Jellyfish (also known as jellies or sea jellies) are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa (over 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa (about 1000-1500 species that make jellyfish and many more that do not)

Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, are also found in fresh water and are less than half an inch in size.

You can see here some stunning photos of glowing jellyfish. Photos were taken in Osaka Aquarium, in Japan.