26 HOMOIOTHERMISM 



70 deg. C. Miss Behre (1918) has found that Planaria 

 will quite readily change its limits of tolerance to high 

 or to low temperatures. Acclimatization of vertebrates 

 has been investigated by Davenport and Castle (1895). 

 They tested toad tadpoles and found that the limit of 

 toleration could be increased several degrees. Loeb and 

 Wasteneys (1912), using Fundulus, demonstrated an in- 

 creased resistance to heat and cold after continued ex- 

 posures to extreme temperatures. Davenport (1897) sug- 

 gested that increased tolerance to extreme temperature 

 may result from lowering of the water content of proto- 

 plasm. Miss Behre believes that an adjustment of meta- 

 bolic rate is an important factor in acclimatization. 

 Hathaway (1928) has shown that continued exposures to 

 high oir low temperatures progressively raised or lowered 

 the limits of tolerance of several species of fishes. He 

 states%lso that fishes which inhabit shallow water (bass, 

 bluegill, and sunfish) undergo a change of tolerance by 

 acclimatization much more readily than the perch, which 

 is typically an inhabitant of deep, cool water. 



Tolerance. — Many animals are very susceptible to 

 temperature changes and succumb if the thermal varia- 

 tion of their environment exceeds a few degrees; other 

 animals resist or endure extreme temperature changes. 

 Those in which the optimum range is narrow are said 

 to be stenothermal. Those which endure the great varia- 

 tion in external temperature are eurythermal. Land 

 animals as a rule are eurythermal to a greater degree 

 than those in the water. In the latter group are such 

 animals as the corals, which live largely in tropical seas. 

 The carp and the goldfish are examples of freshwater 

 forms which usually feed only between about 6 deg. and 

 30 deg. C. Eurythermal animals are illustrated by forms 

 like the brine shrimp {Arternia salina). This crustacean 



