OSMOTIC AND IONIC REGULATION 243 



nesium in Nephrops. It may be noted that the plasma magnesium in this 

 animal is only half that of Carcinus. 



TABLE 7. RATIOS OF CONCENTRATIONS INSIDE MUSCLE CELLS 

 TO THOSE OF PLASMA (ON WATER CONTENT BASIS) 



Na^ K^ Ca^ Mg^ O^ SO^ 



Na„ K„ Ca^ Mg„ Cl^ SO^^ 



Mytilus 0.238 6.7 .... .... 0.513 L600 



Eriocheir 0.071 18.4 .... .... 0.312 



Carcinus* „ 0.109 8.8 0.281 0.68 0.096 



Nephrops 0.052 20.6 0.235 1.97 0.106 0.055 



♦Recalculated from Shaw's (1955b) figures, assuming a water content of 0.945 

 mg./ml. in the plasma. 



Present-day views on the ionic composition of cells (Ussing, 1949; 

 Hodgkin, 1951 ) suggest that most of the potassium is held electrostatically 

 by nondiffusible organic anions, organic phosphates, and protein, and that 

 the low value for sodium is maintained by a process of active extrusion 

 of sodium ions. On this hypothesis cellular potassium stands in Donnan 

 equilibrium with the outside potassium, and 



[K,] [do] 



[Ko] [Ch] 



In Eriocheir these ratios are respectively 18.4 and 3.21, in Carcinus 8.8 

 and 10.5, and in Nephrops 20.6 and 9.39. Only in Carcinus are the theoreti- 

 cal requirements approximately satisfied. It seems that in Eriocheir and 

 Nephrops active uptake of potassium by a metabolic process must be postu- 

 lated, since the Donnan equilibrium is inadequate to explain the ratios, 

 and the binding of large amounts of internal potassium in unionized com- 

 plexes is very improbable. 



The composition of Mytilus muscle (Table 6) as determined by Krogh 

 is inconsistent with the presence of any large amount of organic anions, 

 and the ratios of 6.7 and 2.0, respectively, do not suggest a Donnan 

 equilibrium. 



Intracellular cations may not be evenly dispersed but localized in rela- 

 tion to intracellular structures (Steinbach, 1947; Dubuisson, 1954). In 

 Nephrops the juice pressed from the muscle has the same chloride content 

 as whole muscle, on a water-content basis, but only 74-80% of the sodium, 

 potassium, and acid-soluble phosphorus, and less than 10% of the calcium 

 and magnesium. This suggests that most of the calcium and magnesium is 

 bound to structural proteins not present in the soluble proteins of the 

 juice. 



