题目内容
by day and pale by night. The crab's changing colour follows a regular twenty-four hour plan that exactly
matches the daily rhythm of the sun.
Does the crab actually keep time, or does its skin simply answer to the sun's rays, changing colour
according to the amount of light which strikes it? To find out, biologists kept crabs in a dark room for two
months. Even without daylight, the crab's skin colour continued to change exactly on time.
This characteristic probably developed gradually in answer to the daily rising and setting of the sun, to help
protect the crab from sunlight and enemies. After millions of years it has become completely controlled inside
the living body of the crab.
The biologists noticed 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. From this they discovered that each crab follows not
only the rhythm of the sun but also that of the tides (潮水). The crab's period of greatest darkening is exactly
the time of low tide on the beach where it was caught!
B. protects the crab from the sunlight and enemies
C. keeps the crab warm
D. is of no real use
B. changed colour more quickly
C. changed colour more slowly
D. changed colour on the same timetable
B. over millions of years
C. by the work of biologists
D. both A and B
B. Discoveries in Biology
C. A scientific Study
D. A Living Clock
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