The state, also, of the nervous system of the
higher animals differs much at different times, for instance, a
horse is much more readily startled at one time than at another.
The comparison here implied between the actions of one of the
higher animals and of one so low in the scale as an earth-worm, may
appear far-fetched; for we thus attribute to the worm attention and
some mental power, nevertheless I can see no reason to doubt the
justice of the comparison.
Although worms cannot be said to possess the power of vision, their
sensitiveness to light enables them to distinguish between day and
night; and they thus escape extreme danger from the many diurnal
animals which prey on them. Their withdrawal into their burrows
during the day appears, however, to have become an habitual action;
for worms kept in pots covered by glass plates, over which sheets
of black paper were spread, and placed before a north-east window,
remained during the day-time in their burrows and came out every
night; and they continued thus to act for a week. No doubt a
little light may have entered between the sheets of glass and the
blackened paper; but we know from the trials with coloured glass,
that worms are indifferent to a small amount of light.
Worms appear to be less sensitive to moderate radiant heat than to
a bright light.
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