34 



FRESH-WATER BIOLOGY 



Cold increases the metabolism of warm-blooded animals and 

 decreases that of cold-blooded animals. In the cold-blooded 

 animals a rise of io° C. within limits reasonably compatible with 

 life increases the rate of metaboUsm, or rate of development of 

 young, by two or three times. This is taken as evidence that 

 life is a chemical process because similar changes in temperature 

 have corresponding changes in rate of chemical reaction. 



Thus animals aquatic in their developmental stages and which 

 happen to be in very shallow temporary water are automatically 

 accelerated in development as the sun warms the water, evaporates 

 it and decreases its volume at the same time increasing its tempera- 

 ture. 



Animals react to temperature with considerable precision. Both 

 marine and fresh-water animals can recognize differences of 0.2° C. 

 and will turn back when such slight differences are encountered 

 under experimental conditions. 



Pressure in water increases with depth. The results given by 

 Forel are shown in Table IV. 



TABLE IV 



There is a little less than one atmosphere increase in pressure 

 for each 10 meters of depth. According to this, animals in the 

 deepest parts of a lake like Lake Michigan are living under a 

 pressure of about 375 pounds to the square inch. 



The effect of pressure on organisms was studied by Regnard. 

 Contrary to the popular idea he found that gelatine, agar, and 

 various plants and animals and excised parts of animals take up 

 water, swell and increase in weight under high pressure. This is 

 true even of terrestrial insects. At 400 to 600 atmospheres Para- 

 mecia become swollen and immobile, including the cilia. They 

 recover from ten minutes' exposure. Carp become Hstless at 200 

 atmospheres, die at 300 and become swollen and rigid at 400 

 atmospheres. Salmon ova are destroyed at 400 atmospheres but 



