Commander Hydra Sowerbii was a Cnidarian. His people had left an artefact on Earth several thousand years ago, and when they received a signal from it, he was sent to Earth in his ship, the Water Carrier.

When Hydra reached Earth, he sent his jellyfish soldiers out of the ship. Unfortunately for him, his soldiers were, as the Seventh Doctor pointed out, landing on dry land. Hydra sent his jellyfish into the nearby lake, and brought the Doctor aboard his ship. The Doctor realised the Cnidarians were fresh water, and told Hydra that Earth's seas are salt water.

Back on Earth, Mel worked out that the Cnidarians were vulnerable to salt. She sent Ly, whose life force had been absorbed by the artefact, aboard the Water Carrier with a pack of table salt. Ly opened it into the water of the ship, and Hydra began to choke. The Cnidarians returned home. (PROSE: The Invertebrates of Doom)