SURFACE-SWIMMING FAUNA (INVERTEBRATES). 83 



of the world, In the Atlantic Ocean the phos- 

 phorescence is sometimes so bright that it is 

 possible to read a book on deck by its light 

 alone ; and on a dark night in the Banda seas 

 the water is often like a huge expanse of pale 

 blue smoke studded with diamonds and other 

 lustrous gems. 



These lights are mainly produced by animals 

 which float and drift about on the surface of the 

 water. It is not, as is very commonly supposed, 

 only one or two different kinds of animals that 

 are phosphorescent, but a vast number belonging 

 to many widely different families and of a great 

 variety of form and structure. When the day 

 breaks many of these animals sink down a few 

 fathoms into the darker and cooler strata of 

 water, but a considerable number remain so close 

 to the surface that they can be easily caught in a 

 muslin net dragged after a boat. 



Some of these animals, such as the Jelly-fish, 

 can, during the day, be observed clearly enough 

 from the boat, others can only be seen when the 

 contents of the net are emptied into a glass bottle, 

 and others again are so minute that it requires a 

 strong magnifying glass to detect them at all. Such 

 animals that float or drift in the water without 

 powers of swimming vigorously in one direction 

 or the other, are collectively called the Plankton. 

 In every sea, from the Arctic regions to the 

 Equator, a Plankton will be found. Sometimes 

 it is mainly composed of one species, in other 

 cases it consists of many different species living 

 together. Under certain conditions the water 

 is simply crowded with these organisms, and in 



