Page:Popular Science Monthly Volume 27.djvu/380

364 the animal while walking be turned half round, it will continue its movements as before, and hence will proceed in a direction opposite to its former one. When at rest, some of the feet are used as anchors, and others protruded as feelers.

All species of the Echinodermata, when turned upon their backs, are able to right themselves. The brittle-stars can easily perform the



needful manœuvre by wriggling some of their snake-like arms under the inverted disk, and heaving the whole body over by the mere muscularity of these members. The common star-fish, however, experiences more difficulty, and executes the manœuvre mainly by means of its suckers. It twists round the tip of one or more of its rays until the feet there situated are able to get a firm hold of the floor (Fig. 7,



a), then, by successive action of the feet further back in the series, the whole ray is twisted round (b), so that the under surface of the end is applied flat against the floor (c). The semi-turn or spiral then travels