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 ambulacrai 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. 
—TIf 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 pedicellarize within the circular 
area are physiologically separated from the con- 
tizuous spines and pedicellariz, as regards local 
reflex excitability. That is to say, if any part of 
this circular area be stimulated, all the spines and 
pedicellariz within that area immediately respond 
to the stimulation in the ordinary way; while none 
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