Nervous system

Reproductive system strongly inflated, causing dorsolateral compression of the more ventral oesophagus, and inflation of the central and anterior food groove above. The prostate has thick, muscular walls with the central lumen Bodies of females are usually more voluminous than those of males, owing largely to the presence of the voluminous brood pouch.

Male reproductive system : The large, light orange-brown testis lies posteriorly in the viscera, and is elaborated into two lobes ( Fig. 9). The posterior lobe lies ventrally and in the columellar aspect of the posterior part of the digestive gland, whereas the anterior lobe occupies the columellar aspect of the anterior part of the digestive gland, and is expanded ventrally and dorsally. Both lobes lie in close proximity but are physically separated. Testis and digestive gland are adjacent, not interdigitated; both organs can be differentiated based on colour (light orange-brown and beige, respectively) and texture (lobules of the testis are smaller than the acini of the digestive gland). Small ducts drain both lobes and coalesce into the visceral vas deferens which lies along the columellar aspect of the whorls. Much variation exists in the width of the visceral vas deferens between individuals, but it is roughly half the diameter of the oesophagus. Anteriorly the visceral vas deferens lies dorsal to the oesophagus, and at the anteriormost margin of the anterior testis it connects to the pallial vas deferens which is ~0.3 whorl in length and proximally enlarged to ~5× the width of the distal visceral vas deferens and glandular. The proximal pallial vas deferens lies adjacent to the columellar muscle to the right, and the anterior pericardium and posterior mantle cavity to the left, in a position comparable to that of the pallial oviduct in females. It is white and elongate, broader posteriorly and gradually narrows anteriorly. Its walls are thin, with the central lumen constricted by thick, interdigitating transverse glandular lamellae. At its anterior extremity, which lies at ~½ of the length of the mantle cavity, the proximal pallial vas deferens turns ~180° towards the left into the distal pallial vas deferens. The distal pallial vas deferens is shorter (~½ of the length of the proximal pallial vas deferens) and only slightly wider than the visceral vas deferens. It crosses the mantle floor to the proximal end of the bulging prostate which lies embedded in the mantle floor underneath the food groove. The glandular prostate spans more than ¾ of the cephalic haemocoel and deflects at the neck to the right, narrowing further as it continues into the dorso-ventrally flattened right tentacle, which functions as a penis. The prostate is constricted by thin, transverse, glandular lamellae. At its tip, the penis forms a tentacular hook that curves into a subterminal cavity when not in use. In contrast to the greyish dorsal surface of the penis, the tentacular hook is white, with a terminal gonopore.

Female reproductive system : The small, lobate ovary lies along the columellar aspect below the anterior end of the digestive gland and is drained by a narrow, simple visceral oviduct ( Fig. 10). The visceral oviduct joins the narrower, proximal, descending limb of the renal oviduct at its posterior junction via a broad duct that connects proximally to the complexly branched seminal receptacle. The seminal receptacle is positioned along the left side of the posterior brood pouch below the rectum and is triangular in lateral view with the narrow pointed tip projecting posteriorly. The organ is wedgeshaped in cross section, with the broad, curved dorsal aspect embracing the rectum, and tapering ventrally.

The renal oviduct forms a thick, muscular, ciliated U-shaped tube embedded in connective tissue ventral to the brood pouch. A gono-pericardial duct is lacking. The distal, ascending limb of the renal oviduct is slightly wider in diameter than the proximal limb and opens after a sharp U-turn into a broad, flattened albumen gland, which also lies ventral to the brood pouch, just anterior to the midpoint. The albumen gland connects to the capsule gland, which is a slightly narrower, glandular tube that continues posteriorly embedded in the ventral surface of the brood pouch. The capsule gland opens to the brood pouch posteriorly to the right, and has been observed to contain encapsulated eggs at the junction. The brood pouch contains a longitudinal sperm groove, which extends from the posterior end of the brood pouch to the gonopore. The brood pouch is undivided, with thin, translucent walls; it typically contains four to five large eggs in a single row. Anteriorly, the pallial oviduct narrows abruptly, before terminating near the mantle collar in the vagina. The vagina is a free hanging, muscular tube with a round terminal gonopore.

PHYLOGENETICS

Based on its area of occurrence and its attribution by previous authors, Rivularia would be expected to be a bellamyine, but our phylogenetic reconstruction including various bellamyines indicates that Rivularia is a viviparine ( Fig. 11). This result was unambiguously retrieved for all used methods of phylogenetic inference (i.e. maximum parsimony, maximum likelihood and Bayesian inference). The taxon comes out as sister group to European Viviparus in our phylogenetic reconstruction ( Fig. 11). Furthermore, both of the included viviparid subfamilies form monophyletic clades.