PUIvSATlON WITHOUT MARGINAL SENSE-ORGANS. 2J 



wave then proceeds as is shown by the sequence of arrows and num- 

 bers until it finally returns with lowered amplitude to the center, 

 where it is instantly restimulated and again sent through the circuit 

 with its energy restored. The same conditions apply to figures 31, 

 ii and in. 



When in regular pulsation we always find that the waves of contrac- 

 tion start from a definite place. The position of this center tends to 

 bear a certain relation to the geometrical figure formed by the cuts. 

 It is marked 5 in figs. 8 to iga, and the arrows show the observed 

 courses of the wave of pulsation . Usually the center of pulsation lies 

 near the periphery of the disk at a place where the tissue is widest 

 and least interfered with by cuts, and it also tends to lie upon the axis 

 of bilaterality of the labyrinth of tissue. 



If we stimulate the disk by dropping it upon a glass plate, etc., 

 the waves of pulsation start from the point 5 ; and this is the place 

 where we must touch the disk if we wish to stimulate it into sus- 

 tained pulsation. Wherever we touch the disk with a crystal of 

 K 2 SO 4 , waves of contraction immediately start out from the touched 

 point, but it is usually impossible to establish a permanent center of 



Fig. 28. Fig. 29. 



pulsation at any point other than one upon the geometrical axis of 

 the figure. Centers at other places either cease to initiate pulsations 

 when the effect of the initial stimulus dies out, or the center 

 quickly shifts to the geometrical axis. Sometimes, however, when 

 a disk is stimulated by a severe mechanical shock, two or more per- 

 manent centers of pulsation appear and waves of contraction start 

 out from each independently and interfere where the opposing waves 

 meet one another. Such conditions are shown in figures 14 and 17. 



It will be observed that with the exception of the very elongate 

 spiral (fig. 14) all of the labyrinths formed by the cuts xtz closed circuits, 

 the tissue being merely a more or less complicated circuit, with the 

 center of pulsation at the geometrical center of the figure. After the 

 disk has begun to pulsate we may cut away portions of the laby- 

 rinth, and the part containing the center will still pulsate, provided it 

 remains a closed circuit. Thus the crescent (figure i8a) is cut out 

 from figure 18 and the ring (figure iga) is made from figure 19, 



