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 clockwork, it could be kept in continual slow rotation in a vertical plane. The object of this was to ascertain whether the continuous rotation in a vertical plane would prevent the animal from righting itself (because confusing the nerve-centres which, under ordinary circumstances, could feel by their sense of gravity which was up and which was down), or would still allow the animal to right itself (because not interfering with the serial action of the feet). Well, it was found that this rotation of the whole animal in a vertical plane entirely prevented the righting movements during any length of time that it might be continued, and that these movements were immediately resumed as soon as the rotation was allowed to cease. This, moreover, was the case, no matter what phase of the righting manoeuvre the Echinus might have reached at the moment when the rotation began. Thus, for instance, if the globe were allowed to have reached the position of resting on its equator before the rotation was commenced, the Echinus would remain motionless, holding on with its equatorial feet, so long as the rotation was kept up.