Okay, brace yourself because it sounds worse than it is.
Right up front, know that it won't damage you or the spinach. But do brush it off & leave it in the garden.

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- Ready yet?
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Okay here it is.

Common name is horse-hair worm. It's a predatory nematode. One end can "stand up" and "look" at you! Even though it lacks eyes, it does have a light-sensing organ that detects light & dark.

I just saw one of these today on one of my plants and was pretty creeped out by it. I'm glad to see it's nothing to worry about.

You are just amazing! In 2 1/2 hours I now know what my little beastie is. Jean, thanks so much. And, I'm glad I don't have to worry about it.