Coelenterates are divided into three main classes: hydrozoans, scyphozoans and anthozoans. In hydrozoans the polypoid form predominates and examples are hydras, by-the-wind- sailors and Obelia. In scyphozoans the main phase is the medusoid and the best known example is the common jellyfish (Aurelia). In anthozoans there is only the polypoid form and corals and sea anemones are notable in this group.