292 JELLY-FISH, STAR-FISH, AXD SEA-URCHINS. 



they may be reduced to immediate quiescence — i.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- 

 lari?e — 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 Loven had already brietly mentioned such 

 a plexus as having been observed by him. The 

 plexus consists of cells and fibres, closely distri- 

 buted all over the surface of the shell, immediately 

 under the epidermal layer of cells (Figs. 53, 54, 5o), 

 and it sends fibres all the way up the feet, spines, 

 and pedicellarise. 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. 



