STRUCTURE OF THE MEDUSAE. 25 



ing this stiiniilns. I may add that when nerve-cells 

 are collected into ganglia, they often appear to 

 discharge their energy spontaneously ; so that in 

 all but the very lowest ^nimals, whenever we see 

 apparently spontaneous action, we infer that ganglia 

 are probably present. Lastly, another important 

 distinction must be borne in mind — the distinction, 

 namely, which is to be drawn between muscle and 

 nerve. A stimulus applied to a nerveless muscle 

 can only course through the muscle by giving rise 

 to a visible wave of contraction, which spreads in 

 all directions from the seat of disturbance as from a 

 centre. A nerve, on the other hand, conducts the 

 stimulus without sensibly moving or undergoing 

 any change of shape. Now, in order not to forget 

 this distinction, I shall always speak of muscle- 

 fibres as conveying a visible wave of contraction, 

 and of nerve- fibres as conveying an invisible, or 

 molecular, wave of stinadation. Nerve-fibres, then, 

 are functionally distinguished from muscle-fibres — 

 and also from protoplasm — by displaying the pro- 

 perty of conducting invisible, or molecular, waves 

 of stimulation from one part of an organism to 

 another, so establishing physiological continuity 

 between such parts, without the necessary passage of 

 waves of contraction. 



