This volume presents a broad panorama of the current status of research of invertebrate animals considered belonging to the phylum Cnidaria, such as hydra, jellyfish, sea anemone, and coral. In this book the Cnidarians are traced from the Earth’s primordial oceans, to their response to the warming and acidifying oceans. Due to the role of corals in the carbon and calcium cycles, various aspects of cnidarian calcification are discussed. The relation of the Cnidaria with Mankind is approached, in accordance with the Editors’ philosophy of bridging the artificial schism between science, arts and Humanities. Cnidarians' encounters with humans result in a broad spectrum of medical emergencies that are reviewed. The final section of the volume is devoted to the role of Hydra and Medusa in mythology and art.