156 The Life Story of the Fish 



the gills also means low temperature. So by a long process 

 of reasoning we arrive at a conclusion which we already 

 know: that the fish is a "cold-blooded" animal. 



Or, if we wish to impress our friends, we can call it a 

 poikilo-thermal animal. In either case, what we mean is that 

 it is an animal whose body temperature is about the same as 

 the surrounding medium. This is why fish are so sensitive 

 to changes in the water. If it were not for the comparative 

 constancy of their environment, it is hard to see how they 

 could survive. Natural bodies of water not only undergo no 

 sudden and violent changes of temperature, but their sea- 

 sonal range is small, compared to what mammals often have 

 to withstand. Mammals may have to face variations of as 

 much as one hundred degrees between midsummer and 

 midwinter, whereas fish rarely encounter a range even half 

 as large. For the black bass the extremes are probably not 

 over fifty degrees apart, for most of the trouts forty de- 

 grees, for the tuna thirty. And when the thermometer 

 approaches what is for them the lower limit, many fish go 

 into a state of hibernation. They lie at the bottom. No move- 

 ment is made. No food is taken. The heart slows down, and 

 all the life processes are at a low rate. The fish is very nearly 

 in a state of suspended animation. 



