326 Agnatic Societies 



Few of these can thrive in the waters of the limnetic 

 region of a lake ; but there is at k ast one member of the 

 first group that takes advantage of an abundant supply 

 of food in lake waters, migrates out, and develops 

 enormously, overshadowing in numbers sometimes the 

 truly limnetic forms. It is Chydorus sphczricus. It is 

 rather a littoral than a limnetic species, yet it often 

 abounds in the open lakes, following a rich development 

 there of blue-green algae suitable for its fo< d. 



SPATIAL RELATIONS 



A large part of the animal life of the littoral region is 

 disposed in relation to upper and lower surfaces of the 

 water. This grouping by levels is due to gravity. 

 Where the air rests upon the water, making available 

 an unlimited supply of oxygen, there at the surface are 

 aggregated forms that require free air for breathing. 

 Where the water rests upon the solid earth, there at the 

 bottom are the forms that hide or burrow in the ground. 



Plants and animals differ most markedly here. Light 

 is the prime requisite and source of energy for chloro- 

 phyl-bearing plants. It is not light but oxygen that 

 holds many animals at the surface of the water; and 

 it is indifference to light that allows many other animals 

 to dwell in the obscurity of the bottom. 



Life on the bottom has a number of advantages among 

 which are the following : 



1 . Shelter is available. 



2 . Energy is saved when a resting place is found, and 

 continuous swimming is unnecessary. 



3. Gravity brings food down from above. 



4. Hiding from enemies is easier in absence of strong 

 light. 



It has also its perils chief among which are: 



1. Failure of oxygen 1 either of which may result 



2. Excess of silt in suffocation. 



