The Echinodermata or spiny-skinned, are most commonly represented by the sea-urchins and star-fishes of our coasts. In some of the classes locomotion is performed by means of these spines or prickles, which serve as legs. In others, movement is carried on by suckers and tubes as in the star-fishes, these tubes being also the means whereby the animal obtains its food.
Fig. 838.—Sea-urchin (echinus), with and without spines.
They have a digestive system, and possess a curiously horny skin even when spines are absent. The mouth is in the centre. We give an illustration of the sea-urchin, and of a section of a spine, which is a beautiful object when seen under the microscope, for these spines can be made quite transparent when cut across and ground. The shelly covering is porous, and as the animal grows the shell is added to at the edges. Underneath will be found the mouth, which has teeth fitted for devouring the small crustacea. These sea-urchins abound, and their porous shells may be picked up frequently after a storm.
The star-fishes are well known to all searchers amongst the rocks and those who study the shore, and are often taken home for an aquarium. They are very voracious, however, and when one is examined in a glass of sea-water, the observer will detect many suckers protruding from each of the rays. It is by means of these suckers, which are put forth from innumerable little holes called “ambulacral apertures,” that the star-fish makes his way up the rocks and along the ocean bed. The stomach of the star-fish is extensive, and situated in the centre of the rays wherein is a digestive apparatus. The rays are composed of detached but beautifully fitted pieces, so united as to be flexible, and around the mouth and in strong frame-work. The star-fish has no teeth, but manages to dispose of a vast quantity of matter, which if left alone would be injurious in decay.
Thus Nature has provided a shore scavenger to devour what would be harmful, just as the vulture on land eats the carrion. Besides this kind of refuse food, the star-fish eats small crustaceans, and oysters fall victims to him. By embracing the shell the star-fish manages to insert itself, and if it cannot bring the oyster out to its mouth, it will quietly turn out its mouth into the oyster-shell, and save the bivalve any trouble in the matter. Some writers declare that the star-fish stupefies or poisons its victim, and then the shell opens. These asteroidea can reproduce a ray that has been injured or cut off, or they can break themselves to pieces if caught.
Fig. 839.—Spine of echinus (A, natural size; B, a section magnified).
The brittle stars and feather stars appertain to the order of the Ophiuroida or “serpent-armed,” because the rays are more flexible and thin than the common star-fish. But they differ very much from the star-fish in the arrangement, as well as in the shape of their arms. The former possesses rays which form an appendage of the stomach and enclose it. In the brittle stars the rays are limbs, and could be detached without taking the life of the animal, except in so far as to deprive it of means to obtain its food. The body is quite independent of the rays, the mouth occupying the centre, and is surrounded by minute suckers. The stars are much more flexible than the star-fishes, their rays are longer, and serve either as feet, fins, tentacles, or arms.
The crinoids also belong to the Echinodermata, and resemble plants more than star-fish. They are fixed upon a stalk like a flower, growing upright from the sea-bottom, and the body is called a calyx, which is composed of a ventral and dorsal surface. The arms branch out from the calyx, just as a small tree does, and if we can imagine one of the last planted trees on the Thames embankment reduced to half a finger’s length or less, we have a sort of idea of the crinoid “in the rough.”