Everyone likely knows the story of Jonah and the whale, a tall tale even for the Old Testament: no human will make it out of a whale alive. But while humans make for a relatively frail species, many critters possess a bit more moxie than mankind, including a number of animals can survive being eaten alive. Some of these survive by regenerating missing parts of their bodies, and others just pass through the entire predator animal unscathed. These animals eaten alive might be emotionally scarred for life perhaps, but at least they can live another day.
Predators-prey relationships existed practically since life emerged on Earth, so it makes sense at some point certain species would evolve to survive the process. While never fun for these species to be eaten alive, this survival adaption often has researchers wondering how humans might use it for their own medical research. It's true that there are many amazing and incredibly hardy animal species out there, but when it comes to never giving up, these animals may be the best. After all, would you not despair if you were in the belly of a whale?