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Cnidarians were a jellyfish-like species.
Early in Earth's history, the Cnidarians left an artefact in a watery cave. Thousands of years later, it was discovered by a group of university students. The artefact absorbed their life forces, and a signal was received on the Cnidarian home world. Commander Hydra Sowerbii was sent to Earth in his ship, the Water Carrier, to conquer Earth. He was defeated, not only by the Seventh Doctor and Mel, but also by Earth's geography: the Cnidarians were fresh water jellyfish. (PROSE: The Invertebrates of Doom)