COLOUR AND PHOSPHORESCENCE 191 



passes a nerve which controls it — for the luminescence of 

 these animals is under direct control and the light organs 

 can be turned off and on at will like an electric torch. The 

 organs are also well supplied with blood for, since they are 

 shut off completely from the water, they are unable to get 

 the oxygen, which, as we have seen, is necessary for the 

 production of light, except from the blood stream. A 

 typical light organ of this kind is shown in section in Figure 

 40. 



In the chapter on boring life we discussed the bivalve 



Fig. 40. — Light-producing organ of a crustacean, Sergestes. /., lens of 



several layers ; p., layer of dark pigment behind reflector ; ph., 



photogenic cells where light is produced ; r., reflector. (Modified after 



Terao.) 



mollusc known as the Piddock [Pholas) (Fig. 41) which 

 bores its way into rock where it lives with only the tips 

 of its siphons projecting into the water. This creature, 

 strangely enough, is one of the most luminescent of marine 

 animals, a fact which has been known from very early 

 times. There are no special organs but light is produced 

 within minute glands present in five areas on the skin 

 and spreads from these all over the surface of the body. 

 The light is a greenish blue and very powerful. Passing 



