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