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第1段
1 .<-NARRATOR:-> Listen to part of a lecture in a biology class.
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第2段
1 .<-FEMALE PROFESSOR:-> So, OK, we've been talking about frogs...
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2 .and like all amphibians,
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3 .frogs have thin skin,
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4 .which means that they lose moisture through their skin easily.
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5 .Now typically we think of frogs as living in wet environments,
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6 .but for frogs who live in dry places with desert-like conditions,
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7 .this can be a problem.
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8 .Frogs have been able to survive in such areas by having different physical features,
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9 .special dry-climate features
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10 .that help them maintain an adequate level of moisture in their cells and avoid drying out.
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第3段
1 .Some frogs do this by preventing water loss through their skin.
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2 .By creating a sort of covering over their skin,
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3 .they greatly reduce their skin's exposure to the dry air.
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4 .The covering acts like a barrier that locks in moisture.
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5 .For example, some frogs secrete a substance through their skin...
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6 .a fatty substance that they rub all over their skin using their hands and feet,
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7 .which creates a waxy layer all around their bodies that's almost completely watertight.
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第4段
1 .Other frogs maintain an adequate level of moisture through a different physical feature...
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2 .one that allows them to store water inside their bodies for later use.
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3 .A specially modified internal organ inside their bodies
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4 .enables them to have a high water-storage capacity.
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5 .So the frogs are able to absorb and store moisture during wet, rainy times,
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6 .which they can rely on to get through dry periods.
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7 .The aptly named water-holding frog,
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8 .for example, has a bladder that is highly elastic and stretchable.
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9 .When it does rain,
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10 .the frog absorbs water through its skin,
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11 .and its bladder stretches to hold this extra water.
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12 .The water is then slowly released from the special bladder
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13 .into the frog's internal tissues until the next rain,
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14 .which might not be for several months.
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