ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 385 

 4. Pond life. 



The pond is a well defined unit of environment. Aquatic 

 forms of life are hemmed in by its shores. They are easy to 

 find, easy to collect and easy to keep alive and to observe. 

 As it is not difficult to determine the place of each in the 

 pond, the following study should offer opportunities fo- 

 greater definiteness in field observations. 



The animals of the pond are in part forms that have 

 always been aquatic, and in part land forms returned to the 

 water. Among the latter, some have retained their terres- 

 trial mode of breathing; some have become readapted to 



the water, and have respiratory apparatus of a strictly 

 aquatic type. The problem of getting air has been a 

 primary one determining their distribution. 



Collectively the animal life of the pond may be divided 

 into two groups according to whether the air is taken free or 

 dibsolved in the water; easily recognizable ecological sub- 

 divisions will then be those of the following table. Their 

 places are indicated graphically in the accompanying 

 diagram (fig. 224). 



