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 _stimulation_. Nerve-fibres, then, are functionally distinguished from muscle-fibres--and also from protoplasm--by displaying the property 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_. CHAPTER II. FUNDAMENTAL EXPERIMENTS. The naked-eyed Medusæ are very much smaller in size than the covered-eyed, and as we shall find that the distribution of their nervous elements is somewhat different, it will be convenient to use different names for the large umbrella-shaped part of a covered-eyed Medusæa and the much smaller though corresponding part of a naked-eyed Medusa. The former, therefore, I shall call the umbrella, and the latter the swimming-bell, or nectocalyx. In each case alike this portion of the animal performs the office of locomotion, and it does so in the same way. I have already said that this mushroom-like organ, which constitutes the main bulk of the animal, is itself mainly constituted of thick transparent and non-contractile jelly, but that the whole of its concave surface is lined with a thin sheet of muscular tissue. Such being the structure of the organ, the mechanism whereby it effects locomotion is very simple, consisting merely of an alternate contraction and relaxation of the entire muscular sheet which lines the cavity of the bell. At each contraction of this muscular sheet the gelatinous walls of the bell are drawn together; the capacity of the bell being thus diminished, water is ejected from the open mouth of the bell backwards, and the consequent reaction propels the animal forwards. In these swimming movements, systole and diastole follow one another with as perfect a rhythm as they do in the beating of a heart. _Effects of excising the entire Margins of Nectocalyces._