Turritopsis nutricula
From Tardis Wiki, the free Doctor Who reference
Turritopsis nutricula was a breed of jellyfish that was considered to be immortal. Its cells underwent transdifferentiation, a process in which it could revert to a younger stage and grow old again an infinite amount of times. Olivia Colasanto suggested that there might have been jellyfish out there that have lived for thousands and thousands of years. (TV: End of the Road)
