听力原文： Most animals have little connection with animals of a different kind, / unless they hunt them for food. / Sometimes, however, two kinds of animals come together in a partnership / which docs good to both of them. I You may have noticed some birds sitting on the backs of sheep. /This is not because they want a ride, / but because they find easy food in the parasites on sheep. / The sheep allow the birds to do so/ because they remove the cause of discomfort. / So although they can manage without each other, they do better together. / Sometimes an animal has a plant partner. / The relationship develops until the two partners cannot manage without each other. / This is so in the corals of the sea. / In their skins they have tiny plants which act as" dustman", / taking some of the waste products from the corals / and giving in return oxygen which the animal needs to breathe. / If the plants are killed, / or are even prevented from light so that they cannot live normally, / the corals will die.
(1)