=External Features.=--What is the _shape_ of the body? What kind of _symmetry_ has it? Do you find the oral (or mouth) surface? The aboral surface? Where is the body flattened? What is the shape of the spines? What is their use? How are the tube feet arranged? Where do the rows begin and end? Would you think a sea urchin placed upside down in water, could right itself less or more readily than a starfish? What advantage in turning would each have that the other would not have? The name sea urchin has no reference to a mischievous boy, but means sea hedgehog (French _oursin_, hedgehog), the name being suggested by its spines. =Comparison of Starfish and Sea Urchin.=--The water system of the sea urchin, consisting of madreporite, tubes, and water bulbs, or ampullæ, is similar to that of the starfish. The tube feet and locomotion are alike. There is no need for well-developed respiratory organs in either animal, as the whole body, inside and out, is bathed in water. The method of reproduction is the same. [Illustration: FIG. 61.--A SEA URCHIN crawling up the glass front wall of an aquarium (showing mouth spines and tube feet).] [Illustration: FIG. 62.--A SEA URCHIN with spines removed, the limy plates showing the knobs on which the spines grew.] [Illustration: FIG. 63.--SECTION OF SEA URCHIN with soft parts removed, showing the jaws which bear the teeth protruding in Fig. 62.] The starfish eats animal food. The food of the sea urchin is almost exclusively vegetable, hence it needs teeth (Fig. 62, 63); its food tube is longer than that of a starfish, just as the food tube of a sheep, whose food digests slowly, is much longer than that of a dog. [Illustration: FIG. 64.--THE SEA OTTER, an urchin with mouth (_o_) and vent (_A_) on same side of body.] The largest species of sea urchins are almost as big as a child’s head, but this size is unusual. The spines are mounted on knobs, and the joint resembles a ball-and-socket joint, and allow as wide range of movement. Some sea urchins live on sandy shores, other species live upon the rocks. The sand dollars are lighter colored. (Why?) They are usually flatter and have lighter, thinner walls, for there is danger of sinking into the sand. The five-holed sand cake or sand dollar has its weight still further diminished by the holes, which also allow it to rise more easily through the water. The flattened lower surface of both starfish and sea urchin causes the body to remain still while the tube feet are stretching forward for another step. OTHER ECHINODERMS The =sea cucumbers=, or =holothurians=, resemble the sea urchin in many respects, but their bodies are elongated, and the limy plates are absent or very minute. The mouth is surrounded by tentacles (Fig. 65). [Illustration: FIG. 65.--SEA CUCUMBERS.] [Illustration: FIG. 66.--A BRITTLE STAR.]