Slythers looked like large, black, jellyfish-like creatures. Their skins were so thick that they were immune to the stings of the Varga plant. (COMIC: The Only Good Dalek) They moved slowly and regularly groaned. (TV: The Dalek Invasion of Earth)
Towards the end of the invasion, the Supreme Controller used a Slyther to guard the Bedfordshire mines at night, where it killed and ate anything it found. After it attacked a small group of humans consisting of Ian Chesterton, Larry Madison, Wells, and Ashton (killing the last), Ian hit it with a rock, causing it to fall down a pit to its death. (TV: The Dalek Invasion of Earth)
Behind the scenes
- Terry Nation's original script called for the Slyther to be "like a huge black jelly fish" that would remain largely off-screen and never be seen in its entirety.
- The Slyther costume was built by Shawcraft, the same company that built the Daleks. The studio's first version of the costume wasn't considered scary enough, so it was altered between the end of the fourth episode, "The End of Tomorrow", and the fifth episode, "The Waking Ally".
- The Slyther was operated by Nicholas Evans, who was also a Dalek operator for this story.
- The DVD release of The Dalek Invasion of Earth included an easter egg of a minisode featuring a Slyther's "adventures" called Sid's Date.
- The Slythers are seen by many to be the first Doctor Who "monster" — whilst other enemies had been mainly humanoid or robotic the Slyther was the first to attempt at a completely different form of adversary.
- The description of the Slyther in Terrance Dicks's novelisation of The Dalek Invasion of Earth as being "a vast lumpy blob of a body, powerful flailing tentacles, two tiny deep-set eyes shining with malice..." (quote, page 101) is actually that of a Mire Beast from TV: The Chase. It would appear that the BBC sent Dicks the wrong reference photograph.