I5O J. F. MCCLENDOX. 



water. If the sea water was diluted with ] { vol. of isotonic 

 cane sugar solution, the salt content of the coelom fluid is very 

 little lowered in 4 hours, and only traces of sugar appear in it. 

 The result is the same with isotonic urea (which easily pene- 

 trates most plasma membranes). But the salt content of the 

 blood of elasmobranchs and teleosts is about half that of the sea. 



Botazzi and his colleagues observed that the osmotic pressure 

 of the blood of elasmobranchs is about equal to that of the 

 medium, the sails in the blood being supplemented by organic 

 substances, chiefly urea, of which there is 2-3 per cent. 



If elasmobranchs are placed in concentrated sea water, the 

 osmotic pressure of the blood rises, but the ratio of urea to salts 

 remains the same. G. G. Scott found that changes in the density 

 and osmotic pressure of the blood of elasmobranchs accompany 

 changes in the salt content of the medium. 



However, in marine teleosts as well as all fresh- water animals 

 which have been studied in this respect, both salinity and osmotic 

 pressure of the body fluids are very different from that of the 

 medium. 



The osmotic pressure of the blood of marine teleosts is about 

 half that of the sea, but in fresh-water teleosts it is still less 

 (but much greater than the fresh water). This indicates that 

 there must be a change in the osmotic pressure of the blood as 

 the fish ascends a river. Greene 1 observed that it took salmon 

 30-40 days to pass the brackish water, in which time they were 

 acclimatized to fresh water. After being in fresh water 8-12 

 weeks, the osmotic pressure of the blood was reduced only 17.6 

 per cent. This reduction may be partly accounted for by the 

 absorption of the osmotic substances in the blood by the sexual 

 glands. In harmony with this view is the fact that the osmotic 

 pressure of the blood of the female was reduced much more than 

 that of the male. One salmon, that was very weak and probably 

 dying, showed 32 per cent, decrease in A of blood. Sumner 2 

 observed that changes in weight and salt content of marine tele- 

 osts accompany, but. are not proportional to changes in the 

 medium. 



1 U. S. B. F.. 1904, XXIV.. 445; 1909, XXIX., 129; Jour. Ex[>. Zoo!., 1910. 

 IX. 



2 Bull. U. S. B. F., 1905, XXV., 53, and Am. Jour. Physiol., 1907, XIX., 61. 



