294 PRINCIPLES OF ANIMAL BIOLOGY 



Fresh-water Habitats. — Of the many situations occupied by animals, 

 the aquatic habitats have many advantages for purposes of ecological 

 illustration. They are amenable to inexpensive study and have accord- 

 ingly been thoroughly explored. They present a considerable variety of 

 physical features and so accommodate very diverse communities. More- 

 over, they possess an organization which for orderliness is not easily 

 matched in any terrestrial habitat. This organization depends on 

 general principles which render aquatic situations capable of significant 

 classification. 



Besides the properties of water which make it an important constitu- 

 ent of protoplasm (see page 39), an excellent heat reservoir, and a solvent 

 of gases, salts, and other chemical substances, it has certain character- 

 istics which pertain to it in the bulk. These qualities become the quali- 

 ties of the various bodies of water. It may be turbid or clear, which 

 greatly affects the penetration of light. It has considerable weight; 

 hence objects located at great depths are subjected to high pressures. 

 And lastly, being liquid, it is highly mobile and is subject to waves, con- 

 vection currents, and horizontal currents. Most organisms living in 

 water are influenced by one or more of these features, some organisms by 

 all of them. 



The animals in water occupy different situations which are character- 

 istic of different species. Some live on the bottom, others are free in the 

 water and independent of the bottom. The latter include species that 

 float passively or, if they swim, do so in an aimless, undirected fashion; 

 and other species which, like fishes, swim actively and steer themselves in 

 given directions. The other characteristics of aquatic animals are best 

 described in connection with their several habitats. 



Ponds. — Ponds are shallow — usually not over 2 or 3 meters in depth — 

 and heat from the sun penetrates through all the water. The tempera- 

 ture is consequently nearly uniform from surface to bottom, though 

 shading or resistance to currents by vegetation may cause differences 

 of 5°C. between different parts of a pond. Although the water may 

 be stirred completely by wind, wave action is so slight as to cause no 

 important mechanical disturbance. The important gases are almost 

 uniformly distributed through pond water; bright light and abundant 

 algae may increase the oxygen content through photosynthesis, and, when 

 crowded with animals, the water may contain excess carbon dioxide. 

 Abundant rains dilute the chemical content and increase the turbidity 

 temporarily. 



Whether the bottom of a pond is covered with vegetation depends 

 on its depth and tiu'bidity; even when the water is fairly clear, there are 

 few plants beyond 3 meters in depth. In most ponds, however, this 

 permits vegetation throughout their area (Fig. 261). These plants 



