a single remaining ganglion at one end. (The cuts interposed in the parallelogram may for the present be neglected.) Now, if the end-mark a of the nervo-muscular sheet most remote from the ganglion be gently brushed with a camel's-hair brush—i. e., too gently to start a responsive contractile wave—the ganglion at the other end will shortly afterward discharge, as shown by its starting a contractile wave at its own end of the parallelogram b, thus proving that the stimulus caused
Fig. 5.
by brushing the tissue at the other end, a, must have been conducted all the way along the parallelogram to the terminal ganglion b, so causing the terminal ganglion to discharge by reflex action. Indeed, in many cases, the passage of this nervous wave of stimulation admits of being actually seen. For the numberless tentacles which fringe the margin of Aurelia are more highly excitable than is the general contractile tissue of the bell; so that, on brushing the end a of the parallelogram remote from the ganglion, the tentacles at this end respond to the stimulus by a contraction, then those next in the series do the same, and so on—a wave of contraction being thus set up in the tentacular fringe, the passage of which is determined by the passage of the nervous wave of stimulation in the superjacent nervous network. This tentacular wave is here represented as having traversed half the whole distance to the terminal ganglion, and when it reaches that ganglion it will cause it to discharge by reflex action, so giving rise to a visible wave of muscular contraction passing in the direction b a, opposite to that which the nervous or tentacular wave had previously pursued. Now, this tentacular wave, being an optical expression of the passage of a wave of stimulation, is a sight as beautiful as it is unique; and it affords a first-rate opportunity of settling this all-important question, namely—will this conductile or nervous function prove itself as tolerant toward a section of the tissue as the contractile or muscular function has already proved itself to be? For, if so, we shall
