Body Fluids 115 



that the low and more or less stable osmotic pressures that are 

 characteristic of vertebrate blood are survivals of conditions in 

 primitive seas which had an osmotic pressure of perhaps a third 

 what it is today (Wardlaw, 1931); but such arguments are made 

 questionable by the fact that bony fishes appear to have had their 

 origin in fresh water (Barrell, 1916; Case, 1919), though the re- 

 moter ancestors of fishes were probably marine. Blood is not de- 

 rived from the waters of old oceans but from the body fluids of the 

 the inhabitants of those oceans (Dakin, 1912) . 



As animals left the ocean to invade fresh-water and land habitats, 

 there were changes in the character of limiting membranes and the 

 body fluids. In animals there are two lines of defense, so to speak, 

 where changes may be permitted, controlled, or prevented: the mem- 

 branes on the outsides of bodies and those covering cells which may 

 lie deep in the interior. In protozoans and sponges there is little 

 distinction between these, but in metazoans which have inclosed 

 blood and lymph systems the distinction is marked. In aquatic ani- 

 mals the permeability of the external membranes is of primary 

 importance. Many marine animals permit all constituents of sea 

 water to pass in and out of their bodies. Changes in the osmotic 

 pressure of external media in general limit the distribution of fresh- 

 water animals more than those of marine animals. In gaining ability 

 to retain salts in hypotonic media, the latter have apparently lost 

 ability to adjust to changes (Adolph, 1925) . 



A frog tadpole at the time of its metamorphosis undergoes 

 marked changes in regard to the ability of its skin to resist changes 

 in the osmotic pressure of the surrounding medium (Adolph, 1925, 

 1927a, b) . An adult frog apparently has a mechanism in its skin 

 which is under nervous control and which regulates somewhat the 

 passage of water and solutes (Adolph, 1933) . There is general 

 agreement that animals which have gained ability to regulate or re- 

 sist diffusion processes through their external membranes have done 

 so by the addition of controlling mechanisms. An animal that main- 



