from behind forwards, owing to the squeezing out of the blood, and by the time the movement has reached the head of the worm, the hinder part of the cord-like heart is beginning slowly to dilate again with the influx of red blood from the veins. What causes the muscles of the heart to contract at regular intervals? There is no doubt that the “stimulus” which excites the heart muscles to contraction is in these simpler animals merely the tension or strain produced by the presence of a sufficient quantity of blood which has flowed into the heart from the veins. The heart muscle, after its rapid contraction, rests; it has no other rest, no sleep, as have all the other parts of the body. It must rest and take refreshment after each effort. Whilst it rests the blood quietly flows in and dilates the heart’s cavity; then the rested muscular wall of the heart, gently stretched by the recovery after compression of its elastic components, nourished and oxygenated by the blood, is ready for another “stroke,” and again it contracts tightly, emptying its cavity of blood, which is driven into the arteries. So it goes on—effort and rest, effort and rest alternating without cease. Whilst it is the stroke of the heart which causes the blood to flow through the arteries into the finest network of hair-like vessels, what is it that causes the blood to flow on through the collecting veins, to reach the heart, and actually to distend that collapsed cavity after its stroke? It must be remembered that a very low pressure is enough to effect this. In the simplest arrangements of worms and such-like animals, there is probably some pressure transmitted to the blood in the veins by the heart-stroke; but the elasticity of the heart-wall and its necessary tendency to resume its dilated condition after its squeezing by its rings of muscle, is what is chiefly effective in drawing on the blood in the veins into the heart.