(554 



THE AMERICAN NATURALIST [Vol. L 



sures they had acquired during their fresh-water experi- 

 ence. We can thus speculate that in some such way the 

 present osmotic pressures of the blood of marine and 

 fresh-water teleosts were acquired. "Whatever may be 

 the case with the marine and fresh-water teleosts, it is 

 more clearly indicated that the osmotic pressure of the 

 blood of terrestrial forms is derived from fishes which 

 lived in fresh water. The present day anadromous fishes 

 constitute all that remains of a movement which at one 

 time was far more general. 



The chemical composition of the blood throws further 

 light on the question. The osmotic pressure is due to 

 substances dissolved in the blood. These are chiefly salts. 

 Quinton ( '00) states that sodium chloride represents from 

 85 per cent, to 90 per cent, of all. the dissolved salts of the 

 blood. The sodium chloride content can be ascertained 

 from a study of the chlorides which are easily deter- 

 mined. Let us ascertain the changes in the sodium chlo- 

 ride content of the blood of the forms under discussion. 

 In the first place what is the total salt content of sea water. 

 According to Bottazzi ( '97) the total salt content of water 

 from the Mediterranean is 3.78 per cent. The water of 

 the ocean contains about 3.22 per cent, salts. Of course 

 there is some variation. The percentage of salts in fresh 

 water is very small, 0.05 per cent. (Sumner, '05). What 

 is the percentage of salt of the blood of forms living in 

 the sea? Quinton ('00) made forty-nine determinations 

 of the sodium chloride content of the hemolymph of ten 

 species of marine invertebrates belonging to five different 

 groups and found that these averaged 3.24 per cent. He 

 made 26 determinations of the sodium chloride content of 

 the sea water in which these forms lived, and found that it 

 was about 3.31 per cent. According to these researches 

 of Quinton, the blood of the marine invertebrate contains 

 about the same percentage of salts as the water in which 

 they live. Moreover, it follows that the osmotic pressure 

 of the blood is determined almost wholly by the salts of 

 the blood and not by any organic solutes. It was because 



