646 THE AMERICAN NATURALIST [Vol. L 



term think of the fluid circulating in the blood vessels of a 

 vertebrate. The term body fluid is also ambiguous. In 

 an invertebrate it has reference to that part which we call 

 the blood of a vertebrate. In the vertebrate we usually 

 think of the secretion of serous membranes as 1 'body 

 fluid." After all, the subject of discussion in this paper 

 is the fluid by which food is carried to tissues and wastes 

 carried away. Having thus defined the use of the terms, 

 let us examine the osmotic pressures of the blood of vari- 

 ous animals. 



Table I, which follows, shows one hundred and eleven 

 determinations of the osmotic pressure of the blood of 

 representatives of nearly every animal phylum. Many 

 of these determinations are averages. Some of the forms 

 are wholly terrestrial, some live in fresh water, some in 

 either fresh or seawater, some live wholly in the sea. 

 Considerable variation in the osmotic pressure of the 

 blood is shown. 



Of the marine forms given some are found in the Medi- 

 terranean, while others for the most part occur in the 

 ocean or in protected waters connected with it. There is 

 great variation in the osmotic pressure of the blood of 

 forms living exclusively in the Mediterranean. Great 

 variation is shown in the case of those living in the ocean. 

 In some cases in each environment, complete harmony 

 with or rather isotonicity with the environment is ap- 

 parent. In other cases this is not at all evident. For 



