背景介紹：
水中的魚要不要喝水呢？也許你會說，在水里，魚的嘴巴常常一開一合，那當然是在喝水。其實，那是為了讓水快速流過它的鰓，以便吸取水中的氧氣，放出體內的二氧化碳。
事實表明，生活在淡水中的魚，終生都不喝水；而生活在海水中的魚，卻經常需要喝水。這是為什么呢？
這里，我們不妨從滲透壓談起。滲透壓是由溶解在水里的物質形成的；溶解在水中的物質越多，滲透壓也就越高。魚的血液和組織液中含有很多鹽和蛋白質，溶解濃度要比周圍的淡水高，滲透壓也就比淡水大。由于水是從滲透壓低的溶液向滲透壓高的溶液滲透的，因而周圍的淡水會從四面八方滲入淡水魚的體內。結果，這些淡水魚不但不必喝水，還得設法把多余的水份排出體外，不然它們就會被水活活漲死。相比之下，海水中的魚就不同了。海水中的含鹽量要比這些魚體液中的含鹽量高得多，海水中的滲透壓也比魚體內大得多。大海拼命地吮吸著它們體內的水份，使這些魚不得不經常喝水。海水魚喝水的方式是：水份先由口進入腸內，通過腸壁的滲透作用再進入體內，增加體液中的水份。
是不是所有的海水魚都需要喝水呢？也不是。鯊魚和鰩魚，是用另外的方式適應環(huán)境的：它們在體液中保留了尿素。尿素是一種有害的物質，要是別的動物會很快地把它排出體外。為了保存尿素，鯊魚和鰩魚的鰓用一種特殊的粘膜遮蓋起來，使尿素不會滲出。這樣，它們體內的滲透壓就要比海水的滲透壓高得多，它們便能象淡水魚攝取淡水那樣，從大海中得到水。同時，它們也會把多余的水分排出體外。
無論是大多數海水魚的喝水，還是淡水魚和鯊魚、鰩魚的不喝水，都是對環(huán)境的一種適應。
英文原文：
Fish live in water, but do they drink it? The answer is yes, but how fish drink water depends on where they live.
Water gets into a fish's body through osmosis, the process in which water diffuses from a higher to a lower concentration. For example, if there is more water outside of a cell than inside, water will try to flow into the cell until there is the same concentration of water on either side of the cell's membrane. The body of a fish acts the same way, either absorbing or losing water depending on its surroundings.
Whether a fish absorbs or loses water is based on the fact that all fish must maintain a certain amount of salt in their bodies to stay healthy. Fish that live in fresh water have a higher concentration of salt in their bodies than the surrounding water. Consequently, water continuously flows into the fish's body to attempt to dilute the amount of salt in the fish until it is equal to the amount of salt in the surrounding water. Since fish cannot allow their salt content to be diminished, their kidneys work overtime to expel excess water in the form of urine.
Ocean fish have the opposite problem. Surrounded by salt water, their bodies contain a relatively lower concentration of salt than the ocean water. In this case osmosis causes the fish to constantly lose water in order to equalize salt concentration inside and outside the fish. to partially compensate for the water loss, ocean fish actually drink water through their mouths. And to get rid of the excess salt they take in by drinking seawater, they excrete some salt through cells in their gills.
詞匯講解：
1.depend on依賴(依靠, 取決于, 隨...而定)
如： A lot will depend on how she responds to the challenge.
在很大程度上將取決於她對這一難題的反應。
Depend on me.
相信我。
2. diffuse 散播, 使(氣體或液體)慢慢混合
如： The light was diffused by the leaves.
樹葉使光線漫射。
3. absorb吸納, 吸引...的注意, 吞并
如： Dry sand absorbs water.
干沙吸收水份。
Plants can absorb carbon dioxide and release oxygen.
植物可以吸收二氧化碳釋放氧氣。
4. diminish減少, 變小, 減損
如： He likes to diminish the skills of others.
他喜歡貶低別人的技能。
5.expel驅逐, 逐出, 開除
如：The bullet was expelled from the gun.
子彈從槍中射出。
He was expelled from the school.
他被學校開除了。
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