No. 599] OSMOTIC PRESSURE OF THE BLOOD 



047 



example, the average A of twelve species of invertebrates 

 from the Mediterranean is 2.281°, while the average A of 

 the water in which they live is 2.29°. A simple case of 

 adaptation is thus evident. But the bony fishes, teleosts, 

 tell a different story. 



It is worth while to contrast the osmotic pressure of the 

 blood with that of the external medium. To do this we 

 will break up all the forms into groups not according to 

 the environment alone, but also according to relationship. 

 If we should be guided by environment alone, the result 

 would be a confused tangle. Table II shows the average 

 A of these groups selected not only on the basis of re- 

 lationship but also taking into partial consideration the 

 environment. 



6 Elasmobranehs, Ocean, bays = 1.902 ■ 



"A,'" Blood' 3 Aves, ' =0.67 — 



"A," Blood, 8 Mammals, =0.577 — 



From this table it is evident that the blood of the marine 

 invertebrate is isotonic with the water in which it lives, 

 whether this be the Mediterranean or the ocean. As 

 stated above, it appears to be a simple case of adaptation. 

 But in the other cases the relation is not so simple. If 

 we compare the osmotic pressure of the marine teleosts, 

 fresh-water teleosts and the amphibia, etc., with the os- 

 motic pressure of the external medium great differences 

 are evident. And yet it can not be said but what all these 

 forms are adapted to their environment. But it is not 

 enough to make this statement, but to try to explain why 

 such a relationship becomes possible. The isotonicity 

 existing between the blood of marine invertebrates and 



