512 GENERAL CONSIDERATIONS 



540. Fresh-water Animals. — Fresh-water animals show a variety of 

 adaptations to permit swimming and respiration in water and to meet 

 periods of drying and freezing. Animals in rapidly flowing streams 

 develop organs for attachment, including suckers and hooks on their 

 legs, while the larvae are often protected by cases in which they live. 



Lakes of considerable depth show in the winter a circulation involving 

 all of the levels, but in the summer a body of cold, stagnant water 

 remains below while above it is a surface stratum of water acted upon 

 by wind and currents which produce a constant circulation (Fig. 361). 

 The plane separating these two bodies is known as a thermocline. It 

 is established in the spring and destroyed again in the fall when the 

 surface stratum falls to a temperature less than that of the water below, 

 in consequence of which an overturning and a thorough mingling of the 

 two strata take place. The conditions below the thermocline, particu- 



Thermocline 



Fig. 361. — Diagram to illustrate the facts in regard to a thermocline. The arrows 

 above the water show the direction of the wind; those in the lake above the thermocline 

 show the circulation currents in the water. Below the thermocline the water is stagnant. 



larly the low tempierature and the small amount of oxygen, make possible 

 the existence of only a limited fauna. 



541. Terrestrial Faunas. — Owing to the great variety of conditions 

 which exist, terrestrial faunas are more varied than those of either the 

 sea or fresh water. Such faunas include subterranean forms which are 

 influenced by temperature, moisture, degree of aeration, and the chemical 

 composition of the soil. Certain types of animals are restricted to the 

 surface of open ground or the floor of the forest. Still others live at 

 various vegetational levels depending upon the character of the vegeta- 

 tion. Finally, there are the aerial types, which are capable of flight. 

 In terrestrial faunas should be included the cave faunas, among the mem- 

 bers of which there is a tendency toward the loss of eyes, color and organs 

 of flight, while their senses of touch and hearing become very acute. 

 Animals of desert faunas are active mostly at night, so as to escape the 

 heat of the day; frequently develop the power of rapid locomotion; show 

 numerous cases of adaptive coloration; pass off dry excretions so as to 

 conserve water; and generally exhibit a low rate of reproduction. 



