EFFrect or [ons upon TIsSsut 561 
the margin, and inasmuch as the whole Gonionemus ts just 
as immune against the Ca and K ions of the sea-water as 
the margin, it follows that the normal contractions of the 
Gonionemus originate in the part which contains the ganglia. 
It is probable, moreover, that the margin and the center 
must contain the three metal ions (Na, Ca, and EK) in differ- 
ent proportions. That this is only a difference in degree, 
however, is proved by the fact that an increase in the amount 
of K and Ca ions above that of the sea-water will finally 
stop the rhythmical contractions of the margin. On the other 
hand, it is probable that a very small amount of K and Ca 
ions, smaller than that in the sea-water, allows the center to 
beat rhythmically. 
This difference between the margin and the center is not 
the same in all Meduse. If we cut off the margin in an 
Acalepha (for instance, Aurelia aurita), the center begins to 
beat in sea-water a short time after the operation. It is 
possible that a comparative study of the heart-beat would 
reveal similar facts. 
We have thus far shown that the center of a Gonionemus 
is able to beat for about an hour in a pure NaCl solution, 
while the whole Gonionemus or the margin is able to beat in 
a NaCl solution containing in addition a small amount of K 
and Ca ions. How does a whole Gonionemus behave in a 
pure NaCl solution? As stated above, the contractions of 
Gonionemus occur in sea-water in groups followed by long 
pauses. If a Gonionemus be put into a §n NaCl solution, 
the swimming-bell contracts without interruption and the 
rate of contraction increases considerably. It may within 
two minutes reach a rate of 200 contractions per minute, but 
soon ceases to beat. If the £2 NaCl solution be diluted 
with distilled water, the increase in the rate of contractions 
occurs more slowly and the contractions continue longer. If 
we use a solution of 98 c.c. n NaCl+2c.c, 1 CaCl,, it 
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