TECHNICAL NOTES AND BIBLIOGRAPHY 253 



as abundantly as does Prcftopterus but allegedly does 

 not form a cocoon {74}. On the mucous glands of Neo- 

 ceratodus, which does not estivate, the writer has no in- 

 formation. Perhaps the mucus secreted by Protopterus is 

 unique in its capacity to form a parchment-hke cocoon. 

 With respect to the statement that Neoceratodus dies 

 if removed from water to air, one wonders if it would 

 die if kept in a moist atmosphere; certainly Protopterus 

 will die in dry air unless enveloped by the cocoon, and 

 the failure of Neoceratodus to estivate does not prove 

 the inadequacy of its lungs as respiratory organs. 



The origin of excreted ammonia in the lower animals 

 is obscure. In the course of protein metabolism in the 

 mammal, nitrogen is first degraded to ammonia, but this 

 compound is relatively toxic and it is converted by the 

 hver into the neutral and nontoxic urea. Such ammonia 

 as is excreted in the urine of mammals in the mainte- 

 nance of acid-base balance is not that produced directly 

 by protein metabolism, but ammonia that is manufac- 

 tured de novo from glutamine and other amino acids 

 by the renal tubules and added to the urine on its way 

 out of the body {62, p. 401}. 



The recorded data {54} show that many invertebrates 

 excrete their nitrogen chiefly in the form of ammonia, 

 and they have consequently been called 'ammonoteHc' 

 on the assumption that this predominance of ammonia is 

 indicative of a unique type of protein metabolism, dis- 

 tinguishing them from 'ureotehc (urea-excreting) forms. 

 However, the possibility that this ammonia may be 

 formed peripherally and facultatively has been wholly 

 ignored by students of invertebrate physiology, and we 

 beUeve that the characterization *ammonoteUc' is imwar- 

 ranted in many instances, and sometimes misleading— 

 as for example in the earthworm, which excretes am- 

 monia when fasting and urea when fed. 



In the fishes considerable ammonia is excreted by the 

 gills as weU as the kidneys. In our initial study of nitro- 



