1910.] 



Blood in Vertebrates and Invertebrates. 



619 



one may perhaps be in a position to speculate safely on such matters only 

 when careful analyses have been made of the blood plasma of representatives 

 of all the classes of Vertebrates, and specially of the fresh-water fishes. 



It is, nevertheless, certain that the inorganic salts have increased very 

 slowly, much more slowly in the blood plasma of Elasmobranchs than they 

 have in the sea water and, further, that the urea and ammonia salts have 

 attained an extraordinary concentration. The explanation for this slow 

 increase of the salts cannot be found in any inactivity of the epithelial cells 

 of the mucosa of the intestinal tract, for Erwin Herter* found in the urine 

 of Scyllium catulus 0'0415 per cent. Ca and 0"1416 per cent. Mg, as compared 

 with 0"0464 per cent. Ca and 01421 per cent. Mg in the sea water of the 

 locality (Naples) from which the animal was taken. The total amounts of 

 the sodium and potassium were not determined, but the chlorine was esti- 

 mated and found to be 1'3543 per cent., whereas in sea water at Naples it is 

 2"1142 per cent. This discrepancy may easily be explained, for the hydro- 

 chloric acid in the gastric juice in Scyllhiin catulus, according to Eichet,t is 

 from 0'69 to 1'29 per cent., and as the acid of the gastric juice is neutralised 

 in the intestinal tract the chlorine thus contained may pass out with the 

 intestinal excreta, and not by way of the kidneys, while its place in the renal 

 excretion is taken by phosphoric acid, which is exceedingly abundant in the 

 urine of this form. The correspondence between the sea water and the urine 

 as regards the amounts of calcium and magnesium would seem to indicate 

 very clearly that the intestinal tract, and, perhaps, also the gills, in absorbing 

 fluids make little distinction, if any, between the water and the salts of the 

 sea ; in other words, sea water finds its way into the blood stream of the 

 circulation in the Elasmobranchs. 



It follows from this that it is the kidneys which determine the inorganic 

 composition of the blood plasma in these forms. The kidneys not only 

 regulate the total quantity of salts in the blood plasma, but they also 

 maintain the ratios, almost as they obtain in higher Vertebrates, existing 

 in the plasma between the sodium, potassium, calcium, and magnesium, even 

 after long ages of exposure to the ever increasing saline concentration of the 

 sea. That exposure, it is true, has had its result in increasing the total salts 

 of the blood plasma, but the increase is but sufficient to bring the A due to 

 them up to a little more than half of the A of the sea water of the habitat, 

 or of the total A of the plasma. 



It is in this respect that the dogfish differs completely from Limulus, 



* 'Mitth. Zool. Stat, zu Neapel,' 1892, vol. 10, p. 342. 



t ' Journ. de TAnat. et de la Physiol.,' vol. 14, p. 170 ; also ' Comptes Rendus,' vol. 86, 

 p. 676. 



