280 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
But an Echinus will never let go its attachments 
without some urgent reason, seeming to be above 
all things afraid of being rolled about at the mercy 
of currents; and therefore in this case it lets itself 
down almost as slowly as it raised itself up. So 
gently, indeed, is the downward movement effected, 
that an observer can scarcely tell the precise 
moment at which the righting is concluded. There- 
fore, in the downward movement, the feet, which at 
the earlier part of the manceuvre were employed 
successfully in rearing the globe upon its equator, 
are now employed successfully in preventing its too 
rapid descent (Fig. 52). 
Several interesting questions arise with reference 
to these righting movements of Echinus. First of 
all we are inclined to ask what it is that determines 
the choice of the rows of feet which are delegated 
to effect the movements. As the animal has a 
geometrical form of perfect symmetry, we might 
suppose that when it is placed upon its pole, all the 
