@auxanges I asked myself the same question, but no rhyme or reason was visible as far as I (not an anatomist!) was concerned.
There are surely criteria I wouldn't think of since I'm not a curator. But some very *historically* important specimens that I spotted weren't under glass. E.g., the skeleton of this rhinoceros, which is important enough to have its own Wikipedia page (and if you read French, click over to the FR version, which is *immense*): https://en.wikipedia.org/wiki/Rhinoceros_of_Versailles
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