Turritopsis nutricula is a hydrozoan (jellyfish) with a life cycle in which it reverts back to the polyp stage after becoming sexually mature. It is the only known case of a metazoan (animal) capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage (Piraino et al. 1996, p. 302). It does this through the cell development process of transdifferentiation. Theoretically, this cycle can repeat indefinitely, rendering it effectively immortal.