Sites I have bookmarked, but don’t know why – Episode 567:

http://en.wikipedia.org/wiki/Turritopsis_nutricula

Turritopsis nutricula, the immortal jellyfish, is a hydrozoan whose medusa, or jellyfish, form can revert to the polyp stage after becoming sexually mature. It is the only known case of an animal capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage.

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