INTEBACTION OF SAINTS. 49 



ARTIFICIAL SBA-WATi;r AND THE BFFBCTS OF THE SAI,TS OF SEA- 

 WATER, AS A WHOLE, UPON PULSATION. 



In the experiments upon Cassiopea the solutions containing some or 

 all of the chief constituents of sea-water were made up in accordance 

 with the formula given by Dittmar (1884)*, and also according to 

 Van 't Hoff's formula (100 NaCl + 2.2 KCl + 7.8 MgCl^ +3.8 MgSO^ + 

 3 CaClj, all of f^n concentration. 



Medusae pulsate normally in an artificial sea-water made according 

 to Van 't Hoff's formula, but pulsation is somewhat irregular in a 

 sea-water made according to Dittmar's formula. Table 4 shows the 

 results of experiments with Dittmar's formula, and table 5 gives the 

 results obtained by using Van 't Hoff's formula. 



Tables 4 and 5 show the effects upon Cassiopea of various solutions 

 containing one or more of the constituents of sea-water. It will be 

 apparent that magnesium is the chief restrain er of pulsation, and that 

 it prevents the spontaneous contraction of disks deprived of mar- 

 ginal sense-organs and retards pulsation in perfect Medusae. When 

 magnesium is present the absence of calcium quickly stops pulsation, 

 but when magnesium is absent we may have calcium also absent and 

 the Medusae will pulsate for a considerable time. It is apparent, 

 therefore, that calcium assists the NaCl to counteract the retarding 

 influence of magnesium. This is also shown by the fact that Medusae 

 pulsate for a long time in Na + Mg + Ca, whereas all movement ceases 

 very soon in Na + Mg. 



Potassium, however, does not assist the NaCl to resist the stupefy- 

 ing influence of magnesium, for Medusae cease to pulsate almost as soon 

 in Na + Mg + K as they do in Na + Mg. Potassium serves mainly to 

 stimulate movement in connection with both calcium and sodium ; thus 

 Na + K and Na + Ca give temporary pulsations at about normal rate ; 

 whereas Na + Ca + K gives strong pulsations at fully twice the nor- 

 mal rate, but these can not be sustained indefinitely unless magnesium 

 be present to counteract the too powerful stimulating effects of the 

 Na + Ca + K. A Ringer's solution is only a powerful stimulant, and 

 can not sustain pulsation indefinitely unless tempered by magnesium. 

 Potassium has little power to revive pulsation, whereas calcium pos- 

 sesses this power to a marked degree ; thus, when pulsations have 

 ceased in NaCl they can always be revived by calcium, but at best 

 only a very few isolated contractions can be revived by potassium in 

 the amount and proportion found in sea-water. 



*Reports of voyage of H. M. S. Challenger, Chemistry, vol. i, p. 204. 



