290 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
animal move away in the opposite direction. More- 
over, as showing the high degree in which the action 
of the spines is co-ordinated, I may mention that 
there is an urchin-like form of Echinoderm, which is 
called Spatangus, and which differs from the Echinus 
in having shorter feet and longer spines. When, 
therefore, a Spatangus is inverted, it is unable to 
right itself by means of its feet, as these are too short 
to admit of being used for this purpose; but, never- 
theless, the animal is able to right itself by means 
of the co-ordinated action of its long spines, these 
being used successively and laboriously to prop 
and push the animal over in some one definite 
direction. The process takes a very Jong time 
to accomplish, and there are generally numerous 
failures, but the creature perseveres until it eventu- 
ally succeeds. 
Coming now to stimulation with reference to the 
feet, we find that when a drop of acid, or other 
severe stimulation, is applied to any part of a row 
of protruded pedicels, the entire row is immediately 
retracted, the pedicels retracting successively from 
the seat of irritation—so that if the latter be in the 
middle point of the series, two series of retractions 
are started, proceeding in opposite directions simul- 
taneously ; the rate at which they travel is rather 
slow. This process of retraction, however, although 
so complete within the ray irritated, does not extend 
to the other rays. But if the stimulus be applied 
to the centre of the disc, upon the oral surface of 
the animal, all the feet in all the rays are more or 
less retracted-—the process of retraction radiating 
