[Illustration: Sea-Urchin seen from the oral side, showing the zones with the spines and suckers; for the ab-oral side, on the summit of which the zones unite, see February Number, p. 216.] The whole surface of the animal is divided by zones,--ten in number, five broader ones alternating with five narrower ones. The five broad zones are composed of large plates on which are the most prominent spines, attached to tubercles that remain on the surface even when the spines drop off after death, and mark the places where the spines have been. The five small zones are perforated with regular rows of holes, and through these perforations pass the suckers or water-tubes which are their locomotive appendages. For this reason these narrower zones are called the _ambulacra_, while the broader zones intervening between them and supporting the spines are called the _interambulacra_. Motion, however, is not the only function of these suckers; they are subservient also to respiration and circulation, taking in water, which is conveyed through them into various parts of the body. [Illustration: Portion of Sea-Urchin representing one narrow zone with a part of the broad zones on either side and the ab-oral area on the summit.] The oral aperture is occupied by five plates, which may be called jaws, remembering always that here again this word signifies the function, and not the structure usually associated with the presence of jaws in the higher animals; and each of these jaws or plates terminates in a tooth. Even the mode of eating in these animals is controlled by their radiate structure; for these jaws, evenly distributed about the circular oral aperture, open to receive the prey and then are brought together to crush it, the points meeting in the centre, thus working concentrically, instead of moving up and down or from right to left, as in other animals. From the oral opening the ten zones diverge, spreading over the whole surface, like the ribs on a melon, and converging in the opposite direction till they meet in the small space which we have called the ab-oral region opposite the starting-point.