Page:Popular Science Monthly Volume 27.djvu/381

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on down the ray. Usually two or three adjacent rays perform this manœuvre simultaneously, the spirals of the co-operating rays being invariably turned in the same direction, and, when they have proceeded sufficiently far to drag over the remaining rays, these then abandon their hold on the bottom so as not to offer any resistance to the lifting action of the active rays. The whole movement does not occupy more than half a minute.

But it is in the case of echinus that these righting movements become most interesting, from the fact that they are so much more difficult to accomplish. Two, or perhaps three, adjacent rows of suckers are chosen out of the five to accomplish the task. As many feet in these rows as can reach the floor are thrust downward and fastened firmly to it; by their combined action, as by the pull of liliputian ropes, the globe is tilted slightly in their own direction, the anchoring feet in the opposite rows releasing their hold on the floor to admit of this tilting (Fig. 8). The next feet in the active rows are thus enabled to reach the floor, and, when they have established their hold, they assist in increasing the tilt; then the next feet in the series lay hold, and so on, the globe slowly but steadily rising until it stands upon its equator (Fig. 9). The difficulty of raising such a heavy mass into this

PSM V27 D381 Echinus raises itself.jpg
Fig. 9.

position by means of the slender motive power available is manifest not only from the extreme slowness with which it takes place, but because specimens not perfectly strong may fail completely to reach the position of resting on the equator. Moreover, in some cases when this position has been reached with difficulty, the echinus gives itself a breathing-space, as it were, before beginning its descent. It will be perceived that, as soon as the descent begins, gravity is no longer an obstacle but an aid to the righting movement, and it might be anticipated that the echinus would now simply let go all its attach-