Coral polyp

The polyp is supported in a stone-like cup. It makes the cup from substances dissolved in seawater. The cup, called a theca, anchors the polyp to the older layers below and to the polyps around it.
The tentacles catch food, although the coral gets most of its food from tiny single-celled plants that live inside the coral tissues.

adapted from: How it works- Exploring The Oceans, by Stephen Hall