RESPONSES OF POIKILOTHERMS TO VARIATIONS 27 



thrives in salt marshes and in pools which are opposed 

 to the full radiating and evaporating power of the sun 

 for the purpose of making salt. Centipedes are reported 

 to range from hot plains and to the snow-covered peaks 

 of mountains on the island of Cyprus. The common 

 European eel is found from Ireland to the Nile. The 

 American eel is found from the Caribbean to the Great 

 Lakes. The water snake {Tropidonotus natrix) is found 

 from Sweden to Algeria. Many other species of animals 

 range through a variety of temperature conditions. 



The extreme minimum temperatures which poikilo- 

 thermal animals can tolerate are near the freezing point 

 of water. Below this, metabolic activity ceases. Some 

 marine animals in arctic seas live in water which is a 

 degree or two below zero and are protected from freezing 

 by the salts in their body fluids. Extremely low tem- 

 peratures during a considerable part of the year do not 

 prevent the successful existence of land animals if there 

 is a warm season to allow for growth and reproduction. 



The problem of toleration of high temperatures is 

 unlike that of low temperatures for the reason that the 

 effect of the former is principally chemical and usually 

 involves a permanent change in the living protoplasm, 

 while that of the latter is physical and produces a sus- 

 pension of activities which are resumed with the return 

 of favorable circumstances. The toleration of high tem- 

 peratures is usually accomplished by means of altera- 

 tions in the physico-chemical state of the protoplasm. 

 Insects, for example, lose water in hot surroundings. 

 Brues (1927) reports many insects living at 30 to 40 

 deg. C. in hot springs. He found ''blood worms" devel- 

 oping at 50 deg. C. Snails, amphipods, and isopods were 

 also observed living in similar conditions. Vertebrates 

 were found to be rather uncommon, rarely survived in 



