138 How Animals Changed 



character of protoplasms. At times they may be shown to consist 

 of loss of water, changes in metaboUc rate, or other measurable 

 variations, but the factors concerned in many of them are still un- 

 known. 



Animals which have entered brackish water from either sea water 

 or fresh water may be small in size (Schlieper, 1933) . In some of 

 these, oxygen consumption may decrease as much as 25%. The ac- 

 tivity of surface membranes and excretory organs may be inhibited. 

 Other animals that readily adjust themselves to changed salinities 

 usually use more oxygen in diluted sea water for a time, and may 

 become more or less "febrile." A marine animal that migrates from 

 the ocean into fresher water needs more energy and, if it cannot 

 supply it, is inhibited in various ways. Gunter (1947) claims that 

 marine animals tend to grow smaller in lower salinities. Andrews 

 (1925) tested the resistance of sixteen species of littoral animals to 

 variations in temperature, acidity, salinity, stagnation, and light. 

 He found that young animals usually died more quickly than old 

 when there were extreme variations, but that the former had greater 

 capacity for acclimatization if changes were gradual. Young ani- 

 mals probably died quickly on account of their surface-mass rela- 

 tions and more rapid metabolism. Old animals were less resistant 

 to acidity, probably on account of a more limited alkali reserve. 



Growth is probably an autocatalytic process (Robertson, 1923) . 

 Its rate changes abruptly at certain critical temperatures (Crozier, 

 1929) and in response to other environmental changes. Hence va- 

 riations in an animal's environment may cause changes in metab- 

 olism, lack of growth, or death. Rate of growth and metabolic 

 activity commonly change with age (Burge, 1921). Variations in 

 temperature alter equilibrium constants of chemical reactions and 

 the relative proportions of reacting materials. It is therefore usually 

 easier for an animal to adjust its activities to a lower than to a 

 higher temperature (Wardlaw, 1931). As would be expected, 

 aquatic animals vary greatly in their ability to resist the extreme 



