'Eaten Alive' -- Anaconda Used in Failed TV Stunt Was a RINGER

"Eaten Alive" pulled the wool over America's eyes because everyone expected to see a wild snake eat a guy alive -- but instead a captive snake failed to even try eating the show's host.

If you missed it ... naturalist Paul Rosolie said he was planning to be eaten alive by a 25-foot anaconda -- but most of the 4 million people who watched got pissed off because he never entered the snake's belly.

Things started going south when Rosolie couldn't capture a 25-foot snake in the wild of the Amazon. On the show they acknowledged bringing in a ringer -- a snake that had been in captivity.

Sources on the production tell us the stand-in snake turned out to be too small to get the job done -- as far as swallowing Paul -- but was still strong enough to rip his arm out of the socket. He eventually tapped out when that pain became too much to bear.

Last week, on TMZ Live Rosolie explained how he survived the pre-taped stunt ... by hinting the snake had puked him up -- but now it's obvious that was a lie.

For what it's worth ... Discovery says the safety of the snake, and Paul, were always the #1 priority -- and his intention was to be eaten alive.