Starfishes vary much in size, color and general appearance, although all are readily recognizable as starfishes (fig. 22). The number of arms or rays varies from five to thirty or more in different species; some have the interradial spaces filled out nearly to the tips of the rays, making the animal simply a pentagonal disc. In size starfishes vary from a fraction of an inch in diameter to three feet; in color they are yellow or red or brown or purple. [Illustration: FIG. 22.--A group of Echinoderms; the upper one, a starfish, _Asterina mineata_, the one at the right a starfish, _Asterias ocracia_, at the left a brittle-star, species unknown, and at bottom two sea-urchins, _Strongylocentrotus franciscanus_. (From living specimens in a tide-pool on the Bay of Monterey, California.)] =Brittle-stars (Ophiuroidea).=--The brittle-stars, or serpent-stars (fig. 22) as they are also called, resemble the starfishes in external appearance, that is, they are flat and composed of a central disc with radiating arms (always five in number, although each arm may be several times branched). The central disc is always sharply distinguished from the arms, and the arms are usually slender and more or less cylindrical. The distinguishing difference between the brittle-stars and the starfishes is that the body-cavity and the stomach which extend out into the arms in the starfishes are in the brittle-stars limited to the central disc, or to the disc and bases of the arms. The tube-feet also have no suckers at the tips. More than 700 species of brittle-stars are known. They feed on marine shell-fish, crabs and worms. =Sea-urchins (Echinoidea).=--The sea-urchins (figs. 20, 21 and 22) of which more than 300 species are known, have no arms or rays, and they are usually not flat like the starfishes but globular, with poles more or less flattened. As has been noted in the examination of the body-wall or "shell," the radiate character of the body is shown by the five radiating zones of tube-feet. The mouth, with its five strong "teeth," is on the ventral surface, and the anal opening and madreporic opening are on the dorsal surface. The calcareous plates (seen distinctly in a specimen from which the spines have been removed) which constitute the firm part of the body-wall, are more or less pentagonal in shape and are usually firmly united at the edges. The spines which are so characteristic of the sea-urchins vary much in size and number and firmness, but are present in some form on all of them.