The circulatory system is well developed. Worms breathe by their skin,
as they do not possess any special respiratory organs. The two sexes are
united in the same individual, but two individuals pair together. The
nervous system is fairly well developed; and the two almost confluent
cerebral ganglia are situated very near to the anterior end of the body.
_Senses_. Worms are destitute of eyes, and at first I thought that they
were quite insensible to light; for those kept in confinement were
repeatedly observed by the aid of a candle, and others out of doors by
the aid of a lantern, yet they were rarely alarmed, although extremely
timid animals. Other persons have found no difficulty in observing worms
at night by the same means. {18a}
Hoffmeister, however, states {18b} that worms, with the exception of a
few individuals, are extremely sensitive to light; but he admits that in
most cases a certain time is requisite for its action. These statements
led me to watch on many successive nights worms kept in pots, which were
protected from currents of air by means of glass plates. The pots were
approached very gently, in order that no vibration of the floor should be
caused. When under these circumstances worms were illuminated by a
bull s-eye lantern having slides of dark red and blue glass, which
intercepted so much light that they could be seen only with some
difficulty, they were not at all affected by this amount of light,
however long they were exposed to it. The light, as far as I could
judge, was brighter than that from the full moon. Its colour apparently
made no difference in the result. When they were illuminated by a
candle, or even by a bright paraffin lamp, they were not usually affected
at first. Nor were they when the light was alternately admitted and shut
off. Sometimes, however, they behaved very differently, for as soon as
the light fell on them, they withdrew into their burrows with almost
instantaneous rapidity. This occurred perhaps once out of a dozen times.
When they did not withdraw instantly, they often raised the anterior
tapering ends of their bodies from the ground, as if their attention was
aroused or as if surprise was felt; or they moved their bodies from side
to side as if feeling for some object. They appeared distressed by the
light; but I doubt whether this was really the case, for on two occasions
after withdrawing slowly, they remained for a long time with their
anterior extremities protruding a little from the mouths of their
burrows, in which position they were ready for instant and complete
withdrawal.
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