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The Heart and Pericardium
There are few special features calling for remark in this region in Struthiolaria. The two-chambered heart is enclosed in a pericardial sac, lying at the base of the visceral mass on the left side, immediately behind the pallial cavity. The pericardium indents the left side of the large unpaired renal organ. A flattened squamous epithelium lines its cavity—a reminder of its coelomic origin. None of the blood vessels, however, has any squamous lining, though in some of the arteries the longitudinal connective tissue elements have a compact arrangement giving the appearance of a pseudo-epithelium. Nor are the arterial walls themselves muscular, though in a few cases strands of extrinsic muscle are inserted upon the tough collagenous wall of the vessel, as in the proboscideal artery (p. 17). The pericardial lining is never excretory in Struthiolaria, or—as far as is known—in any of the higher prosobranchs. This function is carried out by the renal organ and, to an increasing extent in the higher types, by the digestive gland. A reno-pericardial duct (Text-fig. 2, Rp) leads from the pericardium to the cavity of the renal sac, and this sac was itself originally one of a pair of tubular coelomoducts opening out of the pericardial coelom. The renopericardial
Text-Fig. 1.—Stereogram of portion of the crystalline style sac and of the proximal region of the intestine, showing the location of the phagocytic depot, and its relation to the smaller vessels of the arterial supply. A St S, Anterior artery of the style sac. C. T. Connective tissue containing melanophores. Ext B W, External wall of visceral mass. Int A, Small arteries of intestinal wall. Int Ch, Intestinal channel of style sac. Phag. Phagocytic zone surrounding style sac. Phag 1, Appearance of phagocytic zone at surface of body wall. Phag 2, Phagocytic zone surrounding intestine. P Int, Proximal division of the Intestine. St Sac, Style sac. (See also Morton, J. E., 1951, p. 16.)
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duct runs forward a short distance in the floor of the renal sac, into which it opens by a whitish papilla about a quarter of an inch behind the external renal aperture. The duct is about 50μ in diameter, its epithelium thrown into 5-8 longitudinal folds by differences in the height of the cells. All the cells bear very long cilia, reaching as much as 15μ in length, and their direction of beat seems to be outwards into the renal sac. In Struthioloria, neither the male nor the female retains the genito-pericardial duct, which was, however, reported by Fretter (1941) to survive in several genera of mesogastropods and stenoglossans.
The auricle of the heart is thin-walled and pyriform; its broad anterior end receives the efferent ctenidial vein and a smaller, separately opening, left efferent renal vein. The wall of the auricle incorporates a thin mesh work of muscle fibres, capable of strong contractions. The tapered posterior end of the auricle opens by a valved aperture into the ventricle, which is a much more muscular sac. It is spongy in texture and is built up of a reticulum of interwoven fibres,
Text-Fig. 2.—The renal organ opened along its right side to show the venous supply of the interior. The course of the left efferent renal vein is shown with broken lines, beneath the reflected left lobe of the renal organ. L Afft R V, Left afferent renal vein. L Efft R V, Left efferent renal vein. Pl R R L, Venous plexus of the right renal lobe. R Afft R V, Right afferent renal vein. R Ap, External renal aperture. R Efft R V, Right efferent renal vein. R L, Right lobe of the renal organ. Rm,. Rectum. R P, Reno-pericardial aperture and direction of its duct to pericardium. Subr, Point of origin of afferent renal veins from underlying subrenal sinus. Vas D, Coils of vas deferens. V Vdf, Venous plexus of the renal floor between the underlying coils of the vas deferens.
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more coherent in the outer wall, and extending inwards to run in various directions across the lumen. The narrow apex of the ventricle opens behind into a structure known as the “truncus arteriosus”, which has been described in several other mesogastropods (see Moore, (1899)) and which forms virtually a third chamber of the heart, giving rise at either end to an arterial trunk. It is retropericardial in position, and bulges against the posterior side of the pericardium. Its walls are composed of non-contractile fibrous connective tissue.