One day two young men went to the forest. On the way, one said t             the other, “We’re g           friends. We must help each o         ” “All right!” the other one answered. After a short t         , there was a great noise. It was a big bear. The two men ran

a            quickly. One of them got up into a tree. The other was fat and couldn’t

c           up. He threw himself at the foot of the tree. The bear came near. It looked at the fat man, put i           nose down and smelt him. It thought the young man was dead, so it walked away. The man in the tree came down. He asked his friend, “The bear put its mouth so near to your ear. W         did it say to you?” The fat young man answered, “the bear said, ‘A friend in need is a friend indeed.’ ”

生活史中经过无性生殖和有性生殖两个阶段世代交替的寄生虫有。