BIOLOGY, MARINE — MAYER. 121 



tion waves to proceed unhindered in the direction of the trend of the circu- 

 lar muscle fibers, and at the same time opposes a partial barrier to their 

 transmission across the muscle fibers. The pulsation is effected through the 

 muscle fibers rather than through the nerve fibrillse of the sub-umbrella. 

 It will not start unless the disk be momentarily stimulated, as by a mechani- 

 cal shock or a touch of a crystal of K2SO4, but once started it continues 

 indefinitely without further external stimulation. 



The waves of pulsation all arise from a definite point, and the labyrinth of 

 muscular tissue around this center must form a closed circuit. This laby- 

 rinth may be simplified after the rhythmic movement has started, by cutting 

 parts of it away, and thus forms may be derived which could not have been 

 set into continuous pulsation in the first instance. Any cut which breaks the 

 circuit, however, instantly stops the pulsation, and sustained movement can 

 not be re-started. 



The rate -of pulsation of these disks is fully twice as fast as that of the 

 normal perfect Medusa. This rate remains constant in the pulsating disk, 

 and when pulsation ceases all movement stops instantly, never gradually. 

 The rate is controlled by the center of pulsation, and does not depend upon 

 the shape or complexity of the labyrinth of tissue formed by the cuts. In 

 this respect it differs from the control of the marginal sense-organs for 

 small pieces of tissue with a marginal sense-organ attached pulsate slower 

 than large ones. The tissue of the disks pulsates at the maximum rate at 

 which it is capable of transmitting successive waves of contraction. 



Disks with radial cuts or cuts crossing the trend of the circular muscles 

 can not be made to pulsate continuously. 



The disks of Aurelia and Dactylonietra, if cut as described above, will pul- 

 sate as does the disk of Cassiopea. 



These experiments show that rhythmical pulsation may be initiated and 

 maintained independently of the nervous system, and that it may be myogenic 

 in character. Pulsation must arise from a definite center, but this center 

 may be established at any point in the muscular layer of the sub-umbrella. 

 Once established it remains at a fixed point, while the disk continues to pul- 

 sate. Sustained pulsation occurs only in tissue forming a closed circuit,* and 

 may therefore depend upon an electrical transmission of energy. These 

 experiments appear to be the first in which paralyzed tissue has been restored 

 to sustained pulsation without subjecting it to the influence of unnatural or 

 injurious solutions. 



If normal perfect Medusae be lifted out of the water and then thrown back, 

 the rate and amplitude of their pulsation suddenly increases. Pulsating disks 

 react in a similar manner, but in their case only the amplitude increases, the 

 rate remaining practically constant. The presence of marginal sense-organs is 

 therefore not necessary either for ordinary pulsations or for the display of 

 "excitement." 



Removal of the mouth-arms and mouth-arm plate of Cassiopea prevents 

 regeneration of marginal sense-organs and lappets if these also be removed. 

 If, however, the stomach cavity be left intact, the margin of the disk will 

 regenerate its sense organs and lappets. This contradicts Zeleny's law that 



*Very elongate and involved spirals may occasionally be set into sustained 

 pulsation, but this is probably due to electrical leakage back to the center 

 through the gelatinous substance of the disk. 



