Page:Popular Science Monthly Volume 27.djvu/383

Rh establish their hold upon it with their sucking-disks. This operation may be witnessed by drawing a piece of sea-weed over a healthy echinus in the water.

The capability of the spines for co-ordinated action is highly remarkable and interesting. Thus, for instance, if an urchin be taken



out of the water and placed upon a table, it is no longer able to use its feet for walking, as the suckers can act only under water. Yet the animal is able to progress slowly by means of its spines, which are used to prop and push the globe-like shell along in some continuous direction. If a lighted match be held in front of the moving animal, as soon as the echinus comes close enough to feel the heat, all the spines begin to make the creature move away in the opposite direction. There is an urchin-like form of echinoderm called spatangus, which differs from the echinus in having shorter feet and longer spines. When, therefore, a spatangus is inverted it is unable to right itself