THE SENSORY RESPONSES OF CHITON 185 



2. Movements of the animal as a whole 



Probably the most striking general reactions of chiton are the 

 suction process, whereby the animal adheres tightly to the rock, 

 and the curling up, armadillo-fashion, which it exhibits when 

 detached. Locomotion has less frequently excited remark 

 (Cooke, '95, p. 400; Heath, '99, p. 579; sep., p. 4). 



The suction power of chiton is well known to collectors (Dallj 

 '07, p. 23). When the animal is disturbed, the girdle is applied 

 to the substrate over its whole length, the shell plates are closely 

 approximated, and suction is also exerted by the foot. The 

 girdle is, however, the most important organ concerned in this 

 protective response. Its efficiency is in part conditioned by its 

 flexibility and by the fine riblets upon its ventral surface, but 

 especially by the fact that it is morphologically differentiated 

 into two concentric rings. This differentiation is exhibited in 

 the coloration of the girdle, a narrow pale line being frequently 

 located inomediately inside the peripheral half of the girdle 

 breadth. When firmly attached, a depression appears along this 

 line, the more peripheral zone of the girdle being applied to the 

 rock, and the inner zone being then sharply arched (figs. 10 and 

 11). On a smooth glass surface a chiton may readily be pushed 

 about from side to side; in this case the foot is not exerting any 

 suction, although the animal seems to be as firmly attached as 

 ever. 



The girdle is important not only for protection as a 'holdfast,' 

 but also because it prevents the entrance of rain-water and of 

 sand into the gill channels. Rain-water is quite toxic for Chiton, 

 killing it in about four hours when the animal is placed upon 

 its dorsal surface in a liter of such water. However, chitons will 

 live for twelve hours or longer completely submerged in rain- 

 water, provided the foot and girdle are free to come completely 

 into contact with a solid surface; we have already noticed the 

 completeness with which a foul solution can be excluded in this 

 way. Chiton rarely frequents situations where it might be cov- 

 ered with sand, but occasionally it is left by the receding tide 

 with the girdle and more or less of the back so covered. The 



