Osmotic Eqiiillhration. 241 



The latter table gives us some idea of the changes in salt 

 ratios which have been taking place in the course of evolution. 

 Ignoring the ratios of calcium and potassium, Avhich do not 

 show any mai-ked variation, and confining our attention to the 

 proportion of magnesium to sodium, we see that in the medusa 

 {Aurelia flavid.), and the king crab {Limulus), the proportions 

 are about the same as in sea-water. In the elasmobranch the 

 proportion is 2.46 : 100, while the lobster {Homarus aineric.) 

 gives the proportion 1.72 : 100, teleosts (pollack and cod) give 

 1.46 and 1.41 : 100 respectively, while mammals give 0.81 : 100. 



Magnesium is an element which exerts a markedly toxic 

 action on protoplasm, and it is only natural to suppose that in 

 the course of evolution not only would there be a fight against 

 any alterations in the osmotic pressure and salt ratios of the 

 environment, but that if the organism were compelled to yield, 

 it would yield more in the case of the less toxic than in the case 

 of the more toxic substances. Now the A of an animals body- 

 fluid is due to the presence in solution of organic bodies and 

 salts (and their ions), and it may be assumed that the salts and 

 their ions account for the greater part of this A. 



On looking at the ratio Mg/A for the body fluid of various 

 animals, we see that the dogfish, pollock and mammal give 

 approximately the same ratio, the lobster a slightly higher 

 figure, while the limulus gives a figure approximating that of 

 sea-water. 



Thus we see that, although the lobster and ehismobranch 

 have not been highly enough evolved to resist the concentration 

 of their bodv fluid, which the increasing- concentration of their 



