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-orJihated, I may mention that 

 there is an urchin-like form of Echinoderra, which is 

 called Spatangus, and wdiich differs fi om 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 



