The =Sea Anemone=, like the coral polyp, lives in the sea, but like the fresh-water hydra, it _deposits no limy support for its body_. The anemone is much larger than the hydra and most coral polyps, many species attaining a height of several inches. It _does not form colonies_. When its arms are drawn in, it looks like a large knob of shiny but opaque jelly. Polyps used to be called _zoophytes_ (_plant-animals_), because of their flower-like appearance (Figs. 50, 51). [Illustration: FIG. 51.—SEA ANEMONES. ] CHAPTER V ECHINODERMS (SPINY ANIMALS) THE STARFISH [Illustration: FIG. 52.—Starfish on a rocky shore. ] SUGGESTIONS. Since the echinoderms are aberrant though interesting forms not in the regular line of development of animals, this chapter may be omitted if it is desired to shorten the course.—The common starfish occurs along the Atlantic coast. It is captured by wading along the shore when the tide is out. It is killed by immersion in warm, fresh water. Specimens are usually preserved in 4 per cent formalin. Dried starfish and sea urchins are also useful. A living starfish kept in a pail of salt water will be instructive. [Illustration: 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. [Illustration: FIG. 54.—LIMY PLATES in portion of a ray. ] [Illustration: FIG. 55.—Starfish (showing MADREPORITE). ] [Illustration: 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.