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THE AMERICAN NATURALIST [Vol.L 



showed 1.424 per cent, sodium chloride. Fredericq ( '04) 

 found the blood of Scyllium to contain but 1.71 per cent, 

 salts, while Macallum ('10) found the blood of the dog- 

 fish, Acanthias vulgaris, contained 1.7739 per cent, sodium 

 chloride. In other words the sodium chloride content of 

 the blood of elasmobranchs will account for only about 

 half of its total osmotic pressure. Evidently a great 

 change has come about. "The difference between the A 

 of the serum and that due to salts of the serum depends," 

 as Macallum ('10) says, 



" on urea and other organic solutes." Urea is present in large quanti- 

 ties in the blood of elasmobranchs. 



Staedeler and Frerichs ('58) obtained as much as two ounces from the 

 the liver of a single shark. In '90 von Schroeder found that Scyllium, 

 another dogfish, contained blood with 2.6 per cent. urea. Rodier ('99) 

 computed that one third the osmotic pressure of the blood of elasmo- 

 branchs was due to urea. 



In '13, I found that Mustelus blood contained 1.55 per 

 cent. urea. Macallum ( '10) in Acanthias vulgaris found 

 an average of 2.026 per cent. urea. Due to dissociation, 

 the salts have twice the osmotic pressure, approximately, 

 as the urea, although the urea and salts are present in 

 about equal quantities. But the urea and salts are not 

 sufficient to account for the osmotic pressure of the blood. 

 The difference is due to the presence of ammonia salts, as 

 Macallum found. For example, he found 0.1727 per cent, 

 ammonia in the serum of the dogfish. This would fully 

 account for the remaining percentage of the depression 

 of the freezing point unaccounted for by the presence of 

 the salts and urea. So that we see, that while super- 

 ficially the elasmobranch resembles the marine inverte- 

 brate in the osmotic pressure of the blood, yet below the 

 surface a marked change has taken place. Several ob- 

 servers had noted that the osmotic pressure was slightly 

 greater than that of the sea water. This at least is 

 another indication that the equilibrium is not like that 

 existing between marine invertebrates and the sea. For 

 some reason the elasmobranch has lost in salts. Their 

 place has been taken by nitrogenous solutes. The con- 



