Everybody knows the Star-fish and many people know the Sea-Urchin. An "urchin" is not a name for a naughty little boy, but the French (oursin) for a hedgehog. A Sea-Urchin is therefore a "Sea-Hedgehog," a name very appropriate for a creature armed with prickles. The Greek word echinos also means a hedgehog, so that the long name given to the group means simply hedgehog-skinned. The prickles attain their maximum in the Sea-Urchin, but they are well represented in the Star-fish, while in the Sea-cucumber the general tendency to "prickliness" is much reduced, and represented only by "spicules" (needles) of shelly stuff underneath the skin of the animal.

The largest and the most beautiful of the Sea-Urchins of the English coast is known as the Purple-tipped Sea-Urchin, on account of the beautiful colour of the spines. It lives on rocky coasts, and during very low tides may be seen at home, although it usually takes care not to stray above the water-line. It is a shelly ball with a flat base; its surface is covered with long spines. Its mouth, which is in the centre of the base, shows five wicked-looking teeth peeping out. The shell is pierced by what look like hundreds of minute pin-holes, arranged in a complicated pattern; these are the holes through which it pokes its feet, which greatly resemble those of a Star-fish, being white suckers with a disc at the end. When thrown out to their full length they are, however, much longer than those of the Starfish, for they are naturally obliged to be thrown out to a distance longer than the length of the animal's own prickles. When moored by all its feet, extended from all sides of the shelly ball, the animal presents a curious and pretty sight. Large specimens are almost as big as a child's head, but smaller ones are more common. There is a considerable range of variation in colour; not only are various shades of purple found, but also purplish-red and red. The spines are mounted on something resembling a ball and socket joint, with a ring-shaped pad, so that they have a wide range of movement; if any of the spines are touched they are immediately set back over a considerable part of the neighbouring surface.

Other kinds may be found upon a more sandy shore. These are heart-shaped and much lighter in colour. The shell is thinner and of less weight. These adaptations for lessening the animal's weight enable it to move over sand: the species above described has no occasion for such precautions. When it crawls over rocks and the strong seaweeds that grow on them, there is no fear of its sinking in. The sand-dweller, on the contrary, must take care that it is not swallowed up.

There are Sea-Urchins that carry their precautions against sinking to an extreme degree. These are the Shield-Urchins or ClypeastridÆ, so-called from their flat shape; they include the American forms popularly known as "sand-cakes." The diagram (Fig. 36) shows one of the most curious of these flattened forms adapted for moving over fine sand and ooze, and literally "as flat as a pancake." The mouth is approximately in the centre of the lower surface, B; the upper surface, A, shows a rosette pattern on the top of the shell. This is formed by the rows of holes for the very minute tube feet. In the English Sea-Urchin above described, which is one of the group called (for that reason) Regulares, the rows of holes are uniformly continued all along the rounded sides of the body down to the neighbourhood of the mouth. Here they are much restricted, forming merely a rosette at the top of the shell: hence they are described as circumscript or "petaloid." The excretory aperture is shown in the photograph as a smaller dot on one side of the mouth, while in the Echinus, on the contrary, it is at the top of the shell. The five odd-looking, elongated holes are a curious individual peculiarity of this Sea-Urchin. It has already been explained that the Shield-Urchins are flattened in order to distribute their weight; these holes are a contrivance for still further reducing the weight in comparison with the area. This is when the animal is lying quiet at the bottom of the water, but when it moves about what effect will the presence of the holes produce? Flattened animals are usually supposed to derive an advantage from the fact that they sink more slowly through depths of water; as in lying upon the ground, their weight is distributed, and they float, as it were, in the same stratum of water without sinking further down. This creature, on the contrary, has apparently feared lest it should move too slowly when it moves in a vertical direction, and it presents us with an arrangement by means of which its sinking through water is facilitated. Water will pass readily through the five holes as the animal goes either up or down, and the resistance of the whole flat area to the water is thus reduced and vertical movement rendered more easy. Thus, by one and the same contrivance, the animal has lessened its weight when lying quiet, and diminished the resistance it meets with when it moves. The distribution of the holes, moreover, is such as to regulate the animal's position in sinking, and to prevent it from falling "headlong." For although the creature has, strictly speaking, no "head," yet the end nearest the mouth is the thickest and heaviest part of the "cake," and would naturally tend downwards. This tendency is counteracted by the fact that the thicker end is unperforated, while the thinner and lighter end has a large central hole to diminish its resistance and enable it to sink more rapidly.

Adapted for living in sand rather than on rocks, but not so extreme in the peculiarity of their form as the Shield-Urchins, are the Heart-Urchins, already referred to, shaggy-looking creatures whose fine yellowish-white spines give them almost the appearance of being clothed with fur. The excretory aperture is at the narrow end of the "heart," and the mouth at one side of the lower surface towards the wide end. The complicated apparatus of teeth found in other Sea-Urchins is absent in these. They are abundant on sandy shores. During the severe winter of 1894-5, when the Mersey at Liverpool was frozen nearly for one memorable day, and filled with floating ice for many more, I saw the shore beyond New Brighton heaped all along with a bank, often two feet across, of the common Heart-Urchin. These, which afforded a fine feast for the hungry sea-gulls, had been killed by the intense cold, and afterwards washed ashore by the tide. The vast numbers of this creature which exist on that coast were thus unexpectedly brought to light.

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 Coelenterata; 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.

The Sea-Cucumbers, Holothuroidea, are another group of Echinodermata that are represented on our own coasts; by small specimens, however, while the Pacific Ocean furnishes instances of larger size—the Trepangs—which are used by the Chinese as articles of food. The name Sea-Cucumber is given in fanciful comparison to a small Gherkin; presumably one that has been very badly pickled—for the colour of the animal is brownish and by no means green. The mouth of a Sea-Cucumber is surrounded by a circlet of tentacles (partially indicated in the diagram, Fig. 38). The body is elongated and crawls along: the "star" shape, so characteristic of the Echinoderms, is scarcely to be recognised except in cross section, where the longitudinal rows of tube-feet are seen to outline a pentagon. The skeleton of the Sea-Cucumber is of a very meagre description. Instead of forming a rounded case, as in the Sea-Urchin, it consists only of loose pieces of very small size, situated below the skin. The Starfishes are intermediate in this respect. Their "skeleton" consists of a vast number of pieces or "ossicles," which are of fair size, but are not closely united, as in the Sea-Urchin. They are, however, so numerous and so well knit, that the skeleton of a dead Starfish presents the complete outward form of the animal. It must be noted that the ordinary skeleton of the Sea-Urchin is only apparently exterior. As is the case with the ossicles of the Starfish and Sea-Cucumber, the skin lies outside, and the hard particles belong to the middle layer, or mesoderm. In this the skeleton of Echinoderms differs from the "shell" of a crab or lobster, which is formed by a hardening of the skin itself.

The Crinoidea, Encrinites or Stone-Lilies, form another group of the Echinodermata. Though still represented by living forms, they attained their maximum development in past ages. The English "Mountain Limestone" of the Carboniferous period is full of their fossilized remains, which form a marble often used for ornamental purposes. The so-called "Stone Lily" consists of a "head" comparable with the body of a Star-fish or other Echinoderm, which is borne at the end of a long fixed stalk. The marble above named owes its ornamental appearance to the presence of these stalks, often very long, and cut through at every possible angle. The Crinoids have their living representative in English Seas, Antedon, the Feather-Star (Fig. 40). On the side opposite the mouth, where, in the Encrinite, the stalk would be, there are a group of elongated processes called cirrhi, by means of which the animal can attach itself to stones or seaweeds. When not thus fixed, it swims about, by moving its fringed arms, each of which is forked. It will be seen that when the animal is fixed by its cirrhi, it stands mouth upwards, so that its position compared with that of the Starfish or Sea-Urchin is upside down. The young of the Feather-Stars have stalks by which they are fixed, like the Encrinites; but afterwards the stalk is lost.

Among fossil Echinoderms there are two groups of stalked forms which have no living representatives. These are the Cystoidea and the Blastoidea. In both of these the stalk bears, as in Encrinites, a calyx or head, which is comparable, with the body of the free Echinoderms.

The Sea-Urchins possess a swimming larval stage, which goes through remarkable changes after passing out of the two-layered (Gastrula) form. It becomes provided with cilia, which are arranged in bands, and outgrowths of peculiar form are established in the case of the Sea-Urchins, while the larvÆ of the other groups also present characteristic shapes. Within the larva the adult form develops, the outside of the larva being finally thrown off.

In the young Feather-Star, a subsequent stage of the young animal has a stalk, by which, like the Encrinite, it is fixed. This animal therefore is at first free-swimming, afterwards fixed, and again free in its final stage—a remarkable series of changes.

These queer-shaped things, the Sea-Urchins and their allies, are perhaps the last creatures amongst which we should think of looking for relations of the Worms. Yet the earliest stages of the larva are considered to present a certain amount of resemblance to the Wheel-ball larva, which has been referred to elsewhere (pp. 42 and 72). Still more startling fact, these larvÆ have been compared to that of Balanoglossus, the lowest member of the Chordata, and a relation of the Vertebrates themselves (see p. 143).

TABLE SHOWING THE CLASSIFICATION OF THE ECHINODERMATA