Actually Gordon Swissknife is amazingly right!
It is true that while colonial animals (corals etc.) can have their immortality, solitary individuals are doomed to die. Jellyfish usually have a complex life cycle, wherein a colonial stage leads to the sexually mature, solitary, adult stage. Eggs and sperm from solitary, sexual, adult medusa (jellyfish) develop into an embryo and planula larva, and they then form the colonial polyp stage. Medusa is formed asexually from polyps. This medusa have a limited lifespan and die shortly after releasing their gametes.
But the hydrozoan Turritopsis nutricula has evolved a remarkable variation on this theme, and in so doing appears to have achieved immortality. The solitary medusa of this species can revert to its polyp stage after becoming sexually, thus, it is possible that organismic death does not occur in this species!How does Turritopsis accomplish this feat? It can do this because it can alter the differentiated state of a cell, transforming it into another cell type. Such a phenomenon is called transdifferentiation, and it is usually seen only when parts of an organ regenerate. However, it appears to occur normally in the Turritopsis life cycle. In this transdifferentiation process, the medusa is transformed into the stolons and polyps of a hydroid colony. Now I do not claim to now much biology but apparently this animal is some what immortal – look at this site (from which I took the material) and have a look for yourself. http://8e.devbio.com/preview_article.php?ch=2&id=6