§ 5.3 BALANCE OF MONOVALENT ELECTROLYTES AND WATER 207 



increased diuresis, either by a contractile vacuole or some other 

 form of excretory organ. Movement of both salts and water may 

 be controlled by hormones. 



In the second situation, the salts in the blood and tissues tend to 

 become concentrated by water loss, with or without simultaneous 

 salt endosmosis. This can occur when animals migrate to land 

 w^here dehydration and thirst are the main dangers, even though 

 water losses through the skin may be virtually abolished; for 

 losses through the respiratory surfaces and by excretion are 

 inevitable. The main response to these water losses is hormonally 

 controlled antidiuresis, involving reabsorption of water from the 

 urine. In addition to this, the Amphibia and some insects can 

 absorb water through the skin. Dehydration and salt increase also 

 occur when animals move into any hypertonic aquatic medium; 

 for instance, when teleosts migrate back to the sea, after having 

 acquired a lowered salt content during their sojourn in fresh water. 

 Here the necessary dilution and proportion of the salts in the blood 

 is maintained, chiefly by active excretion of sodium and chloride 

 ions either through the gills or the kidneys; there may also be 

 antidiuresis, but the action of hormones that control this in fish 

 is less clear than in tetrapods. 



Although in a stable environment little control may be required, 

 most animals maintain a dynamic equilibrium between these two 

 extremes ; for the normal response to either hydration or dehydration 

 (if carried to the limit) would lead to the opposite state being 

 reached in the tissues. Control that allows of a variable response is 

 demanded in particular of those animals which migrate periodically, 

 like some estuarine crustaceans and fish, between fresh waters and 

 the sea. They must be able to contend alternately with flooding of 

 their tissues with water in one environment, and their dehydration 

 by the high salt concentration of the other. In most of these cases, 

 however, all too little is yet known of the part played by hormones. 



In those mammals, where the position is best understood, 

 four hormones are probably concerned: one antagonistic pair to 

 control the movement of salts and another pair for the water bal- 

 ance. Complete elucidation is still difficuh, not only because the 

 movements of sahs and water are so intimately linked by osmotic 

 forces, but also because the associated hormones, active in_a_given 



