206 EPILOGUE 



The function of excretion, like that of osmotic regulation, is the 

 maintenance of a constant internal environment within the animal 

 body. An aquatic animal surrounded by a large volume of water 

 can excrete toxic nitrogenous compounds with impunity, but the 

 necessity for water economy in terrestrial forms generally requires 

 the excretion of some compound less poisonous than ammonia. 

 Woodlice are essentially ammonotelic since over half of their 

 soluble non-protein nitrogen is excreted in the form of ammonia, 

 but the retention of this primitive character has been attended by 

 a general suppression of nitrogenous metabolism. In higher insects, 

 as in myriapods, birds and reptiles, uric acid is the chief excretory 

 compound. This substance has in the first place been evolved in 

 response to the conditions of embryonic life. Uricotelic meta- 

 bolism is correlated with the possession of 'cleidoic' eggs provided 

 with water and enclosed in a comparatively impermeable shell. 

 Ammonia is toxic as we have seen and is only suitable as an 

 excretory compound for eggs developing in water: it cannot be 

 converted to urea as this would cause ureamia, and in excess upset 

 the osmotic relationships of the egg. Hence uric acid, a non-toxic, 

 highly insoluble substance, has been evolved which can accumulate 

 within the egg without causing any ill effects. 



The chief excretory compound of the Arachnida is guanine 

 which, like uric acid, is very insoluble, but the physiology of its 

 excretion has not been studied in the same detail. 



Throughout the Arthropoda entirely mechanistic patterns of 

 behaviour are found, involving rigid but comparatively simple 

 responses to the stimuli of the environments in which they live. 

 Indeed it would seem that the typical arthropodan nervous system 

 is capable only of stereotyped reflex behaviour patterns and that 

 this perhaps restricts them no less than their rigid integument! 



Nevertheless, within the limits imposed by their basic morpho- 

 logy and physiology, every conceivable type of modification and 

 specialisation occurs within the Arthropoda and herein lies the 

 great interest of these animals to the serious student of natural 

 history. 



