Once upon a time, in a far-off digestive system, all of Man's organs lived peacefully together. Mouth was always willing to turn Man's food into nicely packaged boluses for Pharynx and Esophagus to deliver. Stomach would cook up a nice pot of gastric juice with just a dash of extra hydrchloric acid to turn the boluses into a heaping helping of chyme for Pancreas and Liver to play with. When they were done, they would wish the chyme a happy journey travelling with Small Intestine (packing it some extra bile and trypsinogen for the road). Duodenum, Jejunum, and Ileum would always meet the chyme with open villi before sending it on its way. After ascending, transversing, and descending Large Intestine, the chyme turned into feces and settled down with Rectum, who never complained. All of Man's organs did their jobs day in and day out. Neither rain nor snow nor hepatitis could stop them from completing their rounds, even when the respiratory system would throw wild parties. Life was simple, and they were all happy.
All, that is, except for Appendix. Living in squalor in one of the less friendly parts of Man's colon, Appendix would sit around all day moaning and groaning that it had nothing to do. "It's unfair!" bemoaned Appendix. "All day long I hang off of this stupid cecum and listen to all the dumb conversations it has with Bauhin's valve, and I can't do anything about it! All I can digest is cellulose, and Man hardly ever eats leaves anymore! Plus, all these freeloading bacteria keep camping out near my appendicular arteries and stinking up the place! The fact is, there's nothing for me to do around here anymore, and I'm sick of it. I'm catching the next scalpel outta here, and trying my luck in the real world."
And so, Appendix made his departure from Man. "Surely," Appendix said to himself, "there must be some place out here where I can find a home."
First, Appendix tried settling down in the digestive system of Octopus. He had recalled hearing somewhere about the health benefits of living in an organism with gills, and thought it was worth a shot. Try as he might, however, Appendix couldn't get any sleep with three Hearts pumping blood day in and day out. Branchial or not, those guys never kept quiet.
After that, Appendix did a short stint in Horseshoe Crab's digestive system. Almost immediately, however, he grew sick of it. Not only did the open circulatory system force him to live ridiculously close to his neighbors, the whole idea of hemolymph unnerved him. Appendix was no bigot, being all for free expression and whatnot, but something that functioned as blood and interstital fluid just seemed unnatural to him.
Following that, Appendix set up shop in a library and offered supplementary material to those who were looking for it. In the end, though, this just resulted in a lot of confusion and jealousy.
Finally, exhausted and depressed, Appendix returned to Man's digestive system. "If I'm going to be useless wherever I go," he sighed, "it might as well be at home."
No sooner had Appendix set foot in his old abdominal cavity than he found everything in chaos. "What in the name of hemoglobin is going on here?" he yelled.
Large Intestine, who looked like he might have started crenating any second, spoke up. "It's awful, Appendix! The day you left, a whole bunch of Clostridium difficile bacteria showed up and starting trashing the place. We haven't had it this bad since Man ate that knish!"
"Can't you just flush out the bacteria like you always do when this thing happens?" Appendix asked.
"We'd have to get rid of all the good bacteria, too!" Large Intestine moaned. "There's no other place for them to go!"
Appendix, overcome with a sudden wave of determination, firmly spread his arteries as far as he could stretch them. "Not on my watch, there isn't." As soon as he was in place, gut flora from all over the system started pouring in. Pretty soon, Appendix was hosting so many different kinds of positive bacteria that it would make yogurt jealous. "Now flush!" Appendix shouted.
With a mighty roar, all of the Clostridium difficile were wiped clean from the digestive system. The gut flora, holding on tightly to Appendix, were able to keep from being washed away. As soon as everything quieted down, they were released back into the intestines, safe and sound.
"Three cheers for Appendix!" applauded all of Man's organs. Even Gallbladder, who usually kept to himself, gave Appendix a hearty congratulations. From that day on, Appendix served as a home for all wayward gut flora who needed a place to hide from a bad bout of enteritis or diarrhea, and never felt vestigial again.