听力题 W: Wasn’t that lecture interesting? Of all sea animals, the simple jellyfish, with its clear plastic membrane and strange bell-shaped body, has always intrigued me. M: Professor Ladd sure knows what he’s talking about, but I don’t understand why he said that jellyfish can’t really swim. He carefully explained its distinctive method of locomotion. W: Yes, and it’s fascinating. When the jellyfish expands its bell, water rushes in. When it contracts its bell, the water is expelled in one direction, and the jellyfish moves in the other. M: It’s really unique in the way it uses the principle of jet propulsion to move. But isn’t that swimming? W: Perhaps, but I suppose not in the same sense as fish do. Fish propel themselves with their tails to go wherever they want. You see, a jellyfish can only move upward. Then, as it begins to sink again, it poisons the tiny creatures it uses for food with its long string like tentacles that dangle beneath it. M: Oh, so it can’t swim just anywhere it wants. W: Not at all. A jellyfish is taken wherever tides and currents carry it.