650 THE AMERICAN NATURALIST [Vol. L 



IV. Fresh-water teleosts, amphibians, reptiles, birds and 

 mammals. 



Let us examine each of these groups with regard to 

 their osmotic independence of the external medium. 

 That is, what is the effect of changes in the concentration 

 of the external medium on the osmotic pressure of the 

 blood of these groups. 



First, the invertebrates. Let us recall Quinton 's state- 

 ment that marine invertebrates are still physiologically 

 open to the sea. For when the concentration of the ex- 

 ternal medium is changed, it is found that a change in the 

 osmotic pressure of the blood takes place. Fredericq 

 ( '85 and '04) stated that the change in one was followed 

 by an equal change in the other. In a few hours the new 

 equilibrium is established. If the time of sojourn in the 

 modified sea water was small the equilibrium with it was 

 not completely attained. Moreover, all invertebrates did 

 not adapt themselves with the same rapidity to changes 

 in the external medium. On the whole, provided the ex- 

 ternal change was not too great, it was followed in time 

 by complete equilibrium between the osmotic pressure of 

 the blood and that of the modified sea water. This was 

 true in the case of sea water made dilute by addition of 

 fresh water and sea water made more concentrated by the 

 addition of salt. In other words, the organism possesses 

 no structures which render it independent of the changes 

 in the external medium. There are three structures con- 

 cerned in these changes. First the integument, second, 

 the intestinal wall and third the gill membranes. With 

 the appearance of gills, the body integument apparently 

 is the first structure to become impermeable. The in- 

 testinal wall is the first to show a selective action. 



Second, the elasmobranchs. These had been placed by 

 investigators with the marine invertebrates not only be- 

 cause their blood possessed the same osmotic pressure as 

 the external medium, but it was thought that when the 

 external medium was changed, the same changes occurred 

 in the blood of the elasmobranch. I made extensive ex- 



