Turritopsis nutricula, the Immortal Jellyfish

Turritopsis nutricula is a species of jellyfish that is like other jellyfish in that they swim about and look weird and all, but are unique in that they're biologically immortal. After one of these jellyfish grow up into a sexually mature medusa with families and minivans and RNC memberships, they go bloop-bloop-bloop and turn into a little baby polyp all over again, and have to learn to smoke reefer out of a soda can all over again. They are not immortal in the old-fashioned cool way; they can still get eaten or hit by a bus or whatever, but failing that, they don't die so often. It's hard for scientists to check, seeing as scientists don't live forever, themselves, so all anybody can say for sure is that these jellyfish live for a while in captivity.

BONUS IMMORTALITY INFORMATION:
AlexChiu.com

Alex Chiu is a guy who sells immortality bracelets. His website also answers a couple of big questions ("What makes animals crawl, swim, or walk. Why can humans think. Why are there male and female. Must read"). He also describes his vision of world peace, which involves a nuclear war between China and the United States of America. "Only Alex Chiu has the solution to a unified world," he writes, "unless you can think of a better one."