Rhythmical Pulsation in Scyphomeduse. 12 
Leal 
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 +Ca+ 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 10,8. 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 Fic. 10.—A, and B, successive stages of 
on both sides of those organs which Penal ob mcrineer c CaCl, but 
still initiate them. Even under these lacking magnesium. Upon adding mag- 
conditions, however, when death is EEL es pe at one 
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 + 
KCI -++ CaCl, ; 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 + 3 per cent 54m CaCl,, or 
in any solution Jacking magnesium but containing calcium. Nevertheless, 
the tetanus is not due solely to calcium, for it is far more severe in meduse 
subjected to NaCl + CaCl, + KCl than it is if we leave out the potassium, 
and place the medusa in NaCl + CaCl,. However, calcium is the element 
chiefly responsible for the production of the tetanus, for no tetanus occurs 
in medusz subjected to a solution of NaCl-+-KCl. 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. 
