222 FOUNDERS OF OCEANOGRAPHY 



In the case of Pholas, the light -producing power has been 

 known since classical times, but Panceri (1873) first deter- 

 mined that the light-giving mucus was produced, not from 

 the whole surface that it usually covers, but from five definite 

 patches of the integument. These are, then, external organs 

 formed of simple cellular glands in the deeper layer of the 

 skin, and pouring out the luminous secretion on the surface. 

 Dubois later (1887) showed that this secretion contained the 

 two essential substances luciferine and luciferase, which 

 require to be brought into contact in the presence of water 

 in order to produce light, and that this action was inde- 

 pendent of the life of the PJiolas, and could still take place 

 after the substances had been dried or treated with various 

 reagents. The light-production was, in fact, shown to be 

 a chemical phenomenon which could be produced in the 

 laboratory by substances which were no longer alive, 

 although originally formed by a living animal. 



The colour of the light in Pholas is greenish blue, and 

 very brilHant and persistent even after separation from the 

 body ; but it is difficult to say what the use can be to an 

 animal deeply buried at the bottom of a hole in the rock 

 — unless it be that the luminous secretion spreads from the 

 body up to and around the mouth of the burrow and acts 

 as an attraction to minute swimming organisms, which are 

 then sucked in and used as food. (See Fig. 12.) 



In the highest group of molluscs, the cuttle-fishes, we find 

 both primitive light-producing glands, which eject their 

 secretion into the surrounding water, where the luciferine 

 and luciferase in contact with oxygen generate light (external 

 combustion), and also most elaborate and more deeply 

 placed organs, under nerve-control, with internal combus- 

 tion, the photogenous secretion never leaving the cells in 

 which it is formed. 



The most highly differentiated of these closed photogenous 

 organs show cornea, lens, and reflectors arranged around 

 the central light-producing ceUs, the whole being surrounded 



