The Spraxis Jelloids were a single-celled organisms from the binary quasar Bendalos. According to the Sixth Doctor, they were the longest lived species in the universe, with some biologists theorising they were in fact the first living beings ever evolved.
One Jelloid was guarding a the largest diamond in existence on the planet Generios 15 (one of the Three Great Treasures of Generios, under a 50 million year, non-negotiable contract). He swallowed the Doctor and helped him to retrieve the diamond. (AUDIO: The One Doctor)