When the light from a candle was concentrated by means of a large lens on
the anterior extremity, they generally withdrew instantly; but this
concentrated light failed to act perhaps once out of half a dozen trials.
The light was on one occasion concentrated on a worm lying beneath water
in a saucer, and it instantly withdrew into its burrow. In all cases the
duration of the light, unless extremely feeble, made a great difference
in the result; for worms left exposed before a paraffin lamp or a candle
invariably retreated into their burrows within from five to fifteen
minutes; and if in the evening the pots were illuminated before the worms
had come out of their burrows, they failed to appear.
From the foregoing facts it is evident that light affects worms by its
intensity and by its duration. It is only the anterior extremity of the
body, where the cerebral ganglia lie, which is affected by light, as
Hoffmeister asserts, and as I observed on many occasions. If this part
is shaded, other parts of the body may be fully illuminated, and no
effect will be produced. As these animals have no eyes, we must suppose
that the light passes through their skins, and in some manner excites
their cerebral ganglia. It appeared at first probable that the different
manner in which they were affected on different occasions might be
explained, either by the degree of extension of their skin and its
consequent transparency, or by some particular incident of the light; but
I could discover no such relation. One thing was manifest, namely, that
when worms were employed in dragging leaves into their burrows or in
eating them, and even during the short intervals whilst they rested from
their work, they either did not perceive the light or were regardless of
it; and this occurred even when the light was concentrated on them
through a large lens. So, again, whilst they are paired, they will
remain for an hour or two out of their burrows, fully exposed to the
morning light; but it appears from what Hoffmeister says that a light
will occasionally cause paired individuals to separate.
When a worm is suddenly illuminated and dashes like a rabbit into its
burrow to use the expression employed by a friend we are at first led to
look at the action as a reflex one. The irritation of the cerebral
ganglia appears to cause certain muscles to contract in an inevitable
manner, independently of the will or consciousness of the animal, as if
it were an automaton. But the different effect which a light produced on
different occasions, and especially the fact that a worm when in any way
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