rock-builder

from The Century Dictionary.

• noun One of the invertebrate organisms whose skeletons essentially contribute to the formation of rock masses. Many classes of the invertebrates have at one time or another acted as rock-builders on account of the profusion of their exuviæ. Chief among them are the Foraminifera Radiolaria, corals, and corallines; less prominent are sponges, crinoids, brachiopods, and pelecypods.