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The Venous System
(Text-Fig. 5 and Text-fig. 3.)
Struthiolaria has a well-developed system of closed veins which return blood from the peripheral lacunae to a system of central haemocoelic spaces. All the veins from the visceral mass drain into a narrow, spiral visceral sinus, while the greater part of the blood of the head, foot and trunk is returned to the much larger cephalopedal sinus. Both these spaces open in turn into a third cavity, the subrenal sinus, corresponding to the space called by Pelseneer (1906) the anterior abdominal sinus. This forms the principal collecting and distributing centre for the venous blood, which passes from it indirectly to the auricle, by way of the renal organ, the ctenidium or through the renal organ and the ctenidium together.
The cephalopedal sinus is a wide space lying immediately upon the muscular floor of the trunk, and constituting the lower part of the trunk and occupying the whole floor of the haemocoele. It is formed by two narrowly separated and parallel sinuses, which may be referred to as the right and left cephalopedal spaces (Text-fig. 5, Fig. 4, R Ceph P, L Ceph P). The compartment on the right is arched in transverse section and is a good deal more spacious than the left, from which it is delimited by a thin, muscular septum passing obliquely across the cavity of the trunk. The left compartment is extremely shallow, forming a wide, dorsoventrally compressed cleft. On the floor of the cephalic cavity, just beneath the nerve ring, the right and left spaces of the cephalopedal sinus coalesce to form a single narrow channel which expands again in front into the pedal sinus (Ped S). The latter forms a very large cavity, occupying the whole of the foot, immediately above the muscles of the sole. It is very extensible and—when the foot is fully expanded, becomes flooded with blood from the pedal arteries.
At the posterior end of the trunk, the two compartments of the cephalopedal sinus also converge, forming here a single, narrowly constricted opening into the subrenal sinus (Subr S). This consists of a shallow triangular space, lying beneath the posterior part of the floor of the mantle cavity and extending backwards below the pericardium, the renal organ and part of the gastric complex. Ventrally it is bounded by the posterior end of the columellar muscle as this attaches to the shell. In addition to blood from the cephalopedal sinus in front and
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from the visceral sinus behind, the subrenal sinus appears to receive a smaller amount of venous blood which is conveyed backwards along the floor of the pallial cavity by the wide subcutaneous sinus of the food groove. From the subrenal sinus venous blood is distributed along one of two paths: by renal portal veins or afferent renal veins which open directly from it on the right dorsal aspect, or by the rectal sinus (Rect S), a stout vein running forward along the right side and conveying blood to the pallial roof and the gill.
The visceral sinus (Visc S) runs forward to the subrenal collecting centre from the tip of the spire, being visible without dissection, along the concave side of the visceral mass. Upon either side it receives at right angles some 15-20 hepatogenital veins from the digestive gland and the gonad. These vessels alternate in position with the visceral arteries and meander across the viscera immediately beneath the integument. Further forward, on the left, several stout gastric veins return blood from the stomach.