We are used to the idea of aging in ourselves.We are so used to this that it comes as a surprise to find that there may be some animals that do not age.Sea anemones are an example.Some have been kept for nearly a century without showing any signs of lifelessness.Some kinds of sea worms can even"grow backwards."If kept in the dark and given nothing to eat,they get steadily smaller,They finally end as a ball of cells looking rather like the egg from which they came.Under good conditions the ball will turn back to a worm and start growing again.One could probably keep them growing and un-growing again and again.