These animals are sometimes described as "burrowing" creatures, because they live covered in sand. The term is rather misleading. Far from wishing to burrow, they spend their lives in a constant struggle with sand that closes over them only too readily; and their whole structure is adapted to prevent their sinking in a quicksand.
We began our chapter with the Sea-Urchins, because they are the most important members of the group to which they give their name; but there are forms belonging to the Echinodermata that are more familiar to the ordinary observer—the Starfishes. Those who take an interest in the cultivation of the oyster find them far too familiar—for the starfish is the oyster's deadliest foe, not even excepting man.
The common Starfish, Asterias rubens, may constantly be found among stones, about low-tide mark. Its manner of walking is peculiar and characteristic. On the under surface of each ray are rows of white sucker-like tube-feet, which can either be drawn in or pushed out. By doing each alternately the animal walks. First the feet are extended to their full length; then the terminal sucking disc of each catches hold of the ground. Then the feet are again retracted, while their discs still cling; the effect of this is, naturally, to pull the ray onwards. This process is repeated again and again, until some appreciable degree of movement is effected. The tube-feet are in connection with a system of vessels filled with fluid, known as the Water-vascular System of the Starfish. The fluid is driven on by muscular contractions until the feet are fully extended, and again driven back when the feet are retracted. The Water-vascular System is a structure common to all Echinoderms; and vessels of a comparable character are found in some worms.
How does the Starfish know where it is going? Underneath each ray, near the tip, is a little feeler (or tentacle) and a little eye spot. By means of these it gets an idea where each ray is going to; and, since it often moves but one ray at a time, this is sufficient for it. When necessary, however, the several rays can act in concert with one another.
The rayed form of the Starfishes led to their being at first included in the group of Radiate Animals, along with the tentacle-bearing Cœlenterata; but it has long been recognised that they are animals of much higher structure. Their very larvæ can barely be brought into comparison with animals so simple as the true "radiates."
The Snake-Stars, or Ophiuroidea, are closely allied to the Starfishes. In these the arms are thin and sharply defined from the little central disc, instead of sloping gently out of it, as in the Starfishes. The rapid wriggling movements of the arms have gained for them their very appropriate name. They are also called Brittle Stars, because the arms break off easily, sometimes at the will of the animal. Several kinds of them are common on our shores, although they are not so common as the ordinary Starfishes. [Fig. 37] shows the general form of a Brittle Star.
Fig. 37.—A Brittle-Star, Ophiopteris antipodum.