PHOTOGENIC MARINE ANIMALS. 3 



water stretching far into the darkness. Every stroke of the oar 

 causes a luminous eddy, and minute forms are lifted by the 

 blade, and scintillate brightly as they roll into the water. Nor 

 is the phosphorescence limited to surface-forms, for many 

 luminous types of great interest and beauty are brought up by 

 dredge and trawl, even from great depths. 



Sixteen years ago I gave an address on the subject of the 

 phosphorescence of marine animals to the Biological Section of 

 the British Association at Aberdeen. Since that date noteworthy 

 progress has been made only in two departments, viz. (1) in that 

 of photogenic Bacteria, those minute plant-organisms which play 

 so important a part in causing dead food-fishes, for instance, to 

 gleam in the dark ; and (2) in the extension of our knowledge of 

 luminous fishes. 



As a general rule phosphorescence in marine animals shows 

 itself under four conditions, three of which are connected with 

 structure : — 



(1) The animals present special cells which, under certain 

 circumstances, secrete a phosphorescent mucus. 



(2) The special cells produce light without mucus or other 

 visible secretion. 



(3) The animals emit light under the action of the nervous 

 system without special differentiation of the tissues. 



(4) Their phosphorescence is due to photogenic Bacteria. 



In the first three the light is emitted on the slightest touch, 

 or, in some cases, by simply blowing on the animal producing it. 

 It varies in colour from pale lambent light through several shades 

 of pink, red, green, lilac, and blue ; while Huxley found that 

 Pyrosoma at its brightest was red, but in fading it passed through 

 shades of orange, green, and blue. Further, the light dis- 

 appears on the death of the animal. In the fourth group, on 

 the other hand, the luminosity occurs about a day after the 

 death of the fish. 



Phosphorescence is found in almost all the great subdivisions 

 of marine animals up to and including fishes, viz. in the simplest 

 forms, viz. Protozoa, in Ccelenterates, Worms, Botifers, Crabs, 

 Shell-fishes, and Fishes. Moreover, every variety of marine life 

 is represented, such as pelagic (or swimming and floating), 

 sedentary, and reptant. Some of the phosphorescent animals 



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