听力原文：W: Hi, Tom! M: Judy. I haven't seen you in weeks. Where have you been? W: In Florida. M: What vacationing! While the rest of us studying on the campus in February cold? W: Not exactly. I spent most of my time under water. M: I don't understand. W: I was on a special field trip. I went with my marine biology class. M: So you went scuba diving. What were you looking for? Sunken treasure? W: You might say so. The sea's full of treasures. All kinds of strange fascinating organisms. Our class concentrated on studying plankton. M: I found plankton were too small to be seen. W: That's a common misconception. The term plankton covers a wild variety of freely flowing plants and animals, from microscopic one cell organisms to larger ones, such as the common jellyfish. M: Jellyfish may be large enough to be seen. But they are transparent, aren't they? W: Yes, most planktons have transparent tissues as protected camouflage, it makes them practically invisible to predators. M: But not invisible to your biology class, I hope. W: By concentrating, I was able to see the outlines of lots of different plankton plants and animals. In fact, our professor even took photographs of gastropods, which are small oceanic snails. M: How would the snails show up in the photographs of their transparence? W: We scoured it with harmless green dye since particles of the dyes stuck to their tissues, the snails appeared in green outline in the photographs. M: That sounds like an interesting trip. But I think if I'd been in Florida in February, I'd much rather spend my time just swimming and lying in the sun. (20)