FRESH WATER ANBfALS 



From our point of view water is water, and to most of us the 

 lakes and ponds seem to differ from the sea only in being 

 fresh while the sea is salt; but the animals are sensible of other 

 and more important differences. 



Rivers and streams contain the greater part of the fresh 

 water on the surface of the earth. These are narrow and al- 

 ways comparatively shallow ribbons of more or less rapidly 

 flowing water charged with silt which are ultimately connected 

 with the sea. The water they contain varies in amount with 

 different seasons; in some the shrinkage and expansion is 

 relatively slight, but others in the dry season become reduced 

 to a series of pools, or even completely disappear. The tem- 

 perature, especially in shallow streams, may be subject to 

 great seasonal variation, and small streams may even be com- 

 pletely frozen in the winter months. 



Lakes and ponds and marshes differ from rivers and streams 

 primarily in the absence of a current; they represent surface 

 water from the rains held in a depression, and mostly have an 

 outlet to the sea; if there is no outlet thcA^ are usuallv brack- 

 ish or saline. They may be permanent, temporary or variable, 

 and may completely freeze in wnnter. Some large lakes are 

 very deep and were once connected with the ocean, or with 

 ancient seas. A few of these, with no outlet, like the Caspian, 

 are still salt, but in most the water is now fresh. These all 

 have in their fauna typically marine animals, more or less 

 closely related to present oceanic types. In the Caspian and 

 Aral seas live seals related to those in the Mediterranean and 

 Caribbean seas, and to the Laysan seal. But the seals in 

 Lake Baikal are of an arctic t^-pe. 



In certain plants, especially the bromelias, the bases of the 



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