CHAPTER XII
THE ECHINODERMATA
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.
Fig. 36.—The Five-holed Sand Cake, Mellita pentapora, a flat sea-urchin from the east coast of tropical North America. A, upper surface; B, lower surface; C, side view.
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.