阅读理解。Mostanimalsonlyhaveanimals

     Most animals only have animals of a different kind for food. But sometimes two kinds of animals come
  together in a partnership (伙伴关系) which is good for them. You may have noticed some birds on the
 backs of sheep. This is not because they want a ride, but because they find easy food in the parasites (寄
 生虫) on sheep. The sheep let the birds do so because they can stop the parasites from troubling them.
 So though they can do with it by themselves, they can do better together with each other.
     Sometimes an animal has a plant partner. The relationship develops until the two partners cannot do
 without each other. This is so in corals (珊瑚) of the sea. Inside their bodies they have very small plants,
  which act as "cleaners", taking the useless things from the coral and giving oxygen in return. That is what
 the animal needs to live. If the plants are killed, or are even kept from receiving light so that they cannot
 live as usual, the corals will die.