托福听力TPO17听力文本(Conversation+Lecture)

　　TPO17 Lecture4

　　Biology(Octopus)

　　Narrator

　　Listen to part of a lecture in a biology class.

　　Professor

　　Ok, now I want to talk about an animal that has a fascinating set of defense mechanisms. And that’s the octopus, one of the unusual creatures that live in the sea. The octopus is prey to many species, including humans, so how does it escape its predators? Well, let me back up here a second. Anyone ever heard of Proteous? Proteus was a God in Greek mythology who could change form. He could make himself look like a lion or a stone or a tree, anything you wanted, and he could go through a whole series of changes very quickly. Well, the octopus is the real world version of Proteus. Just like Proteus, the octopus can go through all kinds of incredible transformations. And it does this in three ways: by changing color, by changing its texture, and by changing its size and shape. For me, the most fascinating transformation is when it changes its color. It’s a normal skin color, the one it generally presents, is either red or brown or even grey, and it ’s speckled with dark spots. But when it wants to blend in with its environment to hide from its enemies, it can take on the color of its immediate surroundings: the ocean floor, a rock, a piece of coral, whatever. Charles?

　　Student

　　Do we know how that works, I mean, how they change colors?

　　Professor

　　Well, we know that the reaction that takes place is not chemical in nature. The color changes are executed by two different kinds of cells in the octopus ’ skin, mainly by color cells on the skin’s surface call chromatophores. Chromatophores consist of tiny sacks filled with color dye. There might be a couple hundred of these color sacks per square millimeter of the octopus ’ skin, and depending on the species, they can come in as many as five different colors. Each one of these sacks is controlled by muscles. If the muscles are relaxed, the sack shrinks, and all you see is a little white point. But if the muscle’s contract, then the sack expands, and you can see the colors. And by expanding different combinations

　　Student

　　And just with various combinations of those five colors, they can recreate any color in their environment?

　　Professor

　　Well, they can no doubt create a lot with just those five colors, but you are right, maybe they can’t mimic every color around them, so that’s where the second kind of cell comes in. Just below the chromatophores is a layer of cells that reflect light from the environment, and these cells help the octopus create a precise match with the colors that surround them. The colors from the color sacks are supplemented with colors that are reflected from the environment, and that ’s how they are able to mimic colors with such precision. So, that’s how octopus mimic colors. But they don’t just mimic the colors in their environment; they can alos mimic the texture of objects in their environment. They have these little projections on their skin that allow them to resemble various textures. The projections are called papillae. If the octopus wants to have a rough texture, it raises the papillae. If it wants to have a smooth texture, it flattens out the papillae, so it can acquire a smooth texture to blend in with the sandy bottom of the sea. So the octopus has the ability to mimic both the color and the texture of its environment. And it’s truly amazing how well it can blend in with its surroundings. You can easily swim within a few feet of an octopus and never see it.

　　Student

　　I read that they often hide from predators by squirting out a cloud of ink, or something like that.

　　Professor

　　Yes. The octopus can release a cloud of ink if it feels threatened. But it doesn’t hide behind it, as is generally believed. Um, the ink cloud is ... it serves to distract a predator while the octopus makes its escape. Um, now there’s a third way that octopus can transform themselves to blend in withor mimic their environment, and that’s by changing their shape and size, well, at least their apparent size. The muscular system of the octopus enables it to be very flexible to assume all sorts of shapes and postures. So it can contract into the shape of a little round stone, and sit perfectly still on the seafloor. Or it can nestle up in the middle of a plant and take the shape of one of the leaves. Even Proteus would be impressed, I think.