242 INVERTEBRATE PHYSIOLOGY 



Calculations of intracellular ions have often been made in the past by- 

 assuming that all the chloride of the muscle is extracellular. Thus on this 

 basis, Hayes and Pelluet (1947) considered that in lamellibranchs and in 

 cephalopods the cells contained only potassium — no sodium, calcium, or 

 magnesium. Steinbach (1940) likewise thought that no sodium was pres- 

 ent in the muscles of the sipunculid Phascolosoma. The basic assumption 

 that there is no chloride inside cells seems to be false, as is shown by the 

 data of Table 6. The Carcinus analysis was made directly on isolated 

 muscle fibers, those of Krogh and myself on muscle press juice and whole 

 muscle respectively, corrected for extracellular fluid. In estimations of 

 the latter, Krogh used the sodium thiosulphate and ammonium thiocyanate 

 "spaces," that is, the space into which thiosulphate or thiocyanate injected 

 into the blood distributed itself in the muscle, after allowing several hours 

 for even distribution. I used inulin for an estimate of extracellular space 

 in the Nephrops muscle, and found that 11.5% of the muscle water was 

 probably extracellular. All these "spaces" are lower than the chloride 

 space. 



TABLE 6. INTRACELLULAR COMPOSITION OF MUSCLE OF SOME 

 MARINE INVERTEBRATES 



Na 



Mytilus (Krogh, 1939) 121 



Eriocheir (Krogh, 1939) ZZ 



Carcinus (Shaw, 1955b) 54 



Nephrops (Robertson) 27 



The data show over 50 mE intracellular chloride in these muscles, and 

 considerable quantities of sodium and magnesium, with rather less calcium. 

 The chloride content of Mytilus muscle is half that of the blood, and the 

 sum of the anions greatly exceeds the figures for cations, even allowing an 

 extra 50 mE for calcium and magnesium. Sulphate may have been over- 

 estimated, but the chloride figure is supported by that of Fox (1941) for 

 Mytilus calijornianus, 203 mE/kg. fresh weight in the muscular foot. 

 Krogh's figure on the same basis is about 240 niE/kg. for Mytilus edulis, 

 and the salinity of sea water in his case was slightly greater. Such high salt 

 contents are not universal in lamellibranchs, since Hayes and Pelluet 

 (1947) find only 68.5 niE Cl/kg. in the whole muscle of Mactra. 



A comparison of the ratios of intracellular to extracellular ions is given 

 in Table 7. Relative to the blood, Mytilus has more sodium, chloride, and 

 sulphate and less potassium inside its cells than any of the crustaceans. 

 The accumulation of potassium in Nephrops and Eriocheir is twice that of 

 Carcinus, and the only other ion accumulated in the crustaceans is mag- 



