With regard to the forking of the ventral aorta, it is perhaps worthy of note that in two preparations that carry much injection the right division of the aorta appears as though it sprang from the left.

Dorsal Aorta.—As in Bdellostoma, the efferent branchial arteries do not run direct to the dorsal aorta, but those of each side run to a vessel that may, for the sake of convenience, be called a lateral aorta. This occupies a dorso-lateral position parallel to the dorsal aorta, to which it sends regular communicating vessels. These communicating vessels are three in number on each side. The most anterior of these vessels are behind the second pair of gill-sacs. In “A Treatise on Zoology “(Ray Lankester) Goodrich represents in diagram the circulatory system of Bdellostoma, the diagram being based on the figures of Muller, Jackson, and Klinckowström, to which figures I have no access. The diagram shows that the lateral aortae of Bdellostoma have more connecting branches to the dorsal aorta than those of Heptatrema, and that in Bdellostoma there is communication anteriorly to the first gill-sacs. In front of the gill-sacs the lateral aortae of Heptatrema can be traced forward nearly to the head. Posteriorly they turn inwards about abreast of the last gill-sacs to join the dorsal aorta, which has, of course, received all its blood through them. The turn inwards is made at a noticeable angle, and the last efferent branchial vessel of either side may not be received until after the turn is made (see Plate XVII, fig. 1).

The efferent branchial arteries leave the gill-sacs on the anterior aspect. Those of the anterior pair always, and those of the second pair nearly always, branch just as they leave the gill-sac, the two branches entering the lateral aorta separately. The third, and sometimes even the fourth, efferent artery of one side, or both, may branch in like fashion. In the preparation figured in Plate XVII, fig. 1, the second efferent branchial artery of the left side has a distinctly double origin.

From the median dorsal aorta vessels are given off to supply the notochordal sheath, the pharynx, and the body-wall. These last, the segmental arteries, often alternate with each other, but. posteriorly to the heart a more or less regular arrangement in pairs becomes evident. One segmental artery often suffices for two myomeres.

I have not been able to observe the blood-supply of the pro-nephros. With regard to the nephridial system, each segmental artery that crosses the system sends, typically, a branch to the corresponding glomerulus. In cases—and they are frequent—in which there is no segmental artery corresponding to a glomerulus, a renal artery runs direct from the aorta (see Plate XVI, fig. 2). One case was observed in which a segmental artery sends branches to two glomeruli.

Posteriorly to the heart splanchnic arteries run from the dorsal aorta to the intestine; the splanchnic arteries are numerous, and appear to be one in each segment.

Numerous slender arteries pass from the dorsal aorta to the gonads. These arteries are much more noticeable in cases in which there are many ova forming.

Jugular System.—There is a large right jugular vein lying above the lingual mass and beside the pharynx. It arises well forward, abreast of the fifth slime-gland. The vessels that contribute to it come from the body-wall, the lingual mass, and the pharynx. Anteriorly to the gill-sacs it resembles a sinus rather than a vein. In the diagram for Bdellostoma already referred to, the anterior part of the right cardinal

sinus is shown to communicate with the right jugular—indeed, to form the anterior part of it; but there is no reason to suppose that that is the case in Heptatrema.

When the right jugular of Heptatrema gets as far back as the gill-clefts its ventral position becomes more pronounced, as it has to pass below the incurrent branchial tubes. After passing the posterior end of the lingual mass it receives a vein that serves a considerable part of that mass, especially the left side. This may be referred to as the anterior lingual vein. There next enters a vein formed by the union of a left jugular and another vein from the lingual mass. This latter vein, which may be called the posterior lingual vein, is formed by two veins, one from the lower and right portions of the mass, the other from its upper portion. The left jugular vein is formed by two somewhat slender parallel branches lying below the incurrent tubes of the left gillsacs. They unite 1°5 cm. before the posterior lingual vein is reached. After the vein formed by the union of the left jugular and the posterior lingual has entered, the combined jugular vein passes backward towards the heart. It receives the inferior jugular, a median vessel of considerable size. This vein is itself formed by the union of two veins running in the ventral body-wall below the lingual mass. After entry of the inferior jugular the jugular trunk passes to the posterior end of the sinus venosus, which it enters in close conjunction with the right hepatic vein.

Anterior Cardinal System.—The right anterior cardinal sinus starts beside the notochord at the base of the cranium. It runs backward in the body-wall until it comes abreast of the first gill-sac; then it tends towards the middle line coming to lie beside the dorsal aorta, which, from the fifth gill-sac backwards, lies between it and the left anterior cardinal sinus. It receives many lateral veins. It ends in the portal heart. No part of it communicates with the sinus venosus.

The left anterior cardinal sinus commences in a position corresponding to that of the right. It early receives a large branch from below and beside the pharynx. This suggests that the anterior part of the left jugular may perhaps join the sinus. Posteriorly the left anterior cardinal sinus joins with the posterior cardinal trunk to enter the sinus venosus.

Posterior Cardinal Sinuses, Right and Left.—A median sinus originates just below the notochord in the tail. A right sinus presently separates from this, runs parallel to it for a little way, and rejoins it. This may be repeated once or twice. Finally the two sinuses are well established, and communicate with each other by several wide connecting branches below the dorsal aorta. As the sinus venosus is reached the right sinus gives off a small branch, which crosses the left sinus and runs to the portal heart. The right sinus then joins the left, and the combined trunk, with the left anterior cardinal sinus, joins the sinus venosus.

Segmental veins running from the body-wall enter the corresponding posterior cardinal sinus. Usually there is one such vein to two myomeres. Sometimes the renal vein running from a glomerulus enters one of these segmental veins, but more often it runs direct to the posterior sinus of its side. The renal veins leave the glomeruli on the ventral aspect. The segmental veins pass over the nephridial system dorsally.