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The Blood Vessels of the Renal Organ
The subrenal sinus in Struthiolaria lies immediately beneath the floor of the renal organ, and the afferent renal or renal portal veins are in consequence very short. The lining of the renal sac is infolded to form excretory filaments on two aspects only, and the organ has a bilobed structure, with a left excretory lobe forming its roof and a smaller right lobe forming portion of the floor (Text-fig. 2, R. L). The two lobes are completely separated on the right side by the passage of the rectum and the blood entering the renal organ from the subrenal sinus is at once distributed along one or the other of two distinct systems.
The left afferent renal vein at first proceeds forwards from the subrenal sinus along the anterior part of the floor of the renal sac. It then loops upwards to run back along the whole length of the roof, supplying the filaments of the left excretory lobe over their internal surface. At close and fairly regular intervals, secondary and tertiary afferent vessels are given off at right angles, and the whole system so formed traverses the internal wall of the renal organ, each vessel running along the summit of a renal filament of the corresponding order. This
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system of filaments projects deeply into the cavity of the renal sac, and it would appear in general to be very characteristic of the renal organ of prosobranchs that afferent blood is distributed to the renal filaments by vessels arriving along their summits.
All the blood from the left renal lobe is returned directly to the heart. Blood received at the summit passes through the narrow, cleft-like space between the two sheets of epithelium in each filament. It reaches in this way a series of basal sinuses lying beneath the filaments close to the external wall of the renal organ. These sinuses open directly into a set of 15-20 collecting vessels, (Tr L Eff) which pass across the roof of the renal sac from right to left, to open at right angles into the wide left efferent renal vein (L Efft R V) which runs along the pericardial side of the renal organ. This vessel opens separately into the anterior end of the auricle at some distance from the efferent ctenidial vein.
The second—and smaller—portion of renal blood from the subrenal sinus is distributed through the filaments of the right excretory lobe. There is no single afferent trunk, but a series of vessels which radiate from a common centre through a reticulate mesh of renal filaments. These filaments of the right lobe are not arranged as are those of the left in a regular, monopodial branching pattern, and moreover the afferent vessels do not proceed along the summits. The general direction of blood flow is not vertical from summit to base, but transverse, from right to left, through the spaces within the filaments. The blood from the right lobe of the renal organ is eventually taken into a larger collecting vessel which we may call the right efferent renal vein (R Efft R V). This continues forward along the ventral side of the rectum. In front of the renal organ it breaks up into a close-set plexus over the surface of the rectum, and this becomes continuous with the venous reticulum formed by the branches of the rectal sinus. The blood from the right lobe of the kidney thus joins the rest of the blood destined for the gill and is effectively separated from that of the left lobe, which returns directly to the auricle.