302 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
of the spines or pedicellarize surrounding the area 
are affected. Similarly, if any part of the shell 
external to the circumscribed area be stimulated, 
the spines and pedicellariz within that area are not 
affected. These facts prove that the function which 
is manifested by these appendages of localizing 
and gathering round a seat of stimulation, is exclu- 
sively dependent upon the external nerve-plexus. 
It is needless to add that in this experiment it does 
not signify of what size or shape or by what means 
the physiological island is made, so long as the 
destruction of the nervous plexus by a closed curve 
of injury is rendered complete. In order to ascer- 
tain whether, in the case of an unclosed curve of 
injury, any irradiation of a stimulus would take 
place round the ends of the curve, we made sundry 
kinds of section. It is, however, needless to describe 
these, for they all showed that, after injury of a 
part of the plexus, there is no irradiation of the 
stimulus round the ends of the injury. Thus, for 
instance, if a short straight line of injury be made, 
by drawing the point of a scalpel over the shell, say 
along the equator of the animal, and if a stimulus 
be afterwards applied on either side of that line, 
even quite close to one of its ends, no effect will be 
exerted on the spines or pedicellariz on the other 
side of the line. This complete inability of a 
stimulus to escape round the ends of an injury, 
forms a marked contrast to the almost unlimited 
degree in which such escape takes place in the more 
primitive nervous plexus of the Medusze. 
Although the nervous connections on which the 
