Circulatory and Respiratory Organs (pls. iv. and v.)
The systemic heart (pl. iv. i) lies about the centre of the pervisceral cavity. In shape it is nearly triangular, the base of the triangle lying towards the oral end of the body. The right side is the larger, owing to the great cephalic aorta being given off from this side. Lying at the base of the branchiæ are the large branchial hearts (pl. v., fig. 1f), globosely ovoid in shape, and with their axis forming an acute angle with that of the body. At its inner end each has a small fleshy appendage (pl. v., fig. 1f'), and each is encased in a chamber with transparent membranous walls (id., k).
As it approaches the heart the vena cava (id., b) divides, one branch going on each side of the intestine at its point of flexure and entering the branchial heart on its upper surface. Veins (pl. iv., v, v) are seen converging on the inner side of the mantle to enter the vena cava. These cross from the mantle by the peritoneal membrane which is thickened for the purpose. Similar veins run from the anterior aspect of the body, entering the vena cava with those from the sides of the mantle.
After passing from the branchial heart through the gills the blood enters the systemic heart at the two dilatations (“auricles”) already noted as giving the triangular shape to the heart. The left auricle is the more strongly marked, the right being somewhat obscured by the great development at the point whence the cephalic aorta (pl. v., 1 c) is given off. Shortly from its commencement this aorta gives off branches to the liver. At its opposite end the heart contracts to give off the posterior aorta (id., h), which, shortly after its commencement, divides into three branches, these being borne along reflexions of the peritoneal membrane to the mantle.
The branchiæ (pl. iv., g, g; pl. v., fig. 1) are large and prominent, the tip extending to beyond the base of the funnel. About 70 non-ciliated lamellæ are given off on each side. The continuation of the branch of the vena cava, after passing through the branchial heart, forms the central axis of the gill on the anterior side; and it is attached throughout the whole length of the gill by a suspensory membrane (id., a) to the anterior mantle surface. On the posterior side, the branchial vein forms the central axis. Each lamella is bordered by two capillaries, one running from each of the axes of the gill and meeting at the apex of the lamella, and themselves
borne on a fine suspensory membrane. Across the space between them stretch still smaller plume-like lamellæ (id., fig. 2), each with a small capillary. These sub-lamellæ look like repetitions of the entire gill; but, on microscopic examination, it is seen that they consist of a capillary with a very thin membrane thrown into transverse folds (fig. 3) on each side.
The renal organ (fig. 1 g) is well-developed on the branches of the vena cava. Two long lobes extend over the heart, which they almost hide; and each of these lobes has a well-marked opening, which communicates with its branch of the vena cava. Thus, if air be blown into one of the openings, it inflates its branch of the vena cava, and also the branchial heart into which that branch opens.