108 THE ELEMENTARY NERVOUS SYSTEM 



of these primary cuts without necessarily interrupting the 

 passage of the wave. Another extreme 'tost mav he car- 

 ried out in which the jellyfish is reduced to two concen- 

 tric rings attached to each other by only a small bridge 

 of connecting tissue (Fig. 32). This type of preparation 

 can be made by excising the center of a jellyfish and then 



Fio. 32. Diagram of the jellyfish Aurelia from which the central mass and seven of 

 the eight marginal bodies have been removed. The outline of the bell has been further 

 complicated by a circular incision that has nearly divided the bell into two rings. The arrows 

 show the course of the contraction wave as it emerges from the one remaining marginal body 

 and passes over the bell. (After Romanes, 1893.) 



reducing it further by a circular cut parallel to its outer 

 edge and midway between this edge and the center. The 

 circular cut is so extended as to form a complete circle 

 except for a fraction of an inch over which the outer ring 

 is in organic connection with the inner one. When a wave 

 of contraction is started from the one remaining mar- 

 ginal body on the outer ring, it will, on arriving at the 

 bridge, pass across to the inner one irrespective of the 

 region in the bell at which the bridge may be located, thus 



