They look similar to Human-sized slugs with short arms, wrinkled faces and antennae; and leave a trail of slime behind them as they move. They walk with an erect posture, despite having no legs; similar in some ways to the Alpha Centaurans and the Wirrns. The Gastropod voice is extremely deep and guttural, and they can also communicate telepathically. They (or at least the Gastropod leader) are able to flash a deadly beam of energy from their eyes, and also to assume control of other creatures' bodies and communicate through them.
The species is led by a single monarch, and travel from planet to planet devouring its vegetable resources and enslaving the native species. Before leaving to their next destination, the Gastropods lay hundreds of eggs which are capable of surviving in space vacuum for extended periods; and use advanced technology to cause the planet's star to go supernova, and carry the eggs out in space to the next unfortunate world.
The Gastropod's psychic abilities are powerful enough to control even the mind of an experienced Time Lord; a feat worth of note since the Time Lords themselves are telepaths and presumably resistant to mind control techniques.
- Doctor Who, season 21 (sixth Doctor), "The Twin Dilemma" (1984)