Gill-plate

The gill-plates were an apparent part of Skelloid anatomy, which they used to absorb hydrogen. Banto Zame, posing as the Doctor, seemingly foiled an invasion of the Generios system by the Skelloids by exploiting their life cycle's reliance, using his psychic screwdriver to invert the core of their hydrogen stabilisers. In reality, the Skelloid invasion was nothing but a mere holographic illusion created by him. (AUDIO: The One Doctor)

It is unclear whether or not the Skelloids existed beyond Banto's illusion.