200 PHYSIOLOGICAL REGULATIONS 



gain of water equals its loss, such a state can be any one of a vari- 

 ety. For instance, the cow may be lactating or not (Atkeson and 

 Warren, '34) ; the horse may be working or not (Zuntz and Hage- 

 mann, 1898) ; the man may be on a high-protein diet or not 

 (Vozarik, '06). But none of these states makes the mammal's 

 turnover equal to the frog's; only forced drinking does that. 



The rates of turnover per unit of body weight are greater in 

 aquatic species than in terrestrial ones; in smaller than in larger 

 kinds ; in freshwater than in marine organisms. Among inverte- 

 brates, rates of water turnover are strictly known only for some 

 arthropods, protozoa, and the earthworm. Proportional to turn- 

 over, but regularly lower, are the rates of loss in weight in terres- 

 trial organisms recently denied access to water. These rates of 

 loss may be substituted for rates of turnover, being considered 

 subminimal values of turnover. In that way values for numerous 

 further species, especially of insects (Buxton, '37a; Gunn, '37), 

 become available. Extremely low rates of turnover are found, as 

 0.03% of Bo/hour in mealworm larva, and 0.01%. of Bo/hour in the 

 tortoise (Testudo) of Benedict ('32), rivalling MacDougal's ('12) 

 cactus that lost 0.0004% of Bo/hour. Granted that the losses are 

 evaporative, it is possible to compare the protections against evap- 

 oration enjoyed by these species, and per unit of believed surface 

 area. Then the mealworm larva becomes equal to the tortoise, 

 losing about 0.0003 grams/square centimeter and hour. These 

 particular species have the lowest known requirement for water. 



A deduction from the tenets of physical science would be that 

 any turnover of water occurs only with the transformation of some 

 energy. Otherwise water would move through a circuitous path 

 (through the organism) without fall of potential. Though gain 

 might be passive, loss would require work, or vice versa. Only 

 species without turnover may be suspected of degrading no energy 

 for water exchanges. Yet many have been the attempts to believe 

 that all the processes concerned in water metabolism occur with- 

 out energy exchange. It is sometimes forgotten that an oso- 

 mometer also transforms energy in approaching equilibrium by 

 transport of water. 



Rates of water turnover (in water balance) may be partitioned 

 among diverse paths (tables 20, 10, and 11). 



