What are the main classes into which the phylum is divided? What are some examples of each and in which form (polyp or medusae) are they found?
Coelenterates are separated into three main classes: scyphozoans, hydrozoans, and anthozoans. In hydrozoans the polypoid form predominates and like hydras, by-the-wind-sailors and Obelia. In scyphozoans the main phase is the medusoid and the best known instance is the common jellyfish (Aurelia).