Page:Popular Science Monthly Volume 27.djvu/382

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366
THE POPULAR SCIENCE MONTHLY.

ments and allow itself to roll over into its natural position. But an echinus will never let go its attachments without some urgent reason, seeming to be above all things afraid of being rolled about at the mercy of currents, and therefore it lets itself down almost as slowly as it pulled itself up (Fig. 10).

Single rays separated from a star-fish crawl as fast as the entire animal, and likewise in a determinate direction. They also crawl up

PSM V27 D382 Sea urchin curling up for protection.jpg

Fig. 10.

perpendicular surfaces, and when inverted right themselves as quickly as do the unmutilated creatures. A segment of an echinus bearing a single row of ambulacral feet, when propped up on its ab-oral pole, (Fig. 11) will right itself after the manner of entire animals (Fig. 12). It, however, experiences more difficulty in doing so, and very often fails to complete the manœuvre. Such a segment is, of course, analogous to a single detached ray of a star-fish; but on account of the rigid consistence and awkward shape of the segment — standing erect instead of lying flat — it presents a much more curious appearance in locomotion than does the ray of a star-fish.

Dr. Romanes reports observations which show conclusively that the whole external surface, not only of the soft and fleshy star-fish, but even of the hard and rigid echinus, is everywhere sensitive to stimulation. This sensitiveness, moreover, is highly delicate. If any part of the external surface of an echinus is lightly touched with the point of a needle, all the feet, spines, and pedicellariæ within reach of that part, and even beyond it, immediately close in upon the needle and grasp it tightly. This simultaneous movement of such a little forest of prehensile organs is a very beautiful spectacle to witness. Here we have proof of the function of the pedicellariæ. In climbing perpendicular or inclined surfaces of rock covered with waving sea-weeds, it must be of no small advantage to an echinus to be provided on all sides with a multitude of movable stalks bearing forceps, which can instantly seize a passing frond. The frond being thus arrested, the spines come to the assistance of the pedicellariæ, and both together hold the sea-weed steady till the ambulacral feet have time to