Krux

 was a tourist to Death's Deal. He was a Mimmisite from the planet Seratesh. He was also professionally an arthopologist. He noted that the gastropods where moving again. When he and Donna Noble fell down a hole he used his bioluminescence to help them find the TARDIS. He saved Donna by encasing her within him to stop them both dying when the terraworm ate him. He made to the TARDIS and helped rescue the Tenth Doctor. (AUDIO: Death's Deal)