300 THE BIOLOGY OF MARINE ANIMALS 



collected separately from the urine. In freshwater carp the total excretory- 

 N during fasting amounted to about 50 mg/kg/day. The majority of this 

 nitrogen escaped via the gills, in amounts six to nine times as great as that 

 discharged through the ureters. Branchial excretion consisted of readily 

 diffusible substances, ammonia, urea and amines. Together these sub- 

 stances account for about three-quarters of all the nitrogen lost across the 

 gills. Ammonia-N forms some 50-60% of the total, urea-N and amine-N 

 some 5 % each. The remaining nitrogen is present in trimethylamine oxide. 

 Less diffusible substances are excreted by the kidneys, namely creatine, 

 creatinine and uric acid. Since only a small amount of nitrogen is lost by 

 this route, however, these substances form only a minor proportion of 

 the total excretory-N. 



Although comparable experiments have not been carried out on marine 

 teleosts, it is probable that a similar situation exists in these animals since 

 the nitrogenous output of the kidneys forms only a small proportion of 

 total nitrogen excreted. Ammonia forms 50-60% and urea 10-20% of 

 the total excretory nitrogen of marine species. In the urine, however, 

 ammonia levels tend to be low (around 1-2 %), while creatine and TMO 

 together may constitute two-thirds of the nitrogen lost by this route (6). 



The chief excretory end-product in selachians is urea. In the freshwater 

 sawfish Pristis the total output of urea together with ammonia-N amounts 

 to 450 mg/kg/day, of which three-quarters is urea. The branchial mem- 

 branes, it has been noted, are relatively but not absolutely impermeable 

 to urea, and about 75 % of the total urea is excreted extrarenally, across 

 the gills. The gills are readily permeable to ammonia, most of which (about 

 90%) escapes through this route. In selachians as in teleosts, therefore, 

 most of the excretory-N is discharged extrarenally. Nitrogen output in the 

 urine of selachians is relatively low, about 150 mg/kg/day. Most of this 

 can be accounted for as urea (some 80-90%), the remainder being repre- 

 sented by low levels of ammonia, creatine, etc. Both urea and TMO 

 are conserved by the kidney (47, 48). 



In vertebrates the coelomoducts have given rise to a specialized kidney. 

 The unit of the vertebrate kidney is the nephron consisting of a corpuscle 

 and tubule leading to the ureter. The corpuscle is made up of a Bowman's 

 capsule into which projects a glomerulus of blood capillaries. This is 

 essentially a filtration unit for filtering off blood plasma free of proteins. 

 Primitive vertebrates were apparently freshwater animals, hyperosmotic to 

 the environment, and this kind of kidney in effect serves to pump out excess 

 water. The governing factor in filtration is arterial pressure. All dissolved 

 substances in the blood — salts, glucose, amino-acids and nitrogenous 

 end-products — are likewise passed into the capsule. The tubule, especially 

 the proximal convoluted section, alters the composition of the glomerular 

 filtrate, resorbing useful metabolites and salts and excreting other sub- 

 stances, processes which govern the composition of the urine. 



As outlined on previous pages, the necessity for water conservation has 

 brought about alterations in kidney function in marine fishes (p. 53). 



