GWD1-26

    Recently biologists have been

         interested in a tide-associated

         periodic behavior displayed by

Line         the diatom Hantzschia virgata, a

  (5)       microscopic golden-brown alga that

inhabits that portion of a shoreline

washed by tides (the intertidal zone).

Diatoms of this species, sometimes

called “commuter” diatoms, remain

(10)       burrowed in the sand during high

tide, and emerge on the sand sur-

face during the daytime low tide.

Just before the sand is inundated by

the rising tide, the diatoms burrow

(15)       again.  Some scientists hypothesize

that commuter diatoms know that it

is low tide because they sense an

environmental change, such as an

alteration in temperature or a change

(20)       in pressure caused by tidal move-

         ment.  However, when diatoms are

         observed under constant conditions

in a laboratory, they still display

periodic behavior, continuing to bur-

(25)       row on schedule for several weeks.

This indicates that commuter diatoms,

rather than relying on environmental

cues to keep time, possess an inter-

nal pacemaker or biological clock

(30)       that enables them to anticipate peri-

odic changes in the environment.

A commuter diatom has an unusu-

         ally accurate biological clock, a

consequence of the unrelenting

(35)     environmental pressures to which

it is subjected; any diatoms that do

not burrow before the tide arrives

are washed away.

      This is not to suggest that the

(40)      period of this biological clock is

immutably fixed.  Biologists have

concluded that even though a

diatom does not rely on the envi-

ronment to keep time, environmental

(45)       factors—including changes in the

tide’s hydrostatic pressure, salin-

ity, mechanical agitation, and

temperature—can alter the period

of its biological clock according to

(50)       changes in the tidal cycle.  In short,

the relation between an organism’s

biological clock and its environment

is similar to that between a wristwatch

and its owner:  the owner cannot

(55)       make the watch run faster or slower,

but can reset the hands.  However,

this relation is complicated in intertidal

dwellers such as commuter diatoms

by the fact that these organisms are

(60)       exposed to the solar-day cycle as

well as to the tidal cycle, and some-

times display both solar-day and

tidal periods in a single behavior.

Commuter diatoms, for example,

(65)       emerge only during those low tides

that occur during the day.

楼主把这句话读一下看能解决不

Biologists have

concluded that even though a

diatom does not rely on the envi-

ronment to keep time, environmental

(45)       factors—including changes in the

tide’s hydrostatic pressure, salin-

ity, mechanical agitation, and

temperature—can alter the period

of its biological clock according to

(50)       changes in the tidal cycle.  In short,

the relation between an organism’s

biological clock and its environment

is similar to that between a wristwatch

and its owner:  the owner cannot

(55)       make the watch run faster or slower,

but can reset the hands.  

另外再比较下几个选项也能排除一些了.呵呵