40 



spring, as the wave of rising sea temperature travels 

 northwards, it migrates to the English Channel and the 

 North Sea. This migration is often directly associated 

 with the presence, in large quantities at that season, of 

 a particular kind of copepod in the surface water of the 

 English Channel. 



Phosphorescence. Many marine creatures, ranging 

 from deep-sea fish living in the dark abysses of the ocean 

 to various species of the minute plankton drifting in 

 the surface water, possess phosphorescent organs, which 

 emit light of low intensity similar to that of a glow- 

 worm and firefly. In many cases the light appears to 

 possess some important function, and highly specialized 

 organs are developed. In such cases the light is only 

 emitted in response to some stimulus thus, the 

 phosphorescence of the surface water of the sea, when 

 disturbed by the blade of an oar, is due to the distur- 

 bance of myriads of minute planktonic organisms, 

 equipped with phosphorescent organs, either protozoa or 

 protophyta ; many pelagic copepods are phosphorescent. 

 In other cases, phosphorescence appears to be a more or 

 less accidental by-product of some other process, and 

 of little or no significance. The substance which pro- 

 duces the glow is contained in the slimy secretion 

 produced by the epidermal glands of the fish, and, as 

 phosphorescence can only occur in the presence of 

 oxygen, it is evident that the light is produced by the 

 slow oxidation of this substance. The colour of the 

 light emitted by marine organisms is generally blue or 

 light green, but red and lilac also have been observed. 

 The distribution and colour of the light or lights produced 

 by individual fish vary with the different species. In 

 many cases it would appear that these points of light 

 provide the means by which fish recognize each other 

 in the dark depths of the ocean. Some fishes possess 



