Fig. 53.—Plan of starfish; III, madreporite.
External Features.—Starfish are usually brown or yellow. Why? (See Fig. [52].) Has it a head or a tail? Right and left sides? What is the shape of the disk, or part which bears the five arms or rays? (Fig. [53].) Does the body as a whole have symmetry on two sides of a line (bilateral symmetry), or around a point (radial symmetry)? Do the separate rays have 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 centre 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 centre 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).
Fig. 56.—Water tube System of starfish.
m, madreporite; stc, stone canal; ap, ampulla.
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.