Page:Popular Science Monthly Volume 13.djvu/336

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THE POPULAR SCIENCE MONTHLY.

which we will presently speak. The mouth is in the centre of the under side, and beneath each ray there are a large number of locomotive suckers. An eye is situated at the end of each ray; and on the back, near the junction of two arms, is a sort of water-filter called the madreporic body.

As in all similar cases, the dried specimens give us only a partial idea of the real starfishes, and those who have studied these animals in museums only have little idea of the readiness with which they make their way along the vertical and overhanging surfaces of rocks, and into holes and narrow fissures.

Starfishes are very voracious, and feed mainly on mollusks. They are exceedingly destructive to oysters in many places, and thus come in direct competition with man for the possession of this delicious bivalve. Instead of swallowing their food as other animals do, they turn the stomach out of the mouth and over the animal which they wish to devour!

Starfishes have a wonderful power of reproducing lost parts. If an arm is bitten off by a hungry fish, another grows in its place; and cases are known where all the arms but one have been detached, and the remaining arm and central portion of the body have lived on and reproduced all the destroyed parts. Examples of this may be seen wherever starfishes are abundant.

Starfishes are quite numerous in species, and vary greatly in form and size. The ordinary kinds are only three or four inches across, others a foot.

In the same localities where we find true starfishes, we may confidently expect to find the "serpent-stars" or serpent-tailed starfishes, PSM V13 D336 Serpent star.jpgFig. 23.—Serpent-Star
(Ophiopholis bellis, Lyman).
so called because their arms taper like a snake's tail. They are also called "brittle-stars," because they break so easily.

Many visitors to the sea-shore come away without seeing a single living brittle-star, because the curious echinoderms which bear this name hide under the sea-weeds, and in the dark holes and crevices among the rocks, and are, therefore, found only by those who search carefully for them.

The long, gently-tapering arms, starting out abruptly from a well-defined disk, make the form of serpent-stars very distinct from that of the Asteroidæ or genuine sea-stars, already noticed. And, unlike the true starfishes, they have no interambulacral plates, but a series of large plates envelops the whole of each ray or arm, meeting in a