WATER, SALTS AND MINERALS 



61 



Nevertheless the ionic compositions of body fluids throughout the animal 

 kingdom possess a general pattern of resemblance which is too great to 

 have originated fortuitously. In lower animals the tissue fluids closely 

 resemble sea water (coelenterates, polychaetes). Higher forms tend to 

 accumulate K+ relative to Na + , and to reduce levels of Mg+- and S0 4 = . 

 This trend is most pronounced in decapod crustaceans and vertebrates. 

 Concentrations of ions in body fluids of representative species are shown 

 in Table 2.9. Absolute values depend to some extent on the concentration 

 and ionic composition of the external medium. The relative ionic composi- 

 tion of the body fluids of a series of marine invertebrates and fishes is 

 presented in Table 2. 10 on a chloride basis of 100. This ion is in equilibrium 

 with sea water in most animals, and forms a suitable basis for comparison. 

 Of twenty invertebrates examined by Robertson (112), all had chloride 



TABLE 2.10 

 Relative Ionic Composition of Body Fluids 



* Haemolysis of erythrocytes probably responsible for release of intracellular K - 

 (Data from Robertson (111, 112, 113, 114) and earlier sources) 



