68 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
contraction waves which now originate from the 
ganglion must either become blocked and cease to 
pass round the disc, or they must zigzag round and 
round the tops of these overlapping cuts. Now, 
remembering that the passage of these contraction 
waves is presumably dependent on the nervous 
network progressively distributing the ganglionic 
impulse to the muscular fibres, surely we should 
expect that two or three overlapping cuts, by 
completely severing all the nerve-fibres lying 
between them, ought to destroy the functional 
continuity of these fibres, and so to block the 
passage of the contraction wave. Yet thisis not the 
case ; for even in a specimen of Aurelia so severely 
cut as the one here represented, the contraction 
waves, starting from the ganglion, continued to 
zigzag round and round the entire series of sections. 
The second mode of section to which I have 
alluded is as follows (Fig. 10). The central circle (z) 
stands for an open space cut out of the umbrella; 
the outer circle indicates the margin of the animal, 
with all lithocysts save one (/) removed; and the 
median circular line represents a cut. It will be 
seen that the effect of this cut is almost completely 
to sever the mass of tissue at z from the rest of the 
umbrella, the only connection between them being 
the narrow neck of tissue at 2 Yet, in the case 
to which I refer, the contraction waves emanating 
from / passed in the directions represented by the 
arrows without undergoing any appreciable loss of 
vigour. Upon completing the circular cut at z, the 
ring of tissue (y z) became totally paralyzed, while 
