In other words, it means everything in nature deserves to be studied and admired, whether it’s something low and dirty like a worm, or something beautiful like a leaf dancing in the wind. Can I ask why you are reading this book? Is it required in school? This writing style is frankly unnatural and not representative of the English language written and spoken by natives.

Basically: “All of nature is beautiful and interesting.”

"but calls" = "but does not call" Omitting (for simplicity) the the subordinate phrases, the sentence becomes "There is not a worm nor a leaf but calls for study and admiration", or logically, "There exists no worm or leaf that does not call for study and admiration." This means exactly the same as "Every worm and every leaf calls for study and admiration." Speaking mathematically, the logical structure is this: the sentence "There exists no X or Y such that NOT Z" is logically equivalent to "For all X and Y, Z." Note that "nor" and "or" are usually not really different words. We use "nor" rather than "or" for negative emphasis and clarity, not to change the meaning. It works best when paired with "neither" or "not".