260 Influence of environment on animals 
for organisms, the nutrition of which depends upon the action of 
chlorophyll, it becomes of less importance for organisms devoid of 
chlorophyll. Nevertheless, we find animals in which the formation of 
organs by regeneration is not possible unless they are exposed to 
light. An observation made by the writer on the regeneration of 
polyps in a hydroid, Eudendrium racemosum, at Woods Hole, may 
be 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 Kudendrium 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 experi- 
ments, 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 rami- 
fications 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 
