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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. ( 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. (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. 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. (The Twin Dilemma);
According to The Brilliant Book 2012, a book that contains non-narrative-based information, there was a fossilised Gastropod in Dr Henry Fizzog’s Museum of the Macabre on Sardicktown.
The name for this monster obviously derives from Gastropods, the biological class of slugs and snails.