WATER, SALTS AND MINERALS 67 



primitive marine animals closely resemble oceanic water in ionic proper- 

 ties. The blood of higher forms, particularly vertebrates, differs somewhat 

 from sea water in the relative proportions of various ions. Macallum 

 advanced the hypothesis that the chemical composition of the ancient seas 

 differed profoundly from that of the oceans today. Marine animals were 

 considered to have been originally in ionic equilibrium with sea water. 

 As the oceans changed in composition, the fluids of more primitive animals 

 altered correspondingly, whilst higher animals, which developed closed 

 circulatory systems, tended to retain throughout their subsequent evolu- 

 tion the ionic composition possessed by sea water at the time their circu- 

 latory systems were closed off. The peculiar ionic conditions in the plasma 

 of vertebrates were thus related to the composition of sea water at the time 

 these animals first appeared in the ancient seas. 



This interesting hypothesis has served to underline the essential simi- 

 larity of body fluids and has directed attention to their probable origin. 

 Evidence relating to the chemical composition of the oceans in past ages 

 indicates that they have remained relatively stable throughout most of 

 geological time, and any variations which have occurred have been within 

 rather narrow limits. The problem no longer bears the original stamp that 

 was impressed upon it, since the evidence just reviewed shows that all 

 marine animals can regulate the chemical composition of their cellular and 

 body fluids. Ionic regulation is an evolutionary acquisition and its mode of 

 expression has been modified with time. Peculiarities in the vertebrates 

 must be traced back to their freshwater origin (85, 118). 



Differences in ionic mobilities may be involved to some extent in 

 establishing and maintaining ionic levels in animals, but the primary fac- 

 tors operative must be ascribed to active processes of absorption and 

 excretion of particular ions. These processes result in the creation of ionic 

 steady states, in conjunction with, or independent of, the maintenance of 

 osmotic equilibrium. 



Phyletic Review of Primitive Groups largely in Equilibrium with Sea Water 



We have noted that ionic regulation is slight in inactive members of 

 more primitive groups. The ionic composition of the mesogloea in Amelia 

 differs slightly from that of sea water, and is controlled by the bounding 

 ectodermal and endodermal epithelia. Sulphate is actively eliminated 

 together with associated cations. Active absorption of potassium appears 

 to take place. The chloride increment is apparently a passive consequence 

 of the reduction of sulphate, and thus acts to counterbalance the cations 

 (73, 112). 



Peculiar conditions relating to floating devices have been noted in 

 siphonophores. In various genera, e.g. Agalma and Diphyes, there are 

 special bracts (hydrophyllia) or bells (nectocalyces) which are lighter than 

 sea water. Suggested mechanisms to achieve this condition are hypo- 

 tonicity (reduced internal salt concentration) or accumulation of substances 

 with lower specific gravity than NaCl (77). 



