252 ANIMALS OF LAND AND SEA 



completely blind. Some of the fresh water isopods, too, a 

 group represented by the little sow-bugs or i)ill-bugs of our 

 woods, are eyeless and live in caves, while the only known 

 representative of one type was, together with other equally 

 strange crustaceans, brought up from an artesian well in Texas. 

 All of our fresh water typ^s of larger crustaceans, in fact, ex- 

 cept the mysids of the Great Lakes, have developed forms 

 adapted to life in subterranean waters; and in Europe there is 

 a cave fairy shrimp which is curious in sometimes having 

 normal eyes, sometimes being completely blind, and sometimes 

 being bhnd in one eye only. Many insects, especially certain 

 crickets, and some spiders, abound in caves. For the most 

 part these are types which are found elsewhere in deep holes 

 and crevices protected from the light; but a few seem to be 

 special cave-living types. 



The catalogue of unusual situations inhabited by animals 

 might be extended almost indefinitely, but the preceding 

 examples will suffice to indicate broadly the limits between 

 which animal life exists. 



Active animal life is found at a constant temperature of 

 28.4° F. (at which sea water freezes at the surface, though not 

 when under pressure), at a constant temperature of about 

 157° F., and at all intermediate temperatures, constant or more 

 or less widely varying, ranging, therefore, through about 

 128.6° F. In a dormant or inactive condition animal life can 

 withstand the coldest temperatures on the earth's surface, be- 

 coming active again when the temperature rises above the 

 freezing point. Birds and mammals are independent of the 

 temperature about them; they maintain continually a high 

 temperature within their bodies which are insulated from the 

 air by fur or feathers, and they can spare sufficient energy to 

 transform snow to water, if they do not get the latter from 

 their prey. A few insects, as adults, remain active below the 

 freezing point, but these neither eat nor drink. 



Animal life can exist permanently in complete darkness, in 

 caves, underground, and in the abysses of the sea. 



