30 PULSATION OF JELLYFISHES. 



27, 270. This, I believe, is due to the fact that the contraction 

 wave returns so quickly to the center that an insufficient time elapses 

 before the center is again called upon to restimulate the wave. As 

 Romanes showed, an appreciable interval of time must elapse before 

 tissue which has been in contraction can again contract. 



Very elongate, many-whorled spirals, such as one sees in figure 14, 

 are the only forms not closed circuits that we have succeeded in set- 

 ting into constant pulsation. This occurs only when two or more 

 centers arise simultaneously in the spiral, as in S, S', and S", figure 14. 

 These centers mutually sustain one another, the contraction wave from 

 one being restimulated and reflected back from the other. If one 

 attempts to convert a series of partial rings (fig. 32, A) into a spiral 

 by successive cuts, as shown in the dotted lines, 1-5, (fig. 32, B) the 

 tissue ceases to pulsate as soon as the final cut (5) is made which 

 breaks the last circuit. 



A B 



Fig. 32. Showing that sub-umbrella tissue can not maintain itself in pul- 

 sation unless it has the shape of a closed circuit. If cuts be made as 

 shown in the dotted lines in the order 1, 2, 3, 4, 5, the tissue ceases to 

 pulsate as soon as cut number 5 breaks the last complete circuit. 



It must be borne in mind that cuts through the sub-umbrella tissue 

 heal over in the course of a day or two and will then transmit pulsa- 

 tion more or less imperfectly across the healed lines, and then a spiral 

 will pulsate, for it is, physiologically speaking, only a series of concen- 

 tric rings of readily conducting tissue with numerous more or less 

 imperfect points of conduction between the annuli. Similarly a disk 

 having complete circular cuts through the muscular tissue of the sub- 

 umbrella, such as is shown in figure 26, can not be made to pulsate 

 continuously as a whole until two or three days after the operation, 

 although each annulus may be made to pulsate independently. After 

 several days of healing the cuts will allow a more or less imperfect 

 conduction of impulses across from one ring to another, and the con- 



