STAR-FISH AND SEA-URCHINS. 301 



although still remaining feebly active, no longer 

 taking part in any co-ordinated movement — that 

 ray, therefore, being merely dragged along by the 

 others. 



Under this division it only remains further to be 

 said, that section of the nerve-ring in the disc, or 

 the nerve-trunks of the rays, although, as we have 

 seen, so completely destroying physiological con- 

 tinuity in the rows of ambulacral feet and muscular 

 system of the animal, does not destroy physiological 

 continuity in the external nerve-plexus ; for how- 

 ever much the nerve-ring and nerve- trunks may 

 be injured, stimulation of the dorsal surface of the 

 animal throws all the ambulacral feet and all the 

 muscular system of the rays into active movement. 

 This fact proves that the ambulacra! feet and the 

 muscles are all held in nervous connection with one 

 another by the external plexus, without reference 

 to the integrity of the main nerve-trunks. 



2. Echini. — Section of external surface of shell. 

 — If a cork-borer be applied to the external surface 

 of the shell of an Echinus, and rotated there till 

 the calcareous substance of the shell is reached, 

 and therefore a continuous circular section of the 

 over-lying tissues effected, it is invariably found 

 that the spines and pedicellarias within the circular 

 area are physiologically separated from the con- 

 tiguous spines and pedicellarige, as regards local 

 reflex excitability. That is to say, if any part of 

 this circular area be stimulated, all the spines and 

 pedicellarice within that area immediately respond 

 to the stimulation in the ordinary way; while none 



