Jellyfish (also known as jellies or sea jellies or medusozoa) are free-swimming members of the phylum Cnidaria. Jellyfish are found in every ocean, from the surface to the deep sea. Jellyfish are composed of more than 90% water; most of their umbrella mass is a gelatinous material — the jelly — called mesoglea which is surrounded by two layers of epithelial cells which form the umbrella (top surface) and subumbrella (bottom surface) of the bell, or body (wikipedia).