Nitrogen Excretion 191 



Those animals which excrete significant amounts of amino acids are inefficient, 

 in that they lose potential metabolites. Animals are called ammonotelic when 

 nitrogen (from proteins) is excreted predominantly in the form of ammonia, 

 ureotelic when urea is the main product, uricotelic when the main product is 

 uric acid. The correlation between excretory products and habitat of adult or 

 embryo is so good that nitrogen excretion appears to be an important adaptive 

 character. In the series ammonia, urea, uric acid there is decreasing toxicity. 

 Animals whose eggs are protected against water loss (cleidoic eggs) store non- 

 toxic embryonic waste products and the adults tend to excrete the same 

 products, whereas animals whose embryos can excrete freely into surrounding 

 water (noncleidoic eggs) excrete products which may be more toxic."*' 

 Embryonic habitat is one but not the only factor determining nitrogenous 

 excretion. 



The percentage nitrogen excretion in various forms is given in 1 able 30. 

 Data in this table have been selected from what appear to be the more reliable 

 of the many papers. Detailed summaries are to be found in Delaunay,-'' -■^- -* 

 Baldwin, ''•'' Ncedham, '"' Florkin, •''■''• -^^ and Heidermanns. ""' 



Ammonotelic Animals. Table 30 shows that aquatic invertebrate animals 

 excrete a large percentage of their nitrogen as ammonia. This is a soluble sub- 

 stance which diflFuses readily across most body surfaces into water. In Protozoa 

 some early measurements were invalidated by the presence of extraneous 

 organisms, particularly bacteria, and reports of any quantity of urea and uric 

 acid excretion are to be doubted. Also the amount of ammonia varies widely 

 according to diet. In Paramecium, for example, 91 per cent of excreted nitro- 

 gen was NH:{ when the animals had not been fed, 55 per cent was NH^ when 

 they had been fed glycine, and no NH.s was excreted when they had been fed 

 starch; no urea, uric acid, creatinine, or whole protein was found, but some 

 unidentified nitrogenous substance was excreted. ^^ These results confirm 

 the earlier work of Emery-"*' and show that Weatherby's observations"'^ of urea 

 were not correct. 



Spirostomiim amhiii^uitm excretes large amounts of NH.i when the oxygen 

 tension is less than equilibrium with air. "^^ Glaucoma eliminates the nitro- 

 gen from bacterial food as ammonia about six hours after ingestion. -"' Pro- 

 tozoa can utilize amino acids from the culture medium; the extent to which 

 they excrete amino acids from ingested protein has not been shown. 



Actinians excrete much ammonia. -*"* Various echinoderms, starfish, sea 

 urchins, and sea cucumbers, are ammonotelic, but they also give off consider- 

 able amounts of amino acids into the water. -"• -•'• -^ Sipiinculus excretes 

 ammonia predominantly, as does the fresh-water leech. Polychaete worms are 

 ammonotelic, although some urea and traces of uric acid are reported in 

 Arenicola coelomic fluid. ^'^- '" 



Among molluscs the cephalopods Sepia and Octopus excrete most of their 

 nitrogen as NH^. The pelecypods which have been examined, both marine 

 and fresh-water (except Mytilus^, are also ammonotelic. 1 he gastropod 

 Aplysia is ammonotelic, but excretory products in other gastropods are diverse, 

 as will be shown later. 



Crustaceans give off much amino acid nitrogen, but the predominant excre- 

 tory product of marine and fresh-water crustaceans is ammonia. It has been 

 shown for Eriocheir, and probably it is true for others, that the body surface, 



