e mentioned as an
instance of this. If the stem of this hydroid, which is usually covered
with polyps, is put into an aquarium the polyps soon fall off. If the
stems are kept in an aquarium where light strikes them during the day, a
regeneration of numerous polyps takes place in a few days. If, however,
the stems of Eudendrium are kept permanently in the dark, no polyps are
formed even after an interval of some weeks; but they are formed in a
few days after the same stems have been transferred from the dark to
the light. Diffused daylight suffices for this effect. Goldfarb, who
repeated these experiments, states that an exposure of comparatively
short duration is sufficient for this effect, it is possible that the
light favours the formation of substances which are a prerequisite for
the origin of polyps and their growth.

Of much greater significance than this observation are the facts which
show that a large number of animals assume, to some extent, the
colour of the ground on which they are placed. Pouchet found through
experiments upon crustaceans and fish that this influence of the ground
on the colour of animals is produced through the medium of the eyes.
If the eyes are removed or the animals made blind in another way these
phenomena cease. The second general fact found by Pouchet was that the
variation in the colour of the animal is brought about through an action
of the nerves on the pigment-cells of the skin; the nerve-action being
induced through the agency of the eye.

The mechanism and the conditions for the change in colouration were made
clear through the beautiful investigations of Keeble and Gamble, on
the colour-change in crustaceans. According to these authors the
pigment-cells can, as a rule, be considered as consisting of a central
body from which a system of more or less complicated ramifications or
processes spreads out in all directions. As a rule, the centre of the
cell contains one or more different pigments which under the influence
of nerves can spread out separately or together into the ramifications.
These phenomena of spreading and retraction of the pigments into or from
the ramifications of the pigment-cells form on the whole the basis for
the colour changes under the influence of environment. Thus Keeble
and Gamble observed that Macromysis flexuosa appears transparent and
colourless or grey on sandy ground. On a dark ground their colour
becomes darker. These animals have two pigments in