The heart is essentially an enlargement of the great stem or main blood-vessel which, like the trunk of a tree, has branching roots at one end of it and ordinary branches at the other. The trunk branches, and roots of the “heart-tree” are, of course, hollow blood-holding tubes, not solid fibrous structures, as are the woody branches and trunk of a vegetable tree. Further, the finest rootlets and the finest terminal branches in the case of the heart-tree are connected to one another by the network of very fine branches or by great blood-holding cavities, which occupy all parts of the body of an animal. The enlarged part of the trunk of the tree-like system of blood-vessels—the heart—has powerful muscles forming its walls, the fibres disposed so as to surround the contained chamber. When these muscular fibres contract, they squeeze the walls of the chamber together and drive the blood out of it into the forward branches, called “arteries.” It is prevented from going backwards into the hinder branches called “veins” (which we compare to the roots of a tree) by flaps which are so set on the inside of the great vessel at the entrance to those branches that the flaps are made to move out across the space by the backward current, and thus prevent any backward flow, whilst a forward current merely presses them flat against the wall of the vessel, and thus no obstruction to a forward flow is presented. These flaps are called the valves of the heart. The consequence of this arrangement is that whilst blood flows freely into the heart from the veins or hinder (root-like) set of vessels, it is driven by the muscular contraction of the heart—only in one direction—namely, forwards into the arteries. This movement in one direction is helped in some elongated hearts by the contraction of the wall of the heart beginning behind and spreading quickly forward like a wave. The heart of the common earth-worm and of small transparent worms with red blood like it, which are common in the mud of ponds and rivers and can be easily watched with the microscope so that one can see through their glass-like skin what is going on inside them, shows very beautifully this wave of contraction. The heart in these worms is a long contractile vessel which runs the whole length of the body along the back. You can watch the red blood flowing into it through the veins in each ring or segment of the worm’s body—slowly swelling it out—so that it looks like a long red cord. Then, suddenly, there is a movement like a flash in its rapidity, passing from behind forwards! The walls of the red cord-like heart contract so as to drive the blood forward into the arteries, which also are present in every ring of the worm’s body. At the same time you can see the valves, which hang at the entrance of the veins to the heart, swing with a sudden “chuck” and close those vessels against the driven blood. The red cord becomes colourless progressively