THE SENSORY RESPONSES OF CHITON 209 



The chief ethological importance of Chiton's negative geo- 

 tropism is not that it directs the animal upward, but that it 

 keeps it from going down. 



The palUal nerve cords are easily cut by an incision from the 

 ventral side. A chiton with both pallial nerve strands cut at the 

 level of the cleft between proboscis and foot no longer orients 

 upward; it creeps about slowly and aimlessly, almost invariably 

 moving downward, but in a slanting direction. Sometimes a 

 chiton so prepared turns upward, but in no case did one of the 

 six animals used move upward, as normal individuals consist- 

 ently do. When the pallial cord is cut on one side only, and the 

 animal allowed to attach itself (after a rest) to a vertical glass 

 plate, under water, the long axis of the chiton being horizontal, 

 the characteristic effects are as follows : if the side on which the 

 pallial nerve cord is cut is placed downward, the animal orients 

 downward; if the cut side is uppermost, the chiton orients up- 

 ward, but usually passes beyond 'top center,' and then fre- 

 quently reverses, coming back to an approximation of its origi- 

 nal position, but not quite so nearly horizontal. This reaction 

 is not always clear cut. The final position, in fifteen tests upon 

 twelve different animals, was in all but three cases such that the 

 cut side was downward. In a number of cases there seemed a 

 well-defined failure of the anterior portion of the foot to attach 

 itself to the substratum, so that, the posterior region being 

 fully attached, the anterior end became directed downward in a 

 purely mechanical way owing to the pull of gravity. This 

 might account for the fact that in the chiton with both pallial 

 cords cut the anterior end is in a general way directed down- 

 ward, but would not explain the fact that in most cases with the 

 cut side placed upward the orientation was upward. While not 

 conclusive, the result of these tests is for the most part in har- 

 mony with the idea that the weight of the body, exerting tension 

 on the pedal musculature, supplies the stimulus to geotropic 

 contraction. In the case of an animal placed on a vertical sur- 

 face with the long axis horizontal, the muscles on the stretched 

 side are the ones which contract, as we might predict, and since 

 the animal orients upward, we must suppose this to be in a gen- 



