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bilateral symmetry? The skeleton consists of limy plates embedded in the tough skin (Fig. 54). Is the skin rough or smooth? Hard or soft? Are the projections (or spines) in the skin long or short? The skin is hardened by the limy plates, except around the mouth, which is at the center of the lower side and surrounded by a membrane. Which is rougher, the mouth side, (oral side) or the opposite (aboral side)? Which side is more nearly flat? The vent is at or near the center of the disk on the aboral surface. It is usually very small and sometimes absent. Why a vent is not of much use will be understood after learning how the starfish takes food.

Fig. 54.—Limy Plates in portion of a ray.

Fig. 55.—Starfish (showing Madreporite).

An organ peculiar to animals of this branch, and called the madreporic plate, or madreporite, is found on the aboral surface between the bases of two rays (Fig. 55). It is wartlike, and usually white or red. This plate is a sieve; the small openings keep out sand but allow water to filter through.

Fig. 56.—Water tube System of starfish.

m, madreporite; stc, stone canal; ap, ampulla.

Movements: the Water-tube System.—The water, which is filtered through the perforated madreporite, is needed to supply a system of canals (Fig. 56). The madreporite opens into a canal called the stone canal, the wall of which is hardened by the same kind of material as that found in the skin. The stone canal leads to the ring canal which surrounds the mouth (Fig. 56). The ring canal sends radial canals into each ray to supply the double row of tube feet found in the groove at the lower side of each ray (Fig. 57). Because of their arrangement in rows,the feet are