The corallines secrete a smooth stalk which affords no protection, but they also secrete a coating or sheath which incloses both themselves and the stalk. The coating has apertures through which the polyps protrude in order to feed when no danger is near (Fig. [46]). The red “corals” used for jewelry are bits of stalks of corallines. The corallines (Figs. [47], [48]) are not so abundant nor so important as the coral polyps (Figs. [45], [49]).
Fig. 46.—Red Coralline with crust and polyps (eight tentacles).
Fig. 47.—Sea Fan (a coralline).
Fig. 48.—Organ Pipe “Coral” (a coralline).
Colonies of coral polyps grow in countless numbers in the tropical seas. The coral formed by successive colonies of polyps accumulates and builds up many islands and important additions to continents. The Florida “keys,” or islands, and the southern part of the mainland of Florida were so formed.
Fig. 49.—Upright cut through coral polyp × 4.
ms, mouth; mr, gullet; ls, ls, fleshy partitions (mesenteries) extending from outer body wall to gullet (to increase absorbing surface); s, s, shorter partitions; mb, fb, stony support (of lime, called coral); t, tentacles.