NATURAL HISTORY STUDIES 



by-play of vitality ; it" is one of the residual powers of 

 the living creature ; .land it is probably exhibited 

 in many cases in which we do not and cannot see it. 

 What we need to know is (1 ) the internal physiology 

 of luminescence, and (2) what its use may be in 

 cases where it bears the marks of specialization. 



The luminescence is in some cases indissolubly 

 connected with the cellular metabolism as in 

 Noctiluca, Brittle-stars, and some fishes. When 

 the cells die the light goes out. In other cases the 

 luminescent material is not luminescent until it is 

 exuded from its producer into the water, as in 

 many minute Crustaceans called Copepods. The 

 light does not require contact with life to keep it 

 shining. The trail of the luminous Myriopod 

 shines in the dark, and in some cases (Copepods 

 and Pholads ) the luminous secretion can be dried and 

 yet retain its capacity of giving forth light when it is 

 put into water after several days, weeks, or months. 



In the American Lampyrid beetles, popularly 

 called fire-flies, the light-producing organ consists of 

 two layers. The inner one, white and opaque, 

 seems to serve as a reflector, and perhaps protects 

 the insect from its own brightness. The outer 

 one, yellowish and translucent, is the seat of the 

 actual light -producing process. It is interesting 

 to know that innumerable air-tubes or tracheae 

 penetrate the organ, for this bears out the conclusion 

 otherwise arrived at, that the luminescence is often 

 due to an oxidation. In other cases, however, it 

 seems that the luminescence is associated with 

 a kind of fermenting. Sometimes it is possible 



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