302 JELLY-FISII, STAR-FISH, AN^D SEA-URCHINS. 



of the spines or pcdicellariaB surrounding the area 

 are affected. Similarly, if any part of the shell 

 external to the circumscribed area be stimulated, 

 the spines and pedicellarije within that area are not 

 affected. These facts prove that the function which 

 is manifested by those 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 pedicellarijB 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 Medusse. 



Although the nervous connections on which the 



