198 Comparative Animal Physiology 



the fish returns to water, urea is rapidly excreted and the excretion of ammonia 

 nitrogen increases to its characteristic high proportion. Dipnoans, then, shift 

 their metaboHsm from ammonoteHc to ureoteHc and back, according to the 

 available water. 



Ureotelic Animals: Amphibia. Nitrogen is excreted predominantly as urea 

 in the frog (Table 30), and the liver of both frog and toad can synthesize 

 urea. *^" Calculations of glomerular filtration and tubular reabsorption indi- 

 cate that some urea must be secreted actively by the tubule cells of some 

 amphibian kidneys. ^'■^- "- Young tadpoles excrete over 40 per cent of their 

 nitrogen as ammonia, but this declines rapidly to about 12.5 per cent, the adult 

 level (Bialaszewitz, quoted by Delaunay ^* ). 



Mammalia. Mammals, including the monotremes, are ureotelic. Embryonic 

 wastes diffuse readily into the maternal circulation; embryonic life is aquatic. 

 There is need for conservation of water in adults, however, and urea is con- 

 centrated by as much as 100 times by the reabsorption of water in the kidney 

 tubules. Low concentrations of ammonia are toxic, but urea is tolerated up 

 to 150-250 mg. per cent— ten times normal concentrations. ^^ Urea concen- 

 tration in the blood and urine depends on the rate of protein catabolism. 



Excretion Associated with Transition from Water to Land: Gastropoda. 

 The gastropods present an interesting series with respect to transition from sea 

 to fresh water to land, and accompanying transition toward uric acid excre- 

 tion.'^- ■*^' ''^' ''^^ ^^ Aquatic gastropods, particularly marine ones, excrete much 

 ammonia, terrestrial gastropods excrete little ammonia. The eggs of fresh- 

 water and terrestrial gastropods in general have cleidoic characteristics; some 

 gastropods are ovoviviparous. During development of the egg of Limnaea the 

 uric acid content increases from 0.5 mg. per cent in the cleavage stages, to 1.0 

 mg. per cent when the embryo occupies one fourth to one half of the egg case, 

 to 4.5 mg. per cent when the egg is about to hatch. ^ In hibernation the 

 nephridium stores uric acid to as much as three fourths of the organ weight. ''"' 

 **^ The uric acid content of the nephridium has been correlated with the 

 history of different species and type of egg (Fig. 34). '^^ Needham describes 

 Figure 34 as follows: The uric acid content of the nephridium in marine 

 operculates (lower left) is very low. Terrestrial snails (center top), on the 

 other hand, contain much uric acid, whether they are pulmonates or opercul- 

 ates. The transitional periwinkles (Littorinidae) (left side) are intermediate 

 in uric acid content of the nephridium. A marine pulmonate which is believed 

 to have made a secondary return to the sea (Onchidella celtica) contains prac- 

 tically no uric acid. The fresh-water species (right side) do not provide a clear- 

 cut series, but the operculate Hydrohia jenkinsi has little uric acid and has 

 entered fresh water from the sea relatively recently, whereas Bythnia, Palu- 

 dina, and Limnaea stagnalis contain much uric acid and probably have pene- 

 trated fresh water secondarily from land. Other snails are intermediate, for 

 reasons which are not clear. Snails are unable to produce urea by the ornithine 

 cycle and produce it only from the arginine of their food; hence their only 

 choice in protein degradation is ammonia or uric acid. This group of animals 

 merits further study, but the general trend is from ammonia to uric acid 

 excretion as osmotic stress increases. 



Chelonia. Turtles, like all vertebrates, can form urea by the ornithine cycle, 

 and they excrete much nitrogen as urea. In addition, their ammonia excretion 



