THE AMPHIBIA 99 



when they left it. Against these hazards they evolved 

 an additional defense in the form of a mechanism, as yet 

 poorly understood, by which water absorption through 

 the skin can be functionally increased or reduced. This 

 mechanism is imder the control of another pituitary hor- 

 mone, the water-balance' hormone, which acts to in- 

 crease the rate of water absorption so that a dehydrated 

 frog or toad can rehydrate itself quickly when it returns 

 to water, perhaps even to a moist nest, and yet does not 

 lose water excessively when in the air. The absorption 

 of water through the skin is apparently not an active 

 process, but one depending solely on the passive diffu- 

 sion of water, and therefore it is inferred that the pitui- 

 tary water-balance' principle works by increasing the 

 permeability of the skin to water. It is now generally 

 accepted that changes in permeability are effected by 

 changes in the size of pores' through which v/ater can 

 freely diffuse, but the nature of these 'pores' is wholly 

 unknown. 



The Amphibia also absorb sodium chloride through 

 the skin, even when this salt is present in faint traces in 

 the external medium; but unhke the absorption of water, 

 salt absorption is an active process and one that prob- 

 ably operates specifically on the sodium ion, to the ex- 

 clusion of potassium, calciimi, and ammonia. There is 

 as yet no evidence that it is imder pituitary control. Thus 

 the skin absorbs salt (actively) and water (passively), 

 so that to obtam either salt or water the Amphibia do 

 not need to drink water, which apparently they rarely 

 do. 



As in all naked-skin animals, the outermost layer of 

 the epithehum of the skin tends to become hardened 

 or comified and the exposed cells lose their viability. 

 Where the skin plays an active role in salt and water 

 balance, as in the Amphibia, this process of comifica- 

 tion becomes a handicap— and consequently the animal 

 molts at intervals, casting off the dead outer skin in flakes 

 or large pieces. During molting, both the permeability 



