Corals are a group of marine organisms related to sea anemones and sea jellies. These groups of soft-bodied animals make up the phylum Cnidaria. A primary characteristic of all cnidarians is that they have tentacles with stinging cells called nematocysts. Corals are sessile organisms, meaning that they attach to a substrate or base. Most species of corals are colonial, though some are solitary. One coral colony is made up of anywhere from several to thousands of individuals known as polyps.