STAR-FISH AND SEA-URCHINS. 315 
equator; for this would show that the nerve-centre 
was always persistently, though fruitlessly, endea- 
vouring to co-ordinate the action of the absent feet. 
Further, as proof that the ambulacral feet of 
Echinus are under the control of some centralizing 
apparatus when executing the righting manceuvre, 
we may state one other fact. When the righting 
macenuvre is nearly completed by the rows engaged 
in executing it, the lower feet in the other rows 
become strongly protruded and curved downwards, 
in anticipation of shortly coming into contact with 
the floor of the tank when the righting manceuvre 
shall have been completed (see Fig. 52, p. 280). 
This fact tends to show that all the ambulacral feet 
of the animal are, like all the spines, held in mutual 
communication with one another by some central- 
izing mechanism. 
But the best proof of all that the feet in executing 
the righting manoeuvre are under the influence of 
a co-ordinating centre, is one that arose from an 
experiment suggested to me by Mr. Francis Darwin, 
and which I shall now describe. Mr. Darwin 
having kindly sent the apparatus which his father 
and himself had used in their experiments on 
the geotropism of plants, it was employed thus. 
A healthy Echinus was placed in a large bottle 
filled to the brim with sea-water, and having been 
inverted on the bottom of the bottle, it was allowed 
in that position to establish its adhesions. The 
bottle was then corked and mounted on an upright 
wheel of the apparatus whereby, by means of clock- 
work, it could be kept in continual slow rotation 
