The Yulians were the native inhabitants of Yula. A species of psychic snailmen, the Yulians possessed a Communal Will, to which each individual fed their psyche and in turn fed from the psyches of the others. They were peaceful and very hospitable, and welcomed all visitors to their world.
At the time the Fourth Doctor and Sarah Jane Smith visited Yula, the Communal Will, which was hundreds of thousands of years old, was beginning to destabilise, a result of a psychic attack from space. The Doctor traced the source to the planet Crellium, where he defeated Krem-ling's plan to feed off the Yulians' minds and take over their bodies. Afterwards, the Doctor noticed a change in the Communal Will, a new sense of urgency and purpose. (PROSE: The Vampires of Crellium)