658 



THE AMERICAN NATURALIST 



[Vol. L 



There is a difference of but 0.055°. The A f the salts of 

 the blood of the pollock was 0.737° while the A of the en- 

 tire blood was 0.825, showing a difference of but 0.088°. 

 In other words, almost the entire osmotic pressure of the 

 blood of the teleost is due to the salts. The urea, am- 

 monia or other organic solutes present must be very 

 small and are represented by the difference above men- 

 tioned, namely 0.055° in the case of the cod and 0.098 in 

 the case of the pollock. How different is this condition 

 from that found in the elasmobranch where in one case 

 noted by Macallum, and which is typical, the difference 

 between the a of the saline contents of the blood and the 

 entire blood was 0.961°, a difference as great as the aver- 

 age A of the marine teleost and as stated due to the 

 relatively enormous amount of urea and other organic 

 solutes in the blood of the dogfish. Again the question 

 arises : What brought about this change between the com- 

 position of elasmobranch blood and that of the teleost? 

 Was it due to the migrations to and from fresh water be- 

 fore certain species of teleosts took up their home per- 

 manently in the sea? And yet the marked difference be- 

 tween the two is not alone a difference in salt content. It 

 is far more the absence from the blood of urea, ammonia 

 and other organic solutes. Let us use Macallum 's data 

 as a basis for comparison. The blood of marine teleosts 

 contains about 30 per cent, less salts than the blood of 

 elasmobranchs but it contains 90 per cent, less organic 

 solutes. The distinct loss therefor is in organic solutes. 

 This therefore must have been a significant factor in the 

 evolution of the higher form. Now what is the most ap- 

 parent structural difference between the elasmobranchs 

 and teleosts? It is of course that the skeleton of one con- 

 sists of cartilage and the skeleton of the other is bone? 

 It does not necessarily follow, however, that the power to 

 build a bony skeleton depends on the absence of organic 

 solutes from the blood, nor is there apparently any close 

 connection between them. 

 The fresh-water fishes in all probability agree with the 



