Memory worm

A Memory worm is an alien worm that can remove the memories of those it touches.

Biology
A Memory worm is covered with a substance that can remove an hour's memory from any being who touches it. The substance is also in the worm's fangs; If a worm bites an individual, they could lose years worth of memories.

History
A Memory worm was in the possession of the Doctor in London, 1892. He intended to use it to wipe Clara Oswin Oswald's memory of meeting him. However, his cohort Strax neglected to wear protective gauntlets while handling the worm, losing both his memory and the worm.

The Memory worm was later recovered and used by the Doctor as part of a trap; The Doctor hid the worm inside a modern lunchbox, and gave it to Walter Simeon. The worm bit Simeon, causing him to lose his entire life's memories.

Species