LUMINESCENCE IN THE SEA 219 



by Panceri (1873), Dubois (1887), and others since. The 

 Ught, which varies from greenish blue to violet, is given 

 o£f from most of the segments of the body, and is evidently 

 an external secretion, as it can be rubbed off and spread 

 through the surrounding water. 



The use of the light in the case of Chcetopterus remains 

 a mystery. It wiU probably illuminate the water around 

 the mouth of the tube, and that may possibly attract minute 

 organisms upon which the worm feeds. But, on the other 

 hand, this illumination might well be a source of danger, 

 as indicating to fish the presence of the hidden worm. 

 Dahlgren has recorded that he has seen eels pulling the 

 Chcetopterus out of its tube. This is evidently not a case 

 where the enemy is warned off from its prey by the light. 



Crustacea. — Many of the Crustacea, both high and low, 

 are hght-producing, and the hght-organs range in structure 

 from simple groups of surface cells to the most complicated 

 eye-like internal organs. For the purpose of this brief 

 survey, it must suffice to select three examples — the Ostra- 

 coda, such as Cypridina ; Copepoda, such as Metridia ; and 

 Schizopoda, such as Meganyctiphanes. 



Cypridina and other luminous Ostracods have been 

 observed by many naturahsts, and the minute structure 

 and the bio -chemical processes involved have been especially 

 elucidated by Ulric Dahlgren and E. N. Harvey in America. 

 The Hght-organs are unicellular glands opening above the 

 mouth and discharging the hght-producing, mucus-like, 

 yeUow secretion freely into the water. The Ught is blue 

 in colour, and is only produced at night. Harvey has shown 

 (as Dubois had previously done in the case of the mollusc 

 Pholas) that the secretion contains two distinct substances, 

 which must be brought together in the presence of oxygen 

 and water in order to produce hght. Dubois had named 

 these "luciferine" and "lucif erase" in Pholas. Harvey, 

 finding that his two substances from Cypridina did not corre- 

 spond wholly in their reactions, applied the new terms •* pho- 



