306 JELLY-FISH, STAR-FISH, AND SLA-URCHINS. 
understand how it is that when a portion of the 
external plexus is isolated from the rest of that 
plexus as a result of the cork-borer experiment, the 
island still remains in communication with the 
nerve-centres which preside over the co-ordination 
of the spines, as proved by the fact of the Echinus 
using its spines to escape from irritation applied to 
the area included within the circle of injury to the 
_ external plexus produced by the cork-borer. 
Now, where are these nerve-centres situated ? 
We have just seen that we have evidence of the 
presence of such centres somewhere in an Echinus, 
seeing that all the spines exhibit such perfect 
co-ordination in their movements. Where, then, 
are these centres ? 
Seeing that in a Star-fish the rays are co-ordinated 
in their action by means of the pentagonal ring in 
the dise, analogy pointed to the nervous ring round 
the mouth of an Echinus as the part of the nervous 
system which most probably presides over the 
co-ordinated action of the spines. Accordingly, we 
tried the effect of removing this nervous ring, and 
immediately obtained conclusive proof that this was 
the centre of which we were in search; for as 
soon as the nervous ring was removed, the Echinus 
lost, completely and permanently, all power of co- 
ordination among its spines. That is to say, after 
this operation these organs were never again used 
by the animal for the purposes of locomotion, and 
no matter how severe an injury we applied, the 
Echinus, when placed on a table, did not seek to 
escape. But the spines were not wholly paralyzed, 
