314 JELLY-FISH, STAR-IISH, AND SEA-URCHINS. 
rows of ambulacral 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 
supplied 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 circumstances 
mentioned to remain permanently reared upon its 
