292 JELLY-FISH, STAR-FISH, AND SEA-URCEINS. 
they may be reduced to immediate quiescence—.e. 
retracted—by placing another irritant on the ven- 
tral surface of the disc. Similarly, if retraction has 
been produced by placing the irritant on the ventral 
surface of the disc, activity cannot be again induced 
by placing another drop of the irritant on the dorsal 
surface. : 
Now, if we regard all these facts of stimulation 
taken together, it becomes evident that the external 
organs of an Echinoderm—feet, spines, and pedicel- 
larize—are all highly co-ordinated in their action ; 
and therefore the probability arises that they are 
all held in communication with one another by 
means of an external nervous plexus. Accordingly 
we set to work on the external surface of the 
Echinus to see whether we could obtain any evi- 
dence of such a plexus microscopically. This we 
succeeded in doing, and afterwards found that Pro- 
fessor Lovén had already briefly mentioned such 
a plexus as having been observed by him. The 
. plexus eonsists of cells and fibres, closely distri- 
buted all over the surface of the shell, immediately 
‘> under the epidermal layer of cells (Figs. 53, 54, 55), 
and it sends fibres all the way up the feet, spines, 
and pedicellarie. As it seemed to us important 
to investigate the physiological properties of this 
plexus, Professor Ewart and I made a number of 
further experiments, an account of which will now 
lead us on to the next division of our subject, or 
that of section. 
