n6 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 subnmbrella, 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 -f- 

 KC1 -f- CaCln 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 FlG . j.Annulling of pulsation which occurs 

 pulsation in a ring-shaped Strip of w hen 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. i, 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. I, 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. 



