Cnidaria are organisms with a central stomach surrounded by stinging cells on the end of finger-like projections (tentacles). This phylum includes jellyfish, sea anemones, and corals. Coral heads are the skeletal remains of living corals. They are rough in texture and come in a variety of shapes, sizes, and colors. Corals can be either solitary or colonial. The colonial ones build large reefs. Each hole contains a single organism (polyp) that all live together. 

Sea urchin on top of coral

Coral polyps with crab