THE SENSORY RESPONSES OF CHITON 195 



teriorly and posteriorly beyond the limits of our 'middle half.' 

 This artificial subdivision into quarters is retained in our de- 

 scription, however, since the responses we are considering are 

 most characteristically displayed in the respective 'quarters' of 

 the chiton's surface, although there is some 'overlapping' and, 

 as already stated, the subdivision is by no means an organic one. 



A single touch applied to the midventral surface of the girdle 

 is followed by a local puckering of the girdle toward the source 

 of irritation. The foot, in the region immediately adjacent to 

 the level stimulated, is pushed laterad and dorsad, toward the 

 mantle, tending thus to assist the girdle in covering the gills. 

 This reaction of the foot is not evident when the dorsal surface 

 of the girdle is hghtly stimulated. Unless the tactile stimulation 

 is severe or several times repeated the homolateral side only of 

 the foot is involved in this response. Simultaneously with these 

 movements of the girdle and foot, a contraction of the gill ele- 

 ments occurs opposite the singly stimulated area. This involves 

 five or six ctenidia anterior, and as many more posterior, to the 

 point of excitation. In this reaction the ctenidia are elevated 

 dorsally, the tips are drawn toward their bases (thus throwing 

 each element into a more convex arch) , and at the same time they 

 are drawn somewhat anteriorly. The whole response involves a 

 movement something like the fairly rapid closure of the fingers of 

 one hand. The response spreads in both directions from the 

 level of stimulation, although at ordinary temperatures the 

 propagation wave is difficult to observe because of its rapidity. 



Successive stimulations of the girdle lead to a greater puckering 

 in toward the source of excitation, and to a more pronounced 

 rolling up of the whole body. The foot is locally brought 

 slightly laterad toward the girdle and is drawn dorsad to a 

 considerable extent. This response of the foot is at first confined 

 to the stimulated side, but subsequently spreads to the other 

 side, finally involving the whole substance of the foot at the 

 level of stimulation. Successive touches, 1 to 1.5 seconds apart, 

 lead to a tetanic contracture of the foot and ctenidia; during 

 this phase the animal tends to roll up. A 'refractory period' 

 ^succeeds the application of repeated light touches, until relaxa- 



