484 POPULAR SCIENCE MONTHLY 



tains the medusa in a delicately balanced fluid for it contains poisons 

 and antidotes as does our blood, which exactly counteract one the other. 

 For example, the jellyfish dies in less than two hours if placed in a 

 solution having the amounts and proportions of the common salt and 

 the potassium of the sea-water, but if we simply add the calcium it 

 pulsates very rapidly for more than twenty-four hours. Finally, how- 

 ever, the calcium produces so strong a muscular tetanus that the 

 pulsating tissue is torn literally to shreds; but all of these injurious 

 effects disappear when we add the magnesium, which causes the pulsa- 

 tion to become much slower and more regular, and wholly prevents 

 the calcium from producting tetanus. Another curious fact is that 

 were it not for the presence of the calcium the magnesium would so 

 stupefy the nervous and muscular tissue that no pulsation could arise. 

 This is the more remarkable because magnesium and calcium are both 

 inhibitors of pulsation, yet when both are present they tend in a 

 measure to offset each other, magnesium mainly inhibits the muscles, 

 while calcium stupefies the nerves. 



But to return to our subject, let us carry out some experiments to 

 discover the nature of the stimulus which produces each pulsation of 

 the jellyfish. If we cut a ring from the medusa's disk such as is shown 



in Fig. 3 and leaves a long narrow 

 strip AB attached to it, and then 

 start a contraction-wave traveling 

 e around the ring; every time the wave 

 passes the point A a side-tracked por- 

 tion of the wave will pass along the 



FlG. 3. A PULSATING RING WITH AN ■• » Air, 1T n i ■ n 



"Index-strip" strip from A to B. When each side- 



tracked wave comes to the end B it 

 dies out, for it can not return over the recently stimulated tissue 

 along which it has just passed. Thus we see that the index strip 

 AB simply serves to catch a portion of each wave which passes its 

 base. 



Now suppose we place the ring in a pure solution of magnesium 

 chloride, and allow the index strip AB to remain in natural sea-water. 

 Then the contraction-wave gradually dies out in the pulsating ring, for 

 the magnesium paralyzes the muscles; and at the end of about a 

 quarter of an hour all movement will have ceased in the ring, but long 

 after this we find that the strip AB still continues to transmit con- 

 tractions at regular intervals of time. We see then that whenever the 

 something which produced the contraction in the ring comes around 

 to the point A it is still capable of setting up a contraction in the 

 strip AB, although it can not now cause the muscles of the ring itself 

 to pulsate. 



The explanation is that the stimulus which produces pulsation is 



