1138 PROTEIN METABOLISM [pt. iii 



Another interesting fact which fits in closely with this point of 

 view is that, according to the researches of Przylecki who investigated 

 a large variety of organisms, no animal possesses both uricoligase and 

 uricase. In other words, all those animals which possess the power 

 of making uric acid from amino-acids cannot destroy it, and all those 

 which can destroy it have no power of making it, other than from 

 purines. This certainly looks as if the power of formation of uric acid 

 jfrom amino-acids was an adaptation of evolutionary value, for if 

 uricase and uricoligase were often present together, it would be 

 difficult to suggest that any special advantage was to be gained in 

 certain circumstances from the manufacture of uric acid. 



In Table 163 are collected together a number of figures for nitrogen 

 excretion in various animals. All the older work has been excluded, 

 and, as far as possible, only quantitative investigations of the per- 

 centage distribution of the excretory nitrogen appear. As a general 

 rule, the marine invertebrates excrete most of their nitrogen as am- 

 monia — a simple and easy procedure, considering their environment. 

 But with the increasing complexity of the body, ammonia excretion 

 disappears, for it is incompatible with a kidney, even in a very 

 undeveloped form. Excretory structures — structures which have to 

 live, as it were, in an excretory atmosphere — cannot deal with 

 highly alkaline liquids, and the great disadvantage about simple 

 ammonia excretion is that a constant supply of acid is required to 

 neutralise it. This acid is nothing but waste, and so among the marine 

 invertebrates themselves we see urea superseding ammonia. Among 

 the invertebrates the only ones at present known which have a high 

 percentage of uric acid are the pulmonate gastropods, the snail and 

 the slug, which live on land and have terrestrial embryos. In this 

 connection the concretions of urates in certain snails, which, according 

 to McKinnon, contain bacteria capable of breaking down uric acid, 

 are of special interest. Delaunay himself pointed out that the in- 

 vertebrates could be separated into an aquatic and a terrestrial 

 group, the former excreting much ammonia and the latter Httle, 

 and he also remarked on the association between uric acid and 

 terrestrial life. But this might remain enigmatic if we did not 

 consider the needs of the embryo; able, in the one case, to get rid 

 of its nitrogenous waste easily into the surrounding water, and 

 forced, in the other case, to keep it close at hand in very restricted 

 quarters. Delaunay's generahsation alone would not explain the 



