120 =©Papers from the Marine Biological Laboratory at Tortugas. 
in the opposite direction. When the two waves meet 180° away from their 
common point of origin they interfere with and annul each other, and a 
period of quiescence ensues until another contraction-stimulus is sent forth 
from a marginal sense-organ. 
Under normal conditions the two side waves are of practically equal 
magnitude, and thus one can not overpower the other and travel constantly 
around the circuit in one direction. Such an accident is prevented by the 
interference and consequent suppression of the two waves, one by the 
other; but the protection is not perfect, for on several occasions I have 
started such a wave through a severe electrical or mechanical shock, and 
then the sense-organs, being exhausted by the wave which set them into 
play one after another, were powerless to control the pulsation, and the 
single wave rushed constantly around the subumbrella annulus, causing each 
and every part of the medusa to pulsate successively as it passed. The 
rate of pulsation under these unusual conditions was fully twice that of 
the isolated, recurrent contractions initiated by the sense-organs. 
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Fic. 5.—A, showing that a pulsation-wave may pass across newly 
regenerated tissue (dotted area) which contains no muscles. 
B, showing that a pulsation-wave can not pass through mus- 
cles (ruled area) from which the nervous network has been 
peeled away. 
Such a circuit wave can not take possession of the vertebrate heart, 
for here each wave of contraction normally originates in the region of 
the sinus, then spreads over the auricles, and finally over the ventricle, 
whence it can not immediately return over its path. The pulsations of the 
heart are recurrent, and are rhythmical only in the sense that the separate 
pulsations follow one another, at sensibly equal intervals of time. 
In the Scyphomedusz the pulsation-stimulus is conducted by the dif- 
fuse nervous system of the subumbrella, and this stimulus causes the muscles 
to contract. The stimulus will pass through tissue which contains no muscles 
and can not contract, or through tissue wherein the muscles have been 
rendered incapable of contracting through the effects of distilled water, 
magnesium, curare, carbon dioxide, alcohol, etc. 
On the other hand, the pulsation-stimulus can not pass through or 
be conducted by a muscle from which the nervous connections have been 
