The Composition of the Blood of Aquatic Animals and its Bearings upon the Possible Conditions of Origin of the Vertebrates

Abstract

Forty years ago a suggestion was made (Bunge)¹ that the sodium chloride in the human tissues might be a relic, an inherited feature of some aquatic marine ancestor. The same idea was expressed in certain papers by Quinton,² who stated that so far as salinity is concerned the blood of most animals is an altered sea water. In 1903,³ Macallum, then unaware of the suggestions of Bunge and Quinton, advanced the theory “that the blood plasma of vertebrates and invertebrates with a closed circulatory system is, in its inorganic salts, but a reproduction of the sea water of the remote geological period in which the prototypic representative of such animal forms first made their appearance”. He regarded the differences between the blood salinity of the higher vertebrates and the present-day ocean as due to changes in the composition of the sea water since palæozoic or mesozoic times. Macallum followed up his first statement by more detailed papers, and made some valuable analyses of blood to support his thesis. He is now generally credited with being the originator of this interesting conception. In 1912 I discussed the matter⁴ from the point of view of certain experiments and observations on marine and fresh-water invertebrates, and came to the conclusion that whilst one might accept the well-regulated saline composition of the blood of teleost fishes, amphibia, and the higher groups as being to a certain extent an heirloom of an ocean environment, there was no reason whatever to believe that the favoured salinity associated with independence represented the composition of any particular primeval ocean. At that time I stated that the investigations of osmotic regulation did not preclude the idea that the early vertebrates with an independence in blood salinity had evolved in fresh water.