STAR-FISH AND SEA-URCHINS. 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 shell. The 
same remark applies to the pedicellariz, and the 
explanation is simple. It is the external nervous 
plexus which holds all the spines and pedicellarie in . 
communication with one another as by a network ; 
so that when any part of this network is irritated, 
all the spines and pedicellariz 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-ordinated 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 righting movements 
which are performed by an Echinus when the 
animal is inverted, and it will be remembered that 
in this animal the manceuvre is effected by means 
of the feet alone. At first sight this might almost 
