Jellyfish (or medusae) were some of the first multicellular invertebrates that had evolved by the Middle Cambrian period (550 MYA), related to corals, sea anemones and freshwater hydra polyps.
Unlike them, however, jellyfish lived a pelagic, free-swimming lifestyle, but otherwise they had similar body plan - a geometrically circular shape, centrally dominated by the digestive (gastric) tract and that possesses tentacles armed with poisonous and stinging cells (though it is different with some modern species), with which they capture their prey, other free-swimming saltwater species, including each fish fry as well as each other.
In the seriesEdit
In Walking with Monsters ep.1, Impossible Pictures demonstrate a species of jellyfish swimming through the water as some of the first animals to have ever evolved. Considering that during their last 500 millions of years of evolution these invertebrates had barely changed (though they have diversified somewhat), their appearance in the first part of WWM seems quite appropriate.