The planet Spiridon in the ninth system is populated by a host of dangerous creatures which live in the treacherous jungle - a fungoid organism that squirts spores which infect and quickly spread over any lifeform on which they land, eye-plants which track movement in the jungle, a tentacled plant which creeps toward and then snags its food, and some kind of screeching beast that flies. All in all, the vegetation is more animal-like than it is plant-like. Spiridon itself is covered in ice volcanoes which periodically erupt, releasing the liquid ice that forms the core of the planet. The climate of Spiridon is tropical by day, but sub-freezing at night.
The only intelligent lifeform of the planet are known as Spiridons. They are humanoid, but with one important distinction: they are invisible. The suggestion is that they emit an anti-reflecting light wave until the moment of their death. They breathe heavily, and wear luxurious purple furs that cover their whole body to protect them from the cold. Physically, they have a flared nose, two large forehead ridges that sprout from the bridge of their nose in either direction to the side of their forehead, and their feet are leaf-shaped.
When the Daleks came to Spiridon, they bombarded the planet with a deadly kind of bacteria. Only a handful of Spiridons survived, and these were subjugated by the Daleks, but a few tried to fight back. Their attempts apparently failed until the Doctor arrived on the planet and, with the help of a friendly Spiridon named Wester who had helped Jo recover from a fungal infection, ousted the Dalek army that was on their planet.