STAR-FISH AND SEA-URCTIINS. 307 



or motionless. On the contrary, their power of 

 spontaneous movement continued unimpaired, as 

 did also their power of closing round a seat of 

 irritation on the external surface of the sliell. The 

 same remark applies to the pedicellarine, and the 

 explanation is simple. It is the external nervous 

 plexus which holds all the spines and pedicellariee in 

 comnumication with one another as by a network; 

 so that when any part of this network is irritated, 

 all the spines and pedicellari?e in the neighbourhood 

 move over to the seat of irritation. On the other 

 hand, it is the internal plexus which serves to unite 

 all the spines to the nerve-centre which surrounds 

 the mouth, and which alone is competent to co- 

 ordinate the action of all the spines for the purposes 

 of locomotion. 



It remains to consider whether the ambulacral 

 feet exhibit any general co-c^linated action, and, if 

 so, whether this likewise depends upon the same 

 nerve-centre. 



The fact already mentioned, that during pro- 

 gression an Echinus uses some of its feet for crawl- 

 ing and others for feeling its way, is enough to 

 suggest that all the feet are co-ordinated by a 

 nerve-centre. But in order to be quite sure about 

 the fact of there being a general co-ordination among 

 all the feet, we tried the following experiments. 



I have already described the rii-htino,- movements 

 which are performed by an Echinus when the 

 animal is inverted, and it will be remembered that 

 in this animal the manoeuvre is effected by means 

 of the fet;t alone. At first sicht this midit almost 



