No. 599] OSMOTIC PRESSURE OF THE BLOOD 



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of this relationship that Quinton felt justified in making 

 the statement that the marine invertebrate while anatom- 

 ically closed to the external medium, is yet physiologically 

 open to it. That functionally speaking the marine in- 

 vertebrate is still freely exposed to the sea without, which 

 still practically ebbs and flows through its body. 

 Macallum ('10) says: 



In Limulus, the amount of total salts in the blood, 2.982 per cent., 

 approaches Hint of the sea water, — which may be found along the Atlantic 

 coast. At St. Andrews, New Brunswick, the total salts of the seawater 

 collected in April were 2.417 per cent., but in sea water collected in 

 August, 3.165 per cent. In the blood of the lobster, the total salts as 



. given above for the salinity of the sea water at St. Andrews where the 

 lobsters from which the blood was taken were obtained. The blood of 

 Limulus is but slightly modified sea water. It would appear as- if the 

 sodium chloride of sea water passes freely into the blood of the lobster 

 till the sodium chloride concentration in both is approximately balanced. 



This agrees entirely with the work of Quinton. For 

 some reason, the marine invertebrate has not been able to 

 * keep the sea out. One asks why the question of the per- 

 meability of membranes of fishes to salts is of such inter- 

 est to the comparative physiologist? One answer is that 

 impermeability represents independence of the sea the 

 osmotic pressure of which differs so much from that of 

 fish blood. And this independence is not to be found 

 among the marine invertebrates. 



As shown above, elasmobranch blood possesses the 

 same osmotic pressure as that of the marine invertebrate 

 and that of the sea without. But analysis shows that the 

 osmotic pressure of elasmobranch blood is due to entirely 

 different causes. For example, what is the salt content 

 of elasmobranch blood? It should contain about 3.22 per 

 cent, salts in order that its total osmotic pressure be due 

 to salts. But Rodier ('99) found that the blood of elas- 

 mobranchs did not contain over 1.7 per cent, sodium 

 chloride. Dakin ( '08) found the blood of the dogfish to 

 contain but 1.45 per cent, sodium chloride. My analysis 

 of the blood of another species, Mustelus, at Woods Hole 



