124 HISTOLOGY 



in the crustaceans with light organs. It is fully as efficient a reflector 

 as the connective-tissue form. 



The pigment mantle is an organ whose exact function is rather 

 obscure. It is always found on the proximal surface of the organ, and 

 forms there a thin but perfect layer of branching pigment cells developed 

 ^rom a connective-tissue anlage. In the cephalopod mollusks it is 

 formed from a few very large chromatophores of the characteristic 

 round and flattened shape. It is apparently needless, as the reflector 

 protects the underlying tissues from the light, and, furthermore, this 

 pigment cannot reflect light. 



The lens is often absent, even in very highly developed organs, and 

 more particularly in those that are found in land forms. In its most 

 primitive form it appears as a slight thickening of the usually transparent 

 layers that are placed between the light cells and the exterior. The 

 cells are generally of a rather flattened polygonal shape and of great 

 transparency and refractive power. They form a lens that brings the 

 light to a focus at a very short distance from its point of origin, and not 

 into the parallel rays that would at first be expected. 



The lens is often composed of two divisions, a proximal and a distal 

 one, that differ from one another slightly in their texture. The mean- 

 ing of this difference is not known. 



The structure of these remarkable organs will be found on reflection 

 to be remarkably like that of many of the eyes that we shall study in 

 Chapter XIII. In each case there may be accessory tissues present to 

 handle the light by refraction, reflection, and absorption. The two im- 

 portant differences are the fact that in the one case the specific cells give 

 out light and in the other they receive it, and that in the case of the light 

 organ there is very little direct connection with the central nervous sys- 

 tem, while the eye is from its very nature most closely connected with it. 



The nature of the light has been somewhat touched upon, but it may 

 be well to give some rather more exact details here. It has been the 

 subject of some very exact and convincing experiments by Langley and 

 Very as well as Young. 



Examination with the spectroscope has shown that the green light, 

 produced by the common firefly of the United States and by several 

 other insects, consists of those rays that have the maximum of visibility 

 and the minimum of heat rays and ultra-violet rays. Its appearance is 

 marked by the absence of any of the bands that show a deficiency of 

 waves in the most actinic region. This perfection shows that nearly 100 

 per cent of the energy is transformed into light. The meaning of this 

 becomes clearer when we consider that in the ordinary gas flame only a 

 little under 2 per cent of the energy is converted into light, the rest being 

 dissipated, so far as any use is concerned, as low heat rays ! The light 



