282 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
In these individual specimens, therefore, we must 
conclude that the foot-rows thus employed are 
selected because of some slight accidental prepotency 
or superiority over the others; the animal has, as it 
were, 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. 
Stimulation. 
In now quitting our observations on the natural 
movements of the Echinodermata, and beginning 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 
