of sensitivity have been 

 distinguished: 



a. The most anterior and the most posterior regions of the mantle 

 are about equally sensitive. It is difficult, if not impossible, to 

 detect any constant difference in their reactivity to touch. 



b. At the head end : Surface of the head and palp = inner sur- 

 face of mantle > ventral surface of girdle > dorsal surface of 

 girdle. The extreme outer margin of the girdle is about as 

 sensitive as its ventral surface. 



c. At the caudal end : End of foot = inner surface of mantle > 

 ventral surface of girdle > dorsal surface of girdle. 



d. On the sides : Inside of mantle = ctenidia > edge of foot > 

 ventral surface of girdle > sole of foot > dorsal surface of girdle > 

 dorsal mantle between shell plates, the last judged by its effect 

 upon the approximation of the shell plates. 



e. The shell plates are insensitive to touch. 



It will be seen that in a broad sense the capacity of response to 



