112 How Animals Changed 



to the primitive conditions which must have existed millions of years 

 ago" (Rogers, 1927) . 



The salt systems which bathe the living cells of those animals in 

 which body fluids differ from those in sea water do not have the 

 same proportions of salts as occur in the ocean. Those in fresh- 

 water animals of course differ markedly in salinity and composition 

 from the surrounding medium. However, as Dakin (1912) has 

 pointed out, an aquatic organism is not a closed system which is in- 

 dependent of changes in its environment. Such changes may affect 

 internal media physically or chemically. The bloods of marine ani- 

 mals may differ markedly from those in the surrounding ocean. 

 The salts in sipunculids and starfishes are about the same as those 

 in the sea; but the quantity of those in crab and snail bloods are 

 always at least a little below; and those in fishes are much less and 

 rather constant (Duval, 1924, 1927) . On the whole, the blood of 

 many marine invertebrates is much like the ocean, but vertebrate 

 blood substances always differ from the surrounding medium. 

 "Marine invertebrates which have invaded brackish and fresh waters 

 often present an independence of the blood which is not at all un- 

 like that of vertebrates. For example, in both <-Ke fresh-water cray- 

 fish and a frog immersed in water, there is a constant controlled 

 diffusion inwards of water and a regulated output from the excre- 

 tory organs. A new fresh-water crab (from a river in New South 

 Wales) with which we have been experimenting retains a con- 

 stancy of blood salinity which is less than half that of the ocean 

 from which it undoubtedly wandered, yet its sojourn in fresh water 

 cannot have been of long duration" (Dakin, 1931). 



The salt content of fresh water is not only less than that of the 

 ocean but differs in composition. "In striking contrast to these 

 [sea] solutions, in which sodium is 30 to 50 times more concentrated 

 than the calcium, are the dilute salt systems in which avascular 

 fresh-water animals are found. Two salient characteristics emerge 

 from a review of a number of analyses of different fresh waters: 



