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The Ctenidial Blood Supply
The great bulk of the ctenidial blood in Struthiolaria is carried directly from the subrenal sinus, by a large rectal sinus (Text-fig. 5, Fig. 3, Rect S) which is visible through the external body wall throughout its length. It leaves the right anterior corner of the subrenal sinus and runs forward along the pallial wall, a short distance below the rectum and the genital duct. On its ventral side it receives along its course a little additional blood from the body wall, and from its dorsal side it gives off at right angles a long series of parallel branches running across a narrow strip of the mantle to reach the surface of the rectum. Here, along the right side, that is to say—upon the externally visible wall of the rectum, is located a fine, close-set plexus of tiny vessels, formed by the anastomosing of the ultimate venous branches from the rectal sinus (Rect PleX). Into the same plexus flows also the blood from the right efferent renal vein.
Along the left aspect of the rectum, the blood is again collected by a series of small, transverse vessels which carry it through the subepithelial connective tissue of the hypobranchial gland to the afferent ctenidial vein (Afft Ct V). The hypobranchial gland occupies the whole of the roof of the mantle, between the rectum and the ctenidium. It is very liberally supplied with blood, as are all the mucus-secreting regions in Struthiolaria. The transverse vessels from the rectum anastomose freely, and supply a fine reticulum of small venules, while the
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chief of these vessels continue more or less directly across the hypobranchial gland to the ctenidium. As it leaves the hypobranchial gland, the blood is drained towards the left into the afferent ctenidial vein, a longitudinal collecting vessel which runs along the right side of the ctenidium, and from which venous blood passes directly into the long, narrow blood spaces of the ctenidial filaments (Ct Fil V). The interior of each filament constitutes a single, undivided blood channel. Some amount of re-oxygenation may take place through the membranous, non-ciliated wall of the attached part of the filament; in the stiff rod-like extensions of the filaments, however, the blood space lies for the most part between a pair of chitinous skeletal rods, underlying the epithelium of the filament. The greater part of respiration is evidently pallial (see above p. —). After traversing the filament to the left side of the gill, blood is received into the basictenidial sinus, (B Ct S) which is a long shallow space lying in the pallial roof. It overlies the separate gill filaments for about one-third of their attached length, and over this distance the separate filamentar blood spaces are widely open to the collecting sinus.
The stout efferent ctenidial vein (Efft Ct V) passes along the left or axial edge of the ctenidium, and receives blood through a series of wide apertures from the basibranchial smus. The vein then passes through the posterior wall of the pallial cavity into the pericardium where it opens through the anterior end of the auricle. From the left aspect of the mantle, immediately alongside the gill, a small amount of blood is returned directly to the efferent ctenidial vein, while in addition a narrow subosphradial sinus (S Osph S) communicates with that vessel by a series of short cross connections. This portion of the pallial blood thus returns directly to the heart without diversion through either the renal organ or the ctenidium.