You may find it odd that my first post on this blog dedicated primarily to insects is about something that is most decidedly NOT an insect. However, I think anyone reading about biology will find the immortal jellyfish (Turritopsis nutricula) extremely interesting. Their common name is no hyperbole – they are the only example that this writer knows of true biological immortality. The jellyfish starts life as a “polyp” kind of a larva that lives on the ocean floor and develops. It then transforms into an adult jellyfish. This is completely normal in many species of jellyfish but after this one reproduces, it goes back to it’s polyp stage via “transdifferation”. This is when a non-stem cell turns into a different type of cell. This process could go on indefinitely if not for predation and other factors. One could speculate as to why exactly these animals perform this amazing biological feat. I don’t know that answer this one can only ponder it.
here is some more info http://en.wikipedia.org/wiki/Turritopsis_nutricula