WATER, SALTS AND MINERALS 



69 



magnesium and sulphate ions. Lobsters are peculiar in having low 

 potassium values. Protein occurs in high concentrations, up to 8% of the 

 plasma, and forms indiffusible complexes with calcium. Ionic regulation 

 involves: active absorption by the gills of sodium, potassium and calcium 



TABLE 2.14 



Ionic Regulation of Coelomic Fluid by Arenicola marina in Full Strength 

 and Diluted Sea Water (75-50%) 



(From Robertson (112)) 



against a concentration gradient; inward diffusion of magnesium and sul- 

 phate along the concentration gradient; differential excretion by the anten- 

 nary glands, tending to lower blood magnesium and sulphate, and con- 

 serve sodium and potassium. The gills and integument of the lobster are 

 relatively impermeable to magnesium and sulphate ions, which enter 

 largely through the gut. 



Permeability to ions has been measured in various ways. When Carcinus 

 is placed in dilute sea water, changes take place in the relative proportions 

 of ions in the blood. Sodium and chloride decrease more than potassium 

 and calcium, and blood protein rises. Percentages of sodium, potassium, 

 calcium and chloride increase relative to the medium, while magnesium 

 and sulphate decrease (Table 2.15). The greater divergency between ionic 



TABLE 2.15 



Composition of the Haemolymph of Carcinus maenas 

 in Normal and Dilute (67 %) Sea Water 



(From Webb (142)) 



ratios of the blood and external medium which results is due to increased 

 salt absorption and excretion. The overall effect is a reduction of osmotic 

 gradient, largely owing to an absolute decrease of sodium and chloride and 

 increased ionic regulation. Permeability has also been tested by altering 

 the concentration of ions individually in the medium. When animals are 



