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THE AMERICAN NATURALIST [Vol. L 



their environment has been discussed by Frederieq ( '85- 

 '04), Kodier ('99), Dakin ('08), Garrey ('05) and Bot- 

 tazzi ('97- '06). Now it is held that evolution of life 

 began in the sea. The single celled forms were completely 

 surrounded by the sea and it is easily understood why the 

 osmotic relations would remain primitive in case of these 

 forms. In gastrula type animals, such as coelenterata, 

 practically all cells of the body are bathed directly by the 

 sea and as far as we know these forms also are in osmotic 

 equilibrium with sea water. Now with the appearance of 

 mesoderm and a body cavity much of the body is removed 

 from direct contact with the sea. But the complete equi- 

 librium remains. As Quinton ( '00) says, the marine in- 

 vertebrate, though anatomically independent of the sea in 

 many of its organs, yet it is still physiologically open to 

 the sea which in an osmotic sense still ebbs and flows 

 throughout its body. 



Protoplasm originating in the sea was built up with 

 certain relationships with sea water, which relationships 

 are still maintained throughout all marine invertebrates. 

 May not the sparsity of fresh-water porifera and ccelen- 

 terates and the comparative failure of fresh-water algae 

 be due to the difficulty of maintaining the integrity of 

 protoplasm when all cells of these forms are so freely 

 bathed by fresh water, the osmotic pressure of which is 

 nearly zero ? 



Next above the marine invertebrates is a single case of 

 a cyclostome which is in osmotic equilibrium with the sur- 

 rounding sea water. What the osmotic pressure of the 

 blood of a cyclostome in fresh water is, we have no record. 

 It should be noted here that cyclostomes are now regarded 

 as degenerate fishes and on that account any evidence 

 from these forms as to the higher course of evolution 

 must be treated with care. In the next place we find that 

 eight species of elasmobranchs from the Mediterranean 

 and six from the ocean possess blood which is practically 

 isotonic with the sea water outside. Apparently they do 

 not differ from the marine invertebrates. But it is evi- 

 dent that the osmotic pressure of the blood is slightly 



