Papers from the Marine Biological Laboratory at Tortugas. 53 



If we place normal medusae in a solution having the proportions of lOO 

 c.c. sea-water + i c.c. of 0.64 molecular KCl, their rates are increased and 

 maintained thus for more than 24 hours. This solution gives the potassium 

 ion a concentration i .6 times as great as in natural sea-water. Any further 

 increase in the concentration of the potassium ion causes an initial excite- 

 ment, but this is soon followed by a loss in rate. Rings are not so sensitive 

 and the nerves may maintain a normal rate in a solution having 2.5 times the 

 concentration of the potassium ion in sea-water, but a still further concen- 

 tration causes a decline in rate after temporary, initial excitem.ent. 



It has long been known that weak concentrations of potassium cause 

 initial excitement followed by loss of rate. It is, however, interesting 

 to see that a very weak concentration may be a permanent stimulant, for 

 it shows that potassium acts in a manner similar to that of sodium, its close 

 associate in Group I of the periodic system. Like potassium, sodium 

 itself produces sustained increase in rate in weak excess, but depression in 

 stronger concentrations. Also in stronger concentrations its initial effect 

 is to stimulate, but depression soon follows. Sodium also is slightly toxic, 

 potassium even more so. Thus in all these respects potassium resembles 

 sodium, but its metallic properties are more decided, as indeed we would 

 expect from its higher atomic weight. 



EFFECTS OF CALCIUM. 



It is difficult to determine the effects of the calcium ion per se, for (as is 

 well known) it associates itself or combines with the sodium ion, possibly 

 forming a sodium-potassium ion proteid, and it enables the sodium to 

 counteract the negative effect of magnesium. 



If Cassiopea be placed in a van't Hoff's solution resembling sea-water, 

 but lacking calcium, all movement soon ceases, but pulsation begins to be 

 revived when we add a concentration of calcium equal to 0.4 or 0.5 that 

 found in natural sea-water. Pulsation is most rapid, however, when the 

 concentration of the calcium ion is about 2.5 times that found in natural 

 sea-water. Further additions of calcium cause increasing depression and 

 movements usually cease when the solution contains 7 times the concen- 

 tration of calcium found in sea-water. Such a solution is not toxic, however, 

 for complete recovery takes place almost immediately if the medusse be 

 restored to sea-water. 



It is interesting to see that half the normal concentration of calcium 

 suffices to restore pulsation, for half the normal concentration of sodium 

 will accomplish the same end, provided magnesium, calcium, and potassium 

 be present in normal concentration. 



