The Mutant Spiders

From Clifford Pickover's "Wonders of Numbers":

"Three spiders named Mr. Eight, Mr. Nine, and Mr. Ten are crawling on a Peruvian jungle floor. One spider has 8 legs; one spider has 9 legs; one spider has 10 legs. All of them are usually quite happy and enjoy the diversity of animals with whom they share the jungle. Today, however, the hot weather is giving them bad tempers.
"I think it is interesting," says Mr. Ten, "that none of us have the same number of legs that our names would suggest."
"Who the heck cares?" replies the spider with 9 legs.
How many legs does Mr. Nine have? Amazingly, it is possible to determine the answer, despite the little information given."

...and the answer (and only correct answer) is:
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Mr. Nine has 10 legs