2/0 FROM FISH TO PHILOSOPHER 



mg. per loo cc. (the two fluids being identical in this 

 respect) in fed animals, but had a value of only 40 mg. 

 per 100 cc. in a fasting specimen of C. kempi. This high 

 plasma urea concentration may reflect the naturally oli- 

 guric state, but excessive tubular reabsorption of urea 

 in marine and other reptiles certainly invites further in- 

 vestigation. 



That the marine turtles ingest a little sea water in 

 captivity is indicated by significant concentrations of 

 magnesium and sulfate in the stomach, the concentra- 

 tions of these ions increasing through the anterior and 

 posterior intestine as the concentration of chloride de- 

 creases to neghgible values; the urine contains from 10.4 

 to 69.5 mM. per liter of magnesium, 49.5 to 101 mM. 

 of sulfate, 46 to 56 mM. of chloride, and 30 to 83 mM. 

 of phosphate. However, the intestinal fluid invariably 

 approaches the isosmotic state as it passes down the 

 gastrointestinal tract. The urine is slightly hypotonic to 

 the blood: AU/AP (°C.) = 0.70/0.76 in C. caretta and 

 0.60/0.66 in C. kempi. (A°C. is the osmotic pressure 

 as measured by freezing point lowering.) AU in other 

 specimens of C. kempi was 0.67, 0.65, and 0.64, AP, 

 0.77, 0.78, and 0.83. The data do not suggest that 

 Caretta drinks sea water as a source of free water, but 

 rather that it handles accidental gulps exactly as would 

 be expected in view of the presence of the nasal salt 

 gland (Chapter XI). We infer that the marine turtles, 

 hke the marine birds and mammals, subsist primarily 

 upon metabolic water derived from the food and do 

 not habitually drink sea water. 



The order Chelonia (= Testudinata) branched ofiE 

 from the terrestrial reptiles very early, and by the Upper 

 Jiu-assic had produced marine turtles which may have 

 been descended from fresh-water rather than terrestrial 

 forms, so that the order has possibly had a long fresh- 

 water experience permitting it to become neghgent of 

 water conservation. Moreover, many fresh-water diving 

 turtles possess anal sacs into which water is drawn for 



