Sectom, to make way for real gastropods that are referred to in The Ark in Space.
Talk about it here.
The Gastropods resembled humanoid slugs. They left a sticky trail behind them when they moved, and laid eggs which could remain fertile in space. Apparently only the leader, Mestor, could communicate verbally; the rest obeyed his commands without question. (TV: The Twin Dilemma)
Gastropods had a form of mental powers which they could use to either kill individuals or take over their mind and communicate through them. They could also communicate telepathically. Their leader Mestor managed to open the doors of the Doctor's TARDIS through its psychic power. (TV: The Twin Dilemma)
Jaconda, once a verdant paradise, became a despoiled wasteland once the Gastropod appeared and made the Jacondans slaves in their own world. In 2310, their leader, Mestor, planned to make the Jaconda's sun go supernova so as to scatter Gastropod eggs into space. The Sixth Doctor killed Mestor and ended the reign of the Gastropods. (TV: The Twin Dilemma)