116 Papers from the Marine Biological Laboratory at Tortugas. 
are designed especially to prevent such a circuit-wave from taking possession 
of the organ. 
(4) In Cassiopea the pulsation-stimulus is conducted by the diffuse ner- 
vous network of the subumbrella, and is independent of the muscles which 
may or may not respond to its presence by contraction. In other words, 
conductivity of the pulsating tissue is independent of its contractibility. 
(5) Strong primary nervous and muscular excitement followed by ex- 
haustion and sustained muscular tetanus is produced in Lepas or in Cassi- 
opea by a solution containing the amounts and proportions of NaCl-—+ 
KCl-+ CaCl, found in sea-water. This tetanus may, however, be cured 
and normal pulsation restored by adding the amount and proportion of 
magnesium found in sea-water. Magnesium relaxes the muscles, and pre- 
vents tetanus. It has but little direct effect upon the nervous elements which 
alone transmit the pulsation-stimulus. 
EXPERIMENTS. 
Romanes and Eimer found that if we remove the marginal sense-organs 
of a scyphomedusa, the disk becomes paralyzed and does not pulsate spon- 
taneously in sea-water. In 1906 the writer found, however, that any strip 
of subumbrella tissue of a scyphomedusa cut in the shape of a ring, or closed 
circuit, will pulsate rhythmically in sea-water, provided a contraction-wave 
be once started in the circuit. 
Any stimulus, such as that given 
by contact with any soluble salt of 
potassium, sodium, lithium, barium, 
platinum, hydrogen (acid), or an 
electrical or mechanical shock, will 
produce a contraction-wave in the 
disk of the scyphomedusa Cassiopea 
and will serve to start rhythmical Fic. 1—Annulling of pulsation which occurs 
pulsation in a ring-shaped strip of when two waves of equal magnitude, com- 
° ing in opposite directions, meet each other. 
paralyzed subumbrella tissue. 
The contraction-wave arises from any point upon the ring of paralyzed 
subumbrella tissue which we may choose to stimulate. Two waves of equal 
magnitude may start from the stimulated point and travel in opposite direc- 
tions from their common point of origin, as is shown in fig. 1, A. Under 
these conditions each wave travels around the ring until it meets the other 
wave coming in the opposite direction, as is seen in fig. 1, B. All movement 
ceases when these waves meet, for tissue which has been in contraction can 
not again contract until after an appreciable interval of rest; hence neither 
of the waves can stimulate the tissue which has only upon the instant previ- 
ous been set into contraction by the other wave. Under the conditions de- 
scribed above, therefore, the whole ring gives a single contraction, and then 
ceases to pulsate. 
A B 
