EXCRETION 289 



animals to dry terrestrial conditions both in the egg stage (they lay en- 

 closed, cleidoic eggs) and after hatching. Because of its low solubility, 

 uric acid in solution does not cause toxaemia, and by excreting it in solid 

 form water is conserved for other metabolic functions. The same factors 

 favour uric acid as an excretory product in embryonic stages, when it can 

 be stored as solid material until the time of hatching. Uricotelism must 

 have proved advantageous to those species of birds which have re-invaded 

 the seas. Pelagic maritime birds, of course, have no access to fresh water 

 for drinking purposes. Since the salt content of their blood is only about 

 one-third that of sea water, maintenance of water balance is an important 

 factor in their physiology and, for all practical purposes, presents demands 

 as acute to maritime avian species as those obtaining in arid desert regions. 

 Uricotelism, by reducing the water required in excretion, has been a 

 favourable character under conditions of overt or concealed water scarcity. 

 The osmotic relationships obtaining in marine reptiles probably can 

 be bracketed with those found in birds but, unfortunately, very little is 

 known of the physiology of those forms. Among sea-snakes (Hydrophiidae), 

 some are oviparous (Laticaudinae), others viviparous (Hydrophiinae), 

 and one species, Pelamis platurus, is wholly pelagic in habit. Presumably 

 these forms, of recent evolutionary origin, have retained the uricotelism 

 of terrestrial ophidians. The green turtle Chelonia mydas excretes both 

 urea and ammonia as major nitrogenous end-products, and possibly has 

 undergone little alteration in its mode of nitrogen excretion during pro- 

 gression towards a maritime existence (Table 7.2). Terrestrial forms, on 

 the other hand, appear to be in the process of evolving towards uricotelism 

 at the present time. Marine turtles bury their eggs on sandy shores and 

 during development the production of urea predominates over uric acid, 

 as in the adult. Total nitrogen actually diminishes, and it appears that the 

 turtle egg absorbs water from the moist sand in which it lies and gives 

 off end-products of nitrogen metabolism through the shell (22, 26, 32, 33, 50). 



Ureotelic Fishes and Excretion of Trimethylamine Oxide 



A second mode of nitrogen excretion which has appeared among 

 vertebrates is ureotelism. Teleosts tend to be ammonotelic but many species 

 produce appreciable quantities of urea, amounting to a tenth or a fifth 

 of total excretory nitrogen in some marine forms. A striking peculiarity of 

 marine teleosts is found in the large amounts of nitrogen excreted as 

 trimethylamine oxide (TMO). Trimethylamine oxide is found in muscles, 

 blood and other tissues: concentrations are highest in marine species, and 

 much smaller amounts occur in freshwater teleosts. It also occurs in 

 molluscs and Crustacea (up to 300 mg% in muscles of the lobster Homarus), 

 but marine teleosts alone excrete it as an important nitrogenous end- 

 product (Tables 7.2, 7.3). Variations in concentration of TMO are 

 reported in different species of teleosts. Several factors affect TMO 

 content, among which may be enumerated environment, season, size, age, 

 etc. Thus arctic species of teleosts tend to have higher concentrations of 



M.A. — 10 



