Here the broad zones terminate in five large plates differing somewhat from those that form the zones in other parts of the body, and called ovarian plates, because the eggs pass out through certain openings in them; while the five narrow zones terminate in five small plates on each of which is an eye, making thus five eyes alternating with five ovarian plates. The centre of this area containing the ovarian plates and the visual plates is filled up with small movable plates closing the space between them. I should add that one of the five ovarian plates is larger than the other four, and has a peculiar structure, long a puzzle to naturalists. It is perforated with minute holes, forming an exceedingly delicate sieve, and this is actually the purpose it serves. It is, as it were, a filter, and opens into a canal which conducts water through the interior of the body; closed by this sieve on the outside, all the water that passes into it is purified from all foreign substances that might be injurious to the animal, and is thus fitted to pass into the water-system, from which arise the main branches leading to the minute suckers which project through the holes in the narrow zones of plates. [Illustration: Star-Fish from the ab-oral side.] Now in order to transform theoretically our Sea-Urchin into a Star-Fish, what have we to do? Let the reader imagine for a moment that the small ab-oral area closing the space between the ovarian plates and the eye-plates is elastic and may be stretched out indefinitely; then split the five broad zones along the centre and draw them down to the same level with the mouth, carrying the ovarian plates between them. We have then a star, just as, dividing, for instance, the peel of an orange into five compartments, leaving them, of course, united at the base, then stripping it off and spreading it out flat, we should have a five-rayed star. [Illustration: One arm of Star-Fish from the oral side.]