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 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 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 



