Shell of a sea urchin.
Diagrammatic section of a sea urchin.
Mouth of a sea urchin.
Echinocardium cordatum. A bilaterally symmetrical species, sometimes called the heart-urchin on account of its shape. It has fine spines like bristles, and is common on sandy shores, where it burrows to a depth of about 3 inches. Empty shells are often washed up; they are white and have usually lost their spines. Echinus esculentus, common sea urchin. This species is common on British shores; in the Mediterranean countries it is collected and used for food. Heterocentrotus mamillatus, slate-pencil urchin. A species with very thick spines the lives in the Pacific and Indian Oceans.
Most animals, including ourselves, are made on a plan of bilateral symmetry – they can be divided into two more or less identical halves, a right and a left. The echinoderms are usually radially symmetrical – their parts are arranged around a central axis like the spokes of a wheel or the petals of many flowers. The normal number of radii or 'spokes' is five. Some of the 'cake-urchins' are bilaterally symmetrical, though the five rays can still be seen. This five-symmetry is shared by no other animals.
The mouth of a sea urchin is on its lower surface; inside it there is a peculiar structure, rather fancifully called 'Aristotle's lantern'. It consists of five complicated 'jaws', all joined together to form a cone-shaped structure; each jaw has a sharp tooth at lower end, and the tips of the teeth can be seen through the mouth.
All sea urchins and many starfish also have numerous little two- or three-fingered pincers on their bodies, called pedicellariae. A three-fingered one is shown in the diagram above. They are used for removing mud or sand from their bodies, capturing prey, and for self-defence. The pedicellariae of some tropical sea urchins have poison glands and are formidable weapons.