20 
COLOUR IN NATURE 
CHAP. 
of course could not occur if the propagation of the 
stimulus were rapid. Under ordinary conditions the 
phosphorescence is stimulated by the shock of the 
waves against the sensitive organism, but it may be 
also produced by various chemical stimuli, such as 
fresh water, alcohol, and ether. The action of the 
two latter agents is very interesting. If they come 
into direct contact with the luminous organs they 
extinguish the light instantly, while if they do not 
reach the organs they act as powerful stimulants of 
the light. The luminosity ceases at death, but the 
fluid obtained from the living organism by crushing 
retains its phosphorescent power for some time. 
In the Medusae the luminosity is due to the 
epithelial cells, and in consequence of the delicacy of 
these the light is communicated to surrounding objects. 
In Phyllirhoe the light is produced by the ganglion 
cells of the nervous system, and by certain peculiar 
nerve cells (Muller’s cells), and is again apparently due 
to a special substance occurring in the luminous cells. 
In the Copepoda the phosphorescent substance 
seems to be produced in special glands situated at 
various parts of the body, and the light, according to 
Giesbrecht, is only produced when the substance 
secreted comes into contact with sea-water. In 
Metridia longa, according to Vanhoeffen, the light 
appears especially immediately behind the head, and 
at a point close to the posterior end of the abdomen. 
The living organism examined in light is colourless, 
except at these same points which appear as “ moss- 
green ” spots ; these spots are apparently the phos¬ 
phorescent glands. Other phosphorescent Copepods 
display similar phenomena. 
