STAR-FISH AND SEA-URCHINS. 315 



eqTiator; 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 centralizinof 

 apparatus when executing the righting manoeuvre, 

 we may state one other fact. When the righting 

 maoenuvre 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 riohtinoj manoeuvre 

 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 kindlj^ 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 



