44 



Papers from the Marine Biological Laboratory at Tortugas. 



In still greater dilutions there is a slow decline in rate in a rectilinear ratio, 

 but the decline is not so rapid as it is when all four cations, sodium, mag- 

 nesium, calcium, and potassium, are reduced by adding 0.9 molecular 

 dextrose to the sea-water. Thus the sodium of the normal sea-water is a 

 stimulant to the rate of nerve-conduction. 



As we would expect, the 0.6 molecular NaCl is more stimulating than 

 the lower concentration, 0.487 molecular. 



See fig. 7, full-line curve. 



Table 9 shows the relative rates of nerve-conduction in 20 subumbrella 

 rings of Cassiopea xamachana in sea-water and in sea-water diluted with a 

 solution composed of 0.487 molecular NaCl dissolved in 0.075 molecular 

 dextrose. This solution maintains the normal concentration of NaCl and 

 the normal osmotic pressure of Tortugas sea-water and its addition merely 

 causes a decline in the concentrations of the magnesium, calcium, and 

 potassium of the sea-water. 



As in all other experiments of this series, the perfect medusae are more 

 sensitive to the effects of sodium than are the rings, but the difference in 



