METABOLISM IN THE SEA 177 



perhaps, to about fifty fathoms. Opposed to this 

 class of organisms is another more limited one, 

 which has been called the Nekton. These animals 

 are very active, possessing powerful organs of 

 locomotion, and they swim about in the sea, 

 with, in many cases, very considerable ranges 

 of migration. The fishes themselves are the best 

 examples of nektonic animals, but many other 

 creatures also fall under this category, such as 

 cetaceans like the whales, dolphins, and porpoises, 

 and molluscs like the squids and cuttlefishes. 

 Then contrasting strongly with both these classes 

 is the group of organisms which Hensen has 

 called the Plankton^ under which category we 

 now include the vast assemblage of plants and 

 animals which, possessing little powers of loco- 

 motion, simply drift about in the sea at the 

 mercy of winds, tides, and currents. 



This latter class, by reason of its universal 

 distribution throughout all the seas of the world 

 and throughout the whole body of the water, 

 from the surface to the bottom, is by far the 

 most abundant. Planktonic plants and animals 

 are found everywhere and at all times. Even 

 when the sea-water may appear most pellucid, 

 it contains an abundance of microscopic life, and 

 it is no exaggeration to say that in some places, 

 and at certain times in the year, every drop 

 of water may be inhabited by one or more 

 organisms. Usually the creatures forming the 



12 



