用括号所给词的适当形式填空
We are _______(use) to the idea of _______(age) in ourselves. We are so used to this that it makes us _______(surprise) to find that there may be  some animals that do not age. Sea anemones（海葵）are an example. _______(keep) for nearly a century, some of them don't show any signs of lifelessness. Some kinds of sea worms can even “grow back-wards.” If          _______(keep) in the dark and given nothing _______(eat), they get' steadily smaller.hey finally end as a ball of cells _______(look)rather like the egg from which they came. Under _______(give)  conditions, the ball will turn back to a worm and start _______( grow)  again, One could probably keep them _______(grow) and un-growing again and again.