worms within the lion's body
師子身中の虫; (Jpn shishi-shinchu-no-mushi )
An analogy for those who, despite being followers of Buddhism, destroy its teachings, just as worms born from the carcass of the lion devour the lion.
This analogy appears in the Benevolent Kings Sutra, the Brahma's Net Sutra, the Lotus-like Face Sutra, and elsewhere.
It is intended to point out that members of the Buddhist Order, rather than non-Buddhists, are capable of destroying Buddhism.
For example, the Benevolent Kings Sutra says that the upholders of the three treasures of Buddhism, and not non-Buddhists, will become the destroyers of the three treasures, just as worms within the lion's body devour the lion.
The Lotus-like Face Sutra tells that, though no other creature ventures to eat the flesh of a dead lion, the worms born from the lion's body devour it;
likewise, though the Buddha's teachings cannot be destroyed by outside forces, evil monks who exist within "the body" of the Buddha's teachings can destroy them.