One of the most difficult things for a human to face is the loss of a limb. If a person loses an arm or a leg, he/she must be fitted with an artificial limb.
The situation is very different for a starfish. If a starfish loses an arm, it can grow a new one. 31 Snails can even regrow their heads—imagine what the world would be like if humans could do that. But we can’t. Nor can we grow new limbs or even fingers. That’s why scientists are studying animals that can regrow body parts, that is, regenerate. 32
Many different kinds of animals show some form of regeneration. Most of them are, however, limited to the sort a lizard is capable of, like regrowing a lost tail. A cockroach can grow back a missing limb, but the limb itself can’t generate a new cockroach. 33 Bidirectional regeneration, on the other hand, refers to a situation in which splitting of an animal will result in separate fully functional animals. 34 Cut a hydra in half, and you’ll get two hydras. Cut it into four pieces, and you’ll get four.
35 A single one can be cut into hundreds of pieces and each will grow back into a whole in a week or so. Because of this remarkable ability, one planarian can be created over and over, giving it a sort of immortality. Whether this phenomenon can be achieved in humans will likely require years of research.
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