Rhythmical Pulsation in Scyphomedusce. 



I2 5 



permits of recurrent (" rhythmical.") pulsation, for both magnesium and 

 calcium are inhibitors of pulsation, and reduce the stimulating effect which 

 the NaCl tends to exert. 



We see that, in the absence of calcium, magnesium produces a profound 

 relaxation of the muscles, rendering them incapable of pulsating. An ex- 

 actly opposite effect is produced by the remaining elements Na -f- Ca -f- K. 

 If a Cassiopea medusa be placed in a solution lacking magnesium, but con- 

 taining sodium, potassium, and calcium chlorides, its pulsation is at first 

 greatly increased both in amplitude and rate; but finally the rate and ampli- 

 tude decline and become very slow and slight, while at the same time sus- 

 tained tetanus sets in. This tetanus becomes so severe that after being 24 

 hours in the solution lacking magnesium the circular muscle fibers of the 

 subumbrella are torn across, as is 

 shown in figure 10, A ; and soon there- 

 after the whole medusa-bell is drawn 

 up into a crumpled mass, as is shown 

 in figure 10, B. Under these conditions 

 the medusa may give not more than 3 

 weak pulsations per minute, whereas 

 its normal rate may have been been 80. 

 The pulsations soon become so weak 

 that they do not involve the entire 

 margin, but spread only a little way 

 on both sides of those organs which 

 still initiate them. Even under these 

 conditions, however, when death is 

 imminent, the tetanus may be com- 

 pletely cured and normal pulsation restored by simply introducing any mag- 

 nesium salt in the amount found in sea-water. 



Tetanus and a final lowering of the rate of pulsation is also produced 

 in the rhythmical movement of the branchial arms of Lepas by NaCl -J- 

 KC1 -f CaCU ; and in this case also the tetanus is cured and normal pulsa- 

 tion restored by magnesium. The tetanus is caused mainly by calcium, for 

 it takes place in Cassiopea placed in sea-water -f 3 per cent ?<6m CaCX or 

 in any solution lacking magnesium but containing calcium. Nevertheless, 

 the tetanus is not due solely to calcium, for it is far more severe in medusae 

 subjected to NaCl -f CaCU -f KC1 than it is if we leave out the potassium, 

 and place the medusa in NaCl + CaCl 2 . However, calcium is the element 

 chiefly responsible for the production of the tetanus, for no tetanus occurs 

 in medusae subjected to a solution of NaCl -f KC1. The interesting fact 

 remains true, however, that the most severe and constantly sustained tetanus 

 is produced by a solution containing all three elements sodium, calcium, and 

 potassium. 



FIG. 10. A, and B, successive stages of 

 tetanus produced by a partial sea-water 

 composed of NaCl + KCL + CaCU but 

 lacking magnesium. Upon adding mag- 

 nesium, this tetanus was completely 

 cured, and normal pulsation restored. 



