282 JELLY-FISH, STAR-FISH, AND SEA-UUCIIINS. 



In tliese individual specimens, therefore, we must 

 conclude tliat the foot-rows thus employed are 

 selected because of some slight accidental prepotency 

 or superiority over the others ; the animal has, as it 

 wore, thus much individual character as the result 

 of a slight prepotency of some of its nerve-centres 

 over the others. 



Another question of still more interest arises out 

 of these righting movements, namely, that as to 

 their prompting cause. This question, however, I 

 shall defer till later on, since it cannot be answered 

 without the aid of experiments as distinguished 

 from observation. 



Stmiulation. 



In now quitting our observations on the natural 

 movements of the Echiiiodermata, and beo-innino^ an 

 account of the various experiments which we have 

 tried upon these animals, I shall first take the 

 experiments in stimulation. 



All the Echinodermata seek to escape from 

 injury. Thus, for instance, if a Star-fish or an 

 Echinus is advancing continuously in one direc- 

 tion, and if it be pricked or otherwise irritated 

 on any part of an excitable surface facing the 

 direction of advance, the animal immediately 

 reverses that direction. There is one point of 

 special interest concerning these movements of 

 response to stimulation. The form of the animals 

 and the distribution of the nervous system being, 

 as I have before said, of geometrical regularity, it 

 follows that by applying two stimuli simultaneously 



