Distribution in Depth 311 



but that the constant fall of organic material toward 

 the bottom makes it possible for some animals to dwell 

 in the depths, if they can endure the low temperature 

 and the other conditions found there. There are some 

 animal planctonts, such as species of Cyclops and 

 Diaptomus, that range the water (oxygen being pre- 

 sent) from top to bottom. There are many that are 

 confined during periods of activity to the warmer 

 region above the thermocline. There are a few like 

 Leptodora that seem to prefer intermediate depths, 

 and there are a few 

 (Heterocope, Limno- 

 calanus, Mysis, etc.) 

 that dwell in the cold 

 water below the ther- 

 mocline. 



Collectively, this 

 extraordinary assem- 

 blage of organisms 

 that we know as 

 plancton recalls in 



, ,1 i- r r FIG. 1 86. Leptodora. 



miniature the life of 



the fields. It has, in its teeming ranks of minute 

 chlorophyl-bearing flagellates, diatoms and other algae, 

 a quick-growing, ever-present food supply that, like 

 the grasses and low herbage on the hills, is the mainstay 

 and dependence of its animal population. It has in 

 some of its larger algae the counterparts of the trees 

 that support more special foragers, are less completely 

 devoured, and that, through death and decomposition, 

 return directly to the water a much larger proportion 

 of their substance. It has in its smaller herbivorous 

 rotifers and entomostraca, the counterpart of the hordes 

 of rodents that infest the fields. It has in its large, 

 plant-eating Cladocerans, such as Daphne, the equiva- 

 lent of the herds of hoofed animals of the plains; and 



