THE DRIFTING SWARMS 



It may be difficult at first to realize that the micro- 

 scopic plants which are comprised within the plankton 

 form a vegetation which is sufficient to support the entire 

 animal life of the sea, but this is indeed the case. Invisible 

 to the human eye, although their presence contributes to 

 the colour of sea water, all the vast hosts of planktonic 

 creatures — numbers of them almost invisible to the eye 

 — depend upon such microscopic plants. So, directly or 

 indirectly, do all the invertebrate animals of the sea-beds, 

 all the myriads of shoals offish in the sea, in their infinite 

 variety, and all the aquatic mammals, including the 

 greatest sharks and whales. 



One of the most remarkable facts in oceanographic 

 science is the paradoxical one that the whale — one of the 

 largest, or it may be the largest, creatures in the sea, 

 feeds on some of the sea's smallest creatures, using its 

 wonderful sieve to extract them from the water. 



The microscopic plants in the plankton are largely 

 diatoms, dinoflagellates, blue-green algae and similar 

 lowly organisms. Victor Hensen (1871-1911), the Ger- 

 man physiologist, who is credited with devising the name 

 "plankton", began his study of the biology of drifting 

 life of the ocean with the theory that a conical silk net 

 with meshes of a particular size would catch all the plants 

 in the cylindrical column of water through which it 

 passed. He thought that if he counted, under a micro- 

 scope, all the plants in a unit of sea water it would be 

 possible to calculate the number living below a unit of 

 the sea's surface. But Hans Lohmann quickly pointed out 

 to him that very small, and highly important, members 

 of the phytoplankton (planktonic plants) were passing 

 through the meshes of the very finest silk nets. 



From that moment special filters were devised, which 

 have been continually improved. Man searches deeper 

 and deeper into the infinitesimal, both in the structure 

 of atoms and in the world of planktonic creatures. 



The microscopic plants known as diatoms are so 



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