The Cnidarians sent Hydra to Earth in the Water Carrier to investigate a signal they had received from an artefact they had left on Earth centuries before. Their intent was to conquer Earth.
The Water Carrier was a huge ship. Hydra sent jellyfish to Earth from the ship. Hydra brought the Seventh Doctor aboard by grabbing him with a large metal tentacle.
Inside, the ship was filled with water. Jellyfish of different sizes, colours and tentacles swam inside. The Doctor was given an air bubble while he was aboard. Hydra was defeated when Ly brought a pack of table salt aboard and opened it in the ship's fresh water. (PROSE: The Invertebrates of Doom)