The fiddler crab (蟹) is a living clock. It shows the time of day by the colour o

The fiddler crab (蟹) is a living clock. Itshows the time of day by the colour of its skin, which is dark by day and paleby night. The crab’s changing colour follows aregular twenty—four hour plan that exactlymatches the daily rhythm of the sun.

    Does the crabactually keep time, or does its skin simply answer to the sun’s rays, changing colour according to the amount of lightstrikes it? To find out, biologists kept crabs in a dark room for two months.Even without daylight, the crab’s skin colour continued to changeexactly on time.

    Thischaracteristic probably developed gradually in answer to the daily rising andsetting of the sun, to help protect the crab from sunlight and enemies. Aftermillions of years it has become completely regulated (受控制) inside the living bodyof the crab.

    The biologistsnoticed that once each day the colour of the fiddler crab is especially dark,and that each day this happens fifty minutes later than on the day before. Fromthis they discovered that each crab follows not only the rhythm of the sun butalso that of the tides (潮水). The crab’s period of greatest darkening isexactly the time of low tide on the beach where it was caught!

1.The fiddler crab is like a clock because itchanges colour ______.

   A in a regular 24—hourrhythm             B. in answer to the sun’s rays

   C. at lowtide                            D.every fifty minutes

2.The crab’schanging colour ______.

   A. tells the crabwhat time it is        B. protects the crabfrom the sunlight and enemies

   C. keeps the crabwarm             D. is of no real use

3. When the fiddler crabs were kept in thedark , they ______.

   A. did not changecolour            B. changed colour more quickly

   C. changed colourmore slowly        D. changed colour on thesame timetable

4.The crab’scolour—changing ability was probably developed______.

   A. in the process ofevolution (进化)    B. over millions of years

   C. by the work ofbiologists            D.both A and B

5.The best title for this selection would be______.

   A. The Sun and theTides              B. Discoveries in Biology

  C. A scientificStudy                  D. A Living Clock

试题答案

【答案】

1.A

2.B

3.D

4.D

5.D

【解析】略