o;)6 JELLY-FISH, STAR-FIGH, AND SEA-UKCHINS. 



understand ho'vV it is that when a portion of the 

 external plexus is isolated from the rest of that 

 plexus as a residt 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 b}^ 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 ? 



Seeinor that in a Star-fish the ravs are co-ordinated 

 in their action by means of the pentagonal ring in 

 the disc, 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, 



