Nitrogen Excretion 197 



particularly the gills, is permeable to NH3 and urea; excretion of these sub- 

 stances is not altered by plugging the excretory pores. The kidneys excrete 

 dyes and other less diffusible substances, whereas the gills are the chief route 

 for ammonia excretion. "^ Analysis of the urine is, therefore, inadequate to 

 measure total nitrogen excretion. Excretory products of terrestrial Crustacea 

 have not been examined, but such investigation would be of interest. 



Earthworms are intermediate between aquatic and terrestrial life. The urine 

 collected from worms in moist air contains more urea nitrogen than ammonia 

 nitrogen, -^ but the excretion from earthworms immersed in water is am- 

 monotelic. -^ An explanation of this difference might be that much ammonia 

 is given off from the intestine, not as urine. A better explanation is given by 

 recent observations ^* that in normal fed earthworms urea constitutes less 

 than 10 per cent, whereas after starvation urea becomes as much as 86 per cent, 

 of the nitrogenous excretion. It would be of interest to know the effect of 

 water supply on the urea/ammonia ratio. 



Fishes, Ammonotelic and Ureotelic; Trimethy lamina Oxide Excretion. 

 Ammonia is commonly excreted by animals which have plenty of water to 

 eliminate and which live in an aqueous medium. This holds for fresh-water 

 teleosts, which are ammonotelic, although they also excrete some urea; most 

 of their nitrogenous wastes diffuse out across the gills, and less nitrogen leaves 

 by the urine. ""• **^ Marine teleosts, on the other hand, are confronted with a 

 serious osmotic problem of retaining sufficient water (Ch. 2). They still 

 excrete considerable ammonia, but they also excrete some urea (Table 30). 

 In addition, some marine teleosts excrete as much as one third of their nitrogen 

 as trimethylamine oxide, a soluble nontoxic substance. This substance is 

 absent from the muscles of several fresh-water fish (eel, carp), but is present 



in the muscles of all marine fish tested Pletironectes, Conger, Gadiis, and 



others (Hoppe-Seyler, quoted by Grafflin and Gould ^^ ). Trimethylamine 

 oxide has been reported from cephalopods, but only in marine teleosts where 

 water must be conserved is this substance an important excretory product. 



Elasmobranch fish are ureotelic, and marine species retain a concentration 

 of 2-2.5 per cent of urea in the blood. By this high urea concentration they 

 maintain themselves hypertonic to their medium (Ch. 2). All tissues of a 

 dogfish except brain and blood are able to synthesize urea.^^ "Like marine 

 teleosts, the marine elasmobranchs convert a part of their waste ammonia to 

 trimethylamine oxide. This suggests that they have at some time in the past 

 experienced the same osmotic difficulties as confront the marine teleosts of the 

 present day .... But subsequently, it appears, they evolved the still better 

 trick of making urea, which is even less toxic than trimethylamine oxide, and, 

 by retaining enough of it in their tissues, managed in the end to turn the 

 osmotic gradient to their advantage instead of their detriment.'" '' Excess urea 

 is excreted, largely through the gills. Dogfish eggs contain a large amount of 

 deposited urea; the embryos, however, synthesize quantities of urea, which is 

 stored in the yolk. 



Nitrogen excretion in the lungfish, Protopteriis aethiopicus, has been studied 

 by Smith. '*" When this fish is active in fresh water it excretes nearly three 

 times more nitrogen as NH.s than as urea. When estivating in its mud cocoon, 

 however, it excretes urea which may accumulate to the extent of 1-2 per cent 

 of the body weight in a year; muscle urea increases by some seven times. When 



