SURFACE-SWIMMING FAUNA (INVERTEBRATES). 109 



to contain immense numbers of very minute 

 creatures called Nodiluca. Each of these has a 

 gelatinous consistency, and is the shape of a 

 microscopic cherry, bearing a short whip-like 

 process, called the flagellum, which propels the 

 organism slowly through the water. There 

 seems to be no doubt that, on these occasions, 

 the light is caused by these Noctilucas, but there 

 are many other minute forms which abound on 

 the surface and give off a pale phosphorescent 

 light at night. 



We do not know for certain what may be the 

 use of the phosphorescent light to the organisms 

 that possess the power of emitting it. If we 

 assume that the transparency of the bodies of 

 the pelagic animals has a protective value in 

 the daylight, it is difficult to understand why 

 many of them should become so attractive, 

 as the phosphorescent light makes them, at 

 night. It is probable that the star-like lights 

 of many of the Copepods may serve to attract 

 to one another the two sexes, as it does with 

 the Glow-worms and Fire-flies, but such an ex- 

 planation as this cannot well be accepted in the 

 case of Pyrosoma, which is hermaphrodite, or the 

 Noctilucas, which live together in immense num- 

 bers. There can be little doubt, however, that 

 there is some good reason for it, as it occurs in 

 so many different animals belonging to widely 

 separated families. 



In the neighbourhood of coasts or in shallow 

 water, the surface of the sea usually supports 

 a very large number of animals in a larval 

 or immature state. These creatures live only 



