Yahoo! Green recently published a piece about the turritopsis nutricula species of jellyfish, which may be the only immortal animal.
“Since it is capable of cycling from a mature adult stage to an immature polyp stage and back again, there may be no natural limit to its life span. Scientists say the hydrozoan jellyfish is the only known animal that can repeatedly turn back the hands of time and revert to its polyp state (its first stage of life).”
You can read the article here.