Jellyfish are not actually fish, but rather invertebrates in the phylum Coelenterata, which also includes the hydras, corals, and sea anemones. As with the other coelenterates, jellyfish feature radial symmetry and specialized stinging cells known as nematocysts. Although almost completely marine, there are some freshwater species of jellyfish that occasionally bloom in reservoirs and lakes. Worldwide, there are about 200 different species of jellyfish that have been described. From the tiny spherical thimble jellyfish of the Caribbean Sea to the largest, the Arctic lion's mane with tentacles that stretch over 100 feet in length, the basic body plans for jellyfish (a bell, tentacles and a mouth) are similar. With over 95 percent of the typical jellyfish composed of water, they do not feature hearts, blood, brains, or gills.