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ii. The Posterior or Visceral Aorta
The posterior aorta turns abruptly backwards from the inner end of the truncus arteriosus and plunges deeply between the style sac and the anterior lobe of the digestive gland Near its origin, two smaller arteries arise independently from the truncus. The first is the slender artery of the middle intestine (A Mid I) passing backwards and branching along the wall of the intestinal loop. The other is the anterior rectogenital artery (A Rect G. A) which crosses to the right below the floor of the renal organ and divides into two branches which appear on the surface of the visceral mass. The larger branch passes forward to supply the dorsal surface of the rectum (rectal artery) (Rect A); the smaller branch courses over the albumen gland and receptaculum seminis in the female, and over the vas deferens in the male, after which it too continues along the rectum (artery of the genital duct).
From the posterior aorta itself, branches run to the deep aspect of the stomach, and there is also the branch previously described that forms the
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Text-Fig. 5.—Fig. 3—General view of the venous system of the renal organ, ctenidium and pallial skirt. The right wall of the pallial cavity has been detached from its insertion on the columellai muscle, and has been reflected towards the observer to show the course of the rectal sinus. Fig. 4—Diagram to illustrate the relations of the principal venous sinuses. The two cephalopedal channels are represented as cut across in transverse section. A Abd S, Anterior Abdominal Sinus. A Br Pall. Tributaries of the pallial loop vein from the edge of the mantle skirt. Afft Ct V, Affcrent ctenidial vein. A Pall V, Anterior portion of the palhal loop vein. B Ct S, Basictenidial sinus. B Rect S, Branches of the rectal sinus. Ct Fil V, Blood spaces of the ctenidial filaments. Efft Ct V, Efferent ctenidial vein. Hypobr, Plexus of blood vessels of the hypobranchial gland. L Afft R V, Left afferent renal vein. L Ceph P, Left compartment of the cephalopedal sinus. L Efft R V, Left efferent renal vein. Neph Gl, “Nephridial gland.” P Br Pall, Tributaries from the posterior side of the pallial loop vein Ped S. Pedal sinus. Prob S, Prohoscideal sinus. R Afft R V, Right afferent renal veins R Ceph P. Right compartment of the cephalopedal sinus: R Efft R. V, Right eflerent renal veins. Rect PleX. Plexus of blood vessels on the wall of the rectum Rect S, Rectal sinus. S Osph S, Subosphradial sinus. Subct V. Subcutaneous veins. Subr S, Subrenal sinus. Tr L Eff. Tributaries of the left efferent renal vein. Visc S, Visceral sinus.
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posterior artery of the sorting area. Through the anterior lobe of the digestive gland, a number of arteries run outwards from a deep origin, including a stout hepatorectal artery (H Rect A) giving rise finally to two superficial branches running forward along the rectum. Continuing backwards, the posterior aorta buries itself in the gonad, to emerge at the surface on the right side of the visceral spire. Here it runs along the concave surface to the tip of the spire. At regular intervals it produces deep branches supplying the digestive gland and the gonad, after which they come to the surface at the convex side of the spire and break up into small arterioles just beneath the thin integument. An important function of all the branches of the posterior aorta seems to be the storage of calcium salts: their walls are always conspicuous by their opaque, white coating, and they appear to play some role connected with the calcium metabolism of the adjacent digestive gland. Fretter (1943) has remarked upon a similar storage of calcium salts in the walls of the visceral arteries of Onchidella.