Art. XXVI.—Some Features of the Circulatory System of Heptatrema cirrata Forster.
[Read before the Wellington Philosophical Society, 4th October, 1911.]
Plates XVI, XVII.
During this year I obtained several specimens of Heptatrema cirrata Forster. In this paper I give a short account of the circulatory system, which presents some features of interest. As there are not in New Zealand the publications containing the papers of most of the workers on Myxinoid anatomy, I do not go into any great detail in this paper.
Ten specimens were at different times injected. Injections were made into the ventral aorta, the dorsal aorta, and usually one of the posterior cardinal sinuses. Although the injection of Heptatrema is often very effective, it is apt to be capricious. Usually an injection thrown into one of the posterior cardinal sinuses suffices to fill the whole venous system, but at times such an injection fails in one part or more. I have not yet attempted to inject the lymphatic system, which, from the extent of the subdermal lymph-spaces, is probably extensive and diffuse. I have found gelatine the most suitable injecting vehicle.
The circulatory system of Heptatrema presents, as might be expected, many resemblances to that of Bdellostoma, but it presents also some notable differences. The ventral aorta, for example, branches before any afferent branchial arteries are given off; the jugular system achieves great development; the short subintestinal vein passes direct to the right hepatic portal vein, not to the sinus venosus.
Heart.—The sinus venosus is, as usual, thin-walled, and it presents no considerable dilatation when fully injected. The atrium has thick, spongy, and muscular walls. It dilates greatly when injected, and then often presents a lobulated appearance. The passage from the atrium to the ventricle is guarded by a single pair of deep “pocket “valves. The wall of the ventricle is enormously thick, and its cavity is small. The passage to the short bulbus aortae is guarded by a single “sleeve” valve of peculiar construction. The base of this “sleeve “is attached to the wall of the ventricle, and the “sleeve “projects into the bulbus. It is not, however, free, but each side has a line of attachment to the wall of the bulbus. These lines of attachment are dorso-lateral and ventro-lateral. The terminal portion of the “sleeve “is wider than the basal portion, and the edges of this terminal portion tend to fall together, closing the passage.
Ventral Aorta.—Except for the short bulbus, there is no median portion of the ventral aorta, forking of the aorta taking place immediately. Each division of the aorta runs for about 1 cm. before it gives off the earliest of its afferent branchial arteries. The afferent branchial arteries are long and curved, an arrangement that fully provides for the great dilatation of the pharynx that takes place when large pieces of food are passed along it. Each afferent branchial artery enters the wall of the gill-sac on its posterior aspect just below the point of origin of the excurrent branchial tube. The most anterior of the series of gill-sacs is supplied by the terminal portion of the division of the aorta.
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.
Supra-intestinal Vein.—The supra-intestinai vein runs forward from near the termination of the intestine. It ends full in the portal heart.
Subintestinal Vein.—Veins from the anterior ventral part of the intestine unite to form a short subintestinal vein. The veins upon the surface of the gall-bladder unite to form a cystic vein which joins the subintestinal. The subintestinal vein then enters the right portal vein. It does not carry blood through the liver direct to the sinus venosus, as stated by Goodrich (loc. cit.) for Myxinoids generally.
The portal heart receives blood from three sources — the supra-intestinal vein, the right anterior cardinal sinus, and the right posterior cardinal sinus; while into the right portal vein there flows the sub-intestinal. The portal vein, on leaving the portal heart, forks, right and left veins passing to the corresponding lobes of the liver. These branches enter the liver on the lower (concave) surface.
Explanation of Plates.
Fig. 1. Diagrammatic representation of the circulatory system.
Fig. 2. Part of the dorsal vessels and the nephridial system, from the dorsal aspect.
a. Renal veinlet running from the dorsal aspect of a glomerulus to a segmental vein in the body-wall.
b. Segmental artery sending branches to two glomeruli.
Fig. 1. The efferent branchial vessels and the anterior part of the dorsal aortic system, from the dorsal aspect.
Fig. 2. The afferent branchial system, from the ventral aspect.
Fig. 3. Right afferent branchial vessels, from the right side.
Fig. 4. Anterior part of post-cardinal system, showing connection of right sinus with portal heart.
Fig. 5. The jugular system, dissected from ventral aspect.
|a.l.||Anterior Imgual vein.|
|e.b.t.||Excurrent branchial tube.|
|ef.br.||Efferent branchial tube.|
|i.i.||Inferior jugular vein.|
|i.b.t.||Incurrent branchial tube.|
|j.t.||Combined jugular trunk.|
|l.a.||Left division of anterior aorta.|
|l.a.c.||Left anterior cardinal sinus.|
|l.h.||Left hepatic vein.|
|l.j.||Left jugular vein.|
|l.n.||Left nephridial system.|
|l.p.c.||Left posterior cardinal sinus.|
|l.pt.||Left branch of portal vein.|
|l.v.||Left division of ventral aorta.|
|m.a.||Median division of anterior aorta.|
|p.l.||Posterior Imgual vein.|
|r.a.||Right division of anterior aorta.|
|r.a.c.||Right anterior cardinal sinus.|
|r.c.||Communicating branch from right posterior cardinal sinus to portal heart.|
|r.h.||Right hepatic vein.|
|r.j.||Right jugular vein.|
|r.n.||Right nephridial system.|
|r.p.c.||Right posterior cardinal sinus.|
|r.pt.||Right branch of portal vein.|
|r.v.||Right division of ventral aorta.|