2 PULSATION OF JELLYFISHES. 



The pulsating labyrinth may be simplified after the rhythmic move- 

 ment has started, by cutting parts of it away, or cuts may be made in 

 such manner as to increase its complexity. Any cut which breaks 

 the circuit, however, stops the wave of pulsation, and continuous 

 movement can not again be started. 



The rate of pulsation of the disk is fully twice as fast as that of the 

 normal perfect Medusa. This rate remains constant in the pulsating 

 disk, and when pulsation ceases the movement stops instantly, never 

 gradually. The rate of pulsation in disks deprived of marginal sense- 

 organs depends not upon the area of the tissue forming the circuit, 

 but only upon the length of the circuit. Short circuits pulsate more 

 rapidly than do long ones. In this respect it differs from the con- 

 trol normally exercised by the marginal sense-organs ; for small pieces 

 of tissue with a marginal sense-organ attached pulsate slower than 

 large ones. Moreover, when a sense-organ is present, tissue of any 

 shape will pulsate even if its shape does not form a closed circuit. 



The disks of Aurelia and Dactylometra, if cut as described above, 

 will pulsate as does the disk of Cassiopea. 



These experiments show that the rhythmical pulsation in Medusae 

 must arise from a definite center or centers, 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 pulsate. Sustained pulsation in disks occurs only in tissue forming 

 a complete circuit, and depends upon an electric transmission of 

 energy, and the pulsation is self-sustaining (i.e., sustained by internal 

 stimuli) once it be started by an external, momentary stimulus* 



2. If normal perfect Medusae be lifted out of 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 

 the amplitude only increases, the rate remaining practically constant. 

 The presence of marginal sense-organs is therefore not necessary for 

 the display of "excitement. " 



3. The stimulus which causes pulsation is transmitted by the 

 diffuse nervous or epithelial elements of the sub-umbrella. Newly 

 regenerated sub-umbrella tissue, which lacks muscular elements and 

 can not itself contract, will still serve as a bridge to transmit the 

 stimulus which causes contraction in muscular tissue attached to but 



* Professor W. T. Porter (1897) found that any part of the ventricle of the mamma- 

 lian heart (heart of the dog) will beat for hours if supplied with defibrinated blood 

 through its nutrient artery. Isolated portions of the heart of the hag-fish continue to 

 beat rhythmically for hours even in the absence of nutrition. (See A. J. Carlson, 1905, 

 Amer. Journ. Physiol., p. 220.) 



