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This experiment does not prove that the paramecium felt pain. It shows that they respond to light and can associate that stimulus with another (the pin).
Even the parameciums (tiny flatworms that live in ponds) can feel pain. They did an experiment where they had one and jabbed it with a pin at the same time they shone a bright light on it and it learned to flinch whenever the bright light hit it.
If there was no pain, then why the flinch?