20 THE SEA SHORE 



luminous insects is evidently a signal by means of which they 

 call their mates, and this may be the case with many of the marine 

 luminous animals, but it is evidently not so with those which 

 live in such immense numbers that they are simply crowded 

 together ; nor can it be so with the many luminous creatures that 

 are hermaphrodite. It is a fact, however, that numbers of deep- 

 sea species possess the power of emitting light to a striking extent ; 

 and the use of this power is in such cases obvious, for since the rays 

 of the sun do not penetrate to great depths in the ocean, these 

 luminous species are enabled to illuminate their own surroundings 

 while in search of food, and, in many cases at least, to quench their 

 lights suddenly at such times as they themselves are in danger. 



