kin), and second
a peculiar nervous connection between the retina and the muscular
apparatus. In symmetrically built heliotropic animals in which the
symmetrical muscles participate equally in locomotion, the symmetrical
muscles work with equal energy as long as the photo-chemical processes
in both eyes are identical. If, however, one eye is struck by stronger
light than the other, the symmetrical muscles will work unequally and
in positively heliotropic animals those muscles will work with greater
energy which bring the plane of symmetry back into the direction of the
rays of light and the head towards the source of light. As soon as both
eyes are struck by the rays of light at the same angle, there is no more
reason for the animal to deviate from this direction and it will move in
a straight line. All this holds good on the supposition that the animals
are exposed to only one source of light and are very sensitive to light.

Additional proof for the correctness of this theory was furnished
through the experiments of G.H. Parker and S.J. Holmes. The former
worked on a butterfly, Vanessa antiope, the latter on other arthropods.
All the animals were in a marked degree positively heliotropic. These
authors found that if one cornea is blackened in such an animal, it
moves continually in a circle when it is exposed to a source of light,
and in these motions the eye which is not covered with paint is directed
towards the centre of the circle. The animal behaves, therefore, as if
the darkened eye were in the shade.

(b) THE PRODUCTION OF POSITIVE HELIOTROPISM BY ACIDS AND OTHER MEANS AND
THE PERIODIC DEPTH-MIGRATIONS OF PELAGIC ANIMALS.

When we observe a dense mass of copepods collected from a freshwater
pond, we notice that some have a tendency to go to the light while
others go in the opposite direction and many, if not the majority,
are indifferent to light. It is an easy matter to make the negatively
heliotropic or the indifferent copepods almost instantly positively
heliotropic by adding a small but definite amount of carbon-dioxide
in the form of carbonated water to the water in which the animals are
contained. If the animals are contained in 50 cubic centimetres of water
it suffices to add from three to six cubic centimetres of carbonated
water to make all the copepods energetically positively heliotropic.
This heliotropism lasts about half an hour (probably until all the
carbon-dioxide has again diffused into the a