46 PUI^ATION OF JEI<I,YFISHES. 



The Medusae are, however, inhibited from pulsating by the presence 

 of magnesium, not by the mere absence of calcium ; for if magnesium 

 be absent, calcium may also be absent and the Medusae will pulsate 

 fully two hours. 



A large excess of calcium lowers the rate of pulsation of Cassiopea, 

 after a momentary increase. The inhibitory effect of calcium is, how- 

 ever, far less marked than that of magnesium, or than the final toxic 

 effect of potassium. For example, if we add CaSO^ + CaCOg to sea- 

 water at 82° F., to saturation, normal perfect Medusae of Cassiopea 

 pulsate at about two-thirds their normal rate after being in this solu- 

 tion I2j4 hours. One gram of CaClj in 100 c.c. sea-water also slightly 

 reduces the rate of pulsation without injurious effects, recovery being 

 almost immediate in normal sea-water. Perfect Cassiopea with sense- 

 organs intact when placed in a pure solution of CaClj isotonic with 

 the NaCl of sea-water ceases to pulsate in 10 seconds, and can not be 

 restored to pulsation by being placed in NaCl + K2SO4 in amounts 

 found in sea-water. A. strong solution of K2SO4 in NaCl, however, 

 revives them into active pulsation. Evidently their sensibility to 

 Stimuli is impaired but not destroyed. 



Calcium salts never stimulate the disk of Cassiopea into pulsation, 

 even when placed upon it in concentrated solutions. 



We see that calcium, while not of itself a stimulant, is necessary to 

 pulsation and is a stimulant in connection with sodium and potassium. 

 An excess of calcium tends to retard pulsation, but even a saturated 

 solution of CaSO^ in sea-water exerts no appreciable toxic influence. 

 It is far more important to pulsation than potassium ; for Cassiopea 

 will pulsate for more than an hour with irregular periods of rest and 

 activity in the absence of potassium, but in the absence of calcium 

 pulsation ceases in less than 6 minutes. This importance is due solely 

 to the remarkable ability which calcium has to counteract the inhibit- 

 ing effect of magnesium. 



EFFECTS OF SODIUM UPON PULSATION. 



All of the sodium salts are weak stimulants to the disk of Cassiopea 

 deprived of its marginal sense-organs, producing not very powerful 

 contractions. The sodium salts, however, vary considerably in their 

 stimulating power, NaCl or NaOH giving strong and NajCOa or 

 NajSOi weak contractions. 



The disk of Cassiopea deprived of marginal sense-organs pulsates 

 for about 20 minutes in a pure f^n NaCl solution, and also in NaCl -I- 

 K2SO4 or NaCl4-K2SO,-t-CaS04 or NaCl-l-CaSO,.* 



* The proportions of Na, Ca, and K were such as are found in sea-water. 



