Page:Popular Science Monthly Volume 14.djvu/322

308 the ganglia are collected into these eight marginal bodies, as proved by the fact that on cutting out all the eight marginal bodies paralysis of the bell ensues. Therefore, if the reader will imagine this diagram to be overspread with a disk of muslin, the fibres of which start from one or other of these marginal ganglia, he will gain a tolerably correct idea of the lowest nervous system in the animal kingdom. Now suppose that seven of these eight ganglia are cut out, the remaining one then continues to supply its rhythmical discharges to the muscular sheet of the bell, the result being, at each discharge, two contractile waves, which start at the same instant, one on each side of the ganglion, and which then course with equal rapidity in opposite directions, and so meet at the point of the disk which is opposite to the ganglion. Suppose now a number of radial cuts are made in the disk, according to such a plan as this, wherein every radial cut deeply overlaps those on either side of it. The contractile waves which now originate from the ganglion must either become blocked and cease to pass round the disk, or they must zigzag round and round the tops of these overlapping cuts. Now, remembering that the passage of these contractile 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 contractile wave. Yet this is not the case; for, even in a specimen of Aurelia so severely cut as the one here represented, the contractile waves, starting from the ganglion, continued to zigzag round and round the entire series of sections.

The same result attends other forms of section. Here, for instance, seven of the marginal ganglia having been removed as before, the eighth one was made the point of origin of a circumferential section, which was then carried round and round the bell in the form of a continuous spiral—the result, of course, being this long ribbon-shaped strip of tissue with the ganglion at one end, and the remainder of the swimming-bell at the other. Well, as before, the contractile waves always originated at the ganglion; but now they had to course all the way along the strip until they arrived at its other extremity; and, as each wave arrived at that extremity, it delivered its influence into the remainder of the swimming-bell, which thereupon contracted.