Another flatworms-are-fun story: If you irradiate a flatworm using a nuclear reactor, to kill off its stem-cell equivalents, it sort of "rots" as other cells eventually die and aren't being replaced. If after zap, you insert donor stems, its life can instead proceed normally, and the entire body is eventually refreshed. So ship of Theseus like, you have an individual flatworm, swimming through its life, which ends up being genetically unrelated to its earlier self.