3 14 JELLY-FISH, STAR-IISII, AND SEA-URCHINS. 



rows of ainbulacral feet immediately adhere to the 

 surface and rotate the animal upwards or horizon- 

 tally, until it attains the normal position. But if 

 care be taken to prevent contact with any side of 

 the tank, the mutilated Echinus will remain propped 

 on its equator for days ; it never adopts the simple 

 expedient of reversing the action of its mutilated 

 feet- rows, so as to bring the globe again upon its 

 ab-oral pole and get its unmutilated feet- rows into 

 action. 



From this we may conclude that the righting 

 movements of the pedicels are due, not to the merely 

 serial action of the pedicels, but to their co-ordina- 

 tion by a nerve-centre acting under a stimulus 

 so pplied by a sense of gravity ; for if the move- 

 ments of the pedicels were merely of a serial 

 character, we should not expect that the equatorial 

 position, having been attained under these circum- 

 stances, should be permanently maintained. We 

 should not expect this, because after a while the 

 pedicels, which are engaged in maintaining the 

 globe in its equatorial position, must become ex- 

 hausted and relax their hold, when those next 

 behind in the series would lay hold of the bottom 

 of the tank, and so on, the rotation of the globe 

 thus proceeding in the opposite direction to that 

 in which it had previously taken place. On the 

 other hand, if the righting movements of the pedi- 

 cels are due to co-ordination proceeding from a 

 nerve-centre acting under a sense of gravity, we 

 should expect the animal under the circumstajices 

 mentioned to remain permanently reared upon its 



