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152, (fig. 2, b.) and, at the same time, the stream in the centre commences its ascent, as indicated in the drawing by arrows. When the chambers have attained their greatest state of expansion, the sides of the lower valve are forced upwards by the increased flow of the blood from the section below the valve, the lateral openings are closed, and the main current of the blood is projected through the two valves, as shewn in fig. 3, a.

To each side of the abdominal portion of the dorsal vessel, or that which is divided into separate chambers furnished with valves, are attached several flat triangular muscles, the points of which are fixed to the dorsal plate of the abdomen, (Plate III. fig. 1, h.) These were called by Lyonnet, rather fancifully, the wings of the heart, and their object is to retain that organ in its place, and probably to aid its contractions and expansions. When of sufficient length, (as in the example figured,) they adhere immediately to the arch of the abdomen, but when short, their attachment is by means of a supplementary band of fibres.

As the part of the dorsal vessel, to which these remarks refer, is regarded as the true heart, so the anterior portion, which is a simple continuous tube, may be considered as representing the aorta. It commences where the valves and lateral muscles terminate, passes through the thorax, and terminates