The animal kingdom is full of species that can regrow not only tissue, but entire limbs.
The best-known examples include lizards that regrow their tails and starfish that can produce new arms, but many other species are capable of remarkable regeneration.
Flatworms' ability to repair and regenerate truly amazes. Dice a flatworm any which way and each fragment will in time become a fully functional creature. How flatworms do this is not fully understood. But it’s a quality that has given them fame for being, in the words of 19th-century naturalist John Graham Dalyell, “immortal under the edge of a knife”.
The axolotl, for instance, is famed not only for its ability to regrow missing limbs and its tail, but also its jaw, its heart and parts of its brain. If paralysed, it can even redevelop new neurons and neurological connections, allowing it to walk again.
And then there's the Immortal jellyfish, along with at least five other jellyfish species, dodge death by hitting rewind. Even after a dead medusa has collapsed into a pile of mush, its cells can grow into polyps.
It's like a fragment of butterfly wing turning into a caterpillar.
Immortal jellyfish can still die, from predation and disease, but their regenerating abilities make them tough and successful.
The prize for regeneration, though, must go to the Hydractinia – tiny, predatory animals related to jellyfish and corals.
Also known as ‘snail hair’, they are nibbled by passing fish, with their tentacled heads the usual target. Losing their heads isn’t a problem, though – the animals simply grow new ones.
The key to Hydractinia’s powers of regeneration is that they retain their embryonic stem cells for life. These cells can be ‘called up’ when required to reform new parts of the body.