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. 



