202 



Coviparative Animal Physiology 



before they produce urea, and chicks pass through the entire evolutionary 

 series of ammonia, urea, and uric acid. 



Nitrogen excretion is a labile character. It appears to have changed during 

 the evolution of several groups according to their habitat. The best example 

 of such phyletic change in form of nitrogenous product is in the gastropods. 

 Also the marine teleosts excrete more of their nitrogen as urea than do their 

 fresh-water relatives, and the earthworm excretes more urea than do aquatic 

 annehds. Even individual animals change their excretion with diet and with 

 osmotic stress. The lungfish stores urea in estivation and excretes ammonia 



1 — \ r 



URIC ACID 



ALLANTOIN 



ALLANTOIC ACID 



UREA 



■Hr4 



^MMnH^ 



AMMONIA 



^Hh-irH^ 



— o 



2 E 



C t. — 



'— » a> 

 O t- I- 



Z « 



5 o 



Fig. 36. Comparison of end-products of purine and protein catabolism. From Florkin 

 and Duchateau.'' Unbroken line, protein breakdown; broken line, purine breakdown. 



when active; the bug Rhodnius excretes urea for several hours after a blood- 

 meal, and then uric acid. Ammonia is the principal nitrogenous product of 

 amino acid breakdown in ammonotelic animals; it comes from glutamine in 

 mammals. 



Recent studies on microorganisms indicate their ability to adapt enzymatical- 

 ly to different substrates. Nitrogen excretion in animals is likewise adaptive. 

 Most animals excrete several products, but one predominates. The enzymes 

 for their production may be present in varying degrees, and several enzymatic 

 routes may lead to the same product. Which route and which product become 



