This engraving made in tiie 1800s sfiows 

 several stieils of microscopic animais 

 forming part of ttie planl<ton. It was not 

 until the toiv net was developed in 1828 

 that biologists became aware of the 

 staggering amount of planktonic life. 



in such a way that when it was surfaced there was no means of 

 telling whether the plants and animals in the bucket had been picked 

 up at loo fathoms or two inches from the surface. To overcome 

 this difficulty, researchers eventually found a device for closing the 

 net at any desired depth before they brought it up. As a further 

 refinement they attached several tow nets to a single hawser so 

 that different depths could be fished simultaneously. 



With such equipment the distribution of plankton in depth as 

 well as horizontally has been plotted over wide areas. The results 

 sKow a zoning that corresponds roughly to the temperature of the 

 water. They also show that although planktonic plants and animals 

 drift passively with the currents, many make daily vertical migra- 

 tions, coming toward the surface by night and sinking to some 

 depth by day in order to avoid the bright light. 



The plankton population is not the same everywhere, nor is the 

 population static in one place throughout the year. In the shallow 

 seas, near land, the plankton includes many larvae of animals that, 

 when adult, live fixed to the bottom. Over deep water only the 

 larvae of swimming adults or of planktonic animals are found, and 

 these are different from those found in shallow waters. So a distinc- 



This engraving shows how a deep-sea dredge 

 used aboard the Challenger was lowered by 

 stages to the bottom. A weight traveled 

 down the line by stages (G, G', G") so that 

 when the dredge strucl< bottom it would be 

 positioned to scoop up samples. 



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