Corals are marine invertebrates that belong to the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Almost all corals are colonial organisms, composed of hundreds to hundreds of thousands of individual animals called polyps. Each polyp has a stomach that opens at only one end, surrounded by a circle of tentacles that the polyp uses for defense, to capture small animals for food, and to clear away debris. Corals are classified as either "hard coral" or "soft coral". Hard corals extract abundant calcium from surrounding seawater and use this to create a hardened structure for protection and growth, while soft corals grow wood-like cores for support and fleshy rinds for protection. Most structures that we call "coral" are made up of hundreds to thousands of tiny coral creatures called polyps. Coral reefs are the largest living structure on the planet and the only living structure visible from space. Corals play a crucial role in protecting our planet by providing food and habitat for hundreds of thousands, if not millions, of other species.