120 Water 



kangaroo rat of the Arizona desert may similarly secure the whole of 

 its water supply as a metabolic by-product of its food (Schmidt- 

 Nielsen, 1950). 



Just as important as methods for obtaining water are means for 

 resisting desiccation. Many of the same general types of adaptations 

 having this effect have appeared in the course of evolution amongst 

 both plants and animals. First and most obvious is the possession 

 of a more or less impervious covering for the body. The cuticle of 

 plants, the skin of birds and mammals, the exoskeleton of insects, and 

 the mucous secretion of mollusks are examples. Another means for 

 avoiding the excessive loss of water is the reduction of body surface. 

 The leafless plants of the desert are an instance of this plan carried 

 to an extreme. Other plants fold their leaves or turn them edgewise 

 to the scorching heat of the sun. Many terrestrial invertebrates 

 bury part or all of their bodies to reduce evaporation. It is well 

 known that the termites which attack our houses are killed by exposure 

 to dry air, but these pests can cross exposed foundations by construct- 

 ing runways within which protection from excessive evaporation is af- 

 forded. In arid regions many of the smaller animals are active only 

 during the night when drying is less intense. 



Water may also be saved by reducing the loss in respiratory and 

 excretory systems. Respiratory organs are often enclosed within a 

 cavity as has already been mentioned in relation to the gills of land 

 crabs. Lungs are commonly located well within the body with only 

 a small opening to the exterior. The branching tubular tracheal sys- 

 tem of the insects is another example, and in many forms the size of 

 the external openings of the tracheae may be reduced by the spiracles. 

 In a similar way evaporation from the internal cavities of the plant 

 leaf is restricted by the guard cells of the stomata. 



Reduction in the amount of liquid needed for excretion is a physio- 

 logical adaptation that also aids the body in conserving water. Many 

 animals with little available moisture in their habitats and in their 

 food produce dry faeces. Furthermore, in the interest of saving 

 water, it is desirable for land dwellers to dispose of their nitrogenous 

 wastes with the minimum quantity of liquid urine. Mammals ac- 

 complish something in this direction by reabsorbing part of the water 

 in the urine before it is excreted. The ultimate attainment in this 

 matter is reached by those birds, reptiles, and insects that are able to 

 excrete nitrogenous wastes as solid uric acid practically without 

 water loss. 



If lack of water becomes still more acute, it may be necessary for 

 the organism to go into a dormant condition in which its need for 



