EXCRETION 



293 



consists of NH 3 . The level of nitrogen excretion is considerably lower in 

 terrestrial than in freshwater, littoral and marine species (Table 7.5). Urea 

 and uric acid, at most, form trivial end-products, and amino-acids seldom 



TABLE 7.4 



Uric Acid Content of Nephridia of Snails and Slugs 



(mg/g dry weight) 



TABLE 7.5 



Nitrogen Excretion in Isopods and Amphipods 



Mean values for total non-protein nitrogen excreted 



(From Dresel and Moyle (10)) 



exceed 10%. In these groups excretory adaptation to terrestrial conditions 

 has taken the form of reduction of nitrogen metabolism rather than of 

 transformation of NH 3 to other less toxic products (9, 10). 



Among decapod crustaceans the shore crab Carcinus maenas shows no 

 increase in the relative amounts of urea and uric acid excreted over sub- 

 littoral species. Excretion in high-littoral and terrestrial species has not 

 been investigated. 



ENZYMES INVOLVED IN NITROGEN EXCRETION 

 Protein Degradation 



Protein degradation and synthesis of particular excretory products are 

 catalysed by enzymes at all stages. Amino-acids are deaminated with the 



