Nervous system

Bocxlaer, Bert Van, Strong, Ellen E., Richter, Romy, Stelbrink, Björn & Rintelen, Thomas Von, 2018, Anatomical and genetic data reveal that Rivularia Heude, 1890 belongs to Viviparinae (Gastropoda: Viviparidae), Zoological Journal of the Linnean Society 182, pp. 1-23 : 8-13

Alimentary system

Buccal apparatus: Mouth forming a transverse slit ( Fig. 5B), opening ventrally at the anterior end of the smooth oval snout; buccal apparatus slender and elongate ( Fig. 6A), occupying the length of the snout. Odontophore occupying ~4/5 of the length of the buccal mass, somewhat dorso-ventrally flattened. A small, thin paired jaw flanks the mouth laterally, lying between the anterior ends of the dorsal folds and the oral tube. A thin, narrow, v-shaped subradular organ extends along the ventral surface of the odontophore at the anterior end, below the radula. The buccal pouches form two slight, thin-walled expansions below the dorsal folds, just in front of the buccal ganglia, and are slightly less glandular than the dorsal food groove. Two prominent salivary glands open dorso-laterally alongside the dorsal food groove in the dorsal folds, slightly anterior to the middle of the buccal apparatus; they pass posteriorly through the circumoesophageal nerve ring, where two proximate, compact masses are elaborated between the more anterior cerebral ganglia and the posterior connective of the pleural ganglion and the supra-oesophageal ganglion. Laterally, the salivary glands may overly the nerve ring slightly. Thick buccal retractors extend laterally from the rear of the buccal mass between the cerebro-pedal connectives and the pleural ganglia to insert on the walls of the cephalic haemocoel. The radular sac is short, projecting ventrally slightly past the end of the buccal mass and just through the nerve ring.

Radula: The radula is small (on average ~ 2.3 mm but up to ~ 2.9 mm in length in adults), slender and delicate, taenioglossate with on average ~55 (but up to ~65) rows of teeth. The rachidian is symmetric and possesses a blunt central cusp which can be broad ( Fig. 7A), or narrow ( Fig. 7B–D); it is bordered by three to four pointed cusps on each side that decrease outwardly in size; the flanking cusps are typically longer than the central cusp. The lateral teeth ( Fig. 7A–D) are similar in width but longer than the rachidian, and somewhat asymmetrical. They have a slightly larger, blunter central cusp than that of the rachidian, which is bordered by four to five cusps on either side that decrease outwardly in size, again with the flanking cusps longer than the central cusp. The inner marginal teeth ( Fig. 7A–D) are more asymmetrical than the lateral teeth, with long slender shafts (half the width of the lateral teeth and rachidian) and a cutting edge with one large, rounded inner cusp, bordered by one to two inner pointed cusps, which may be lacking, and two to three outer pointed cusps. The outer marginal teeth ( Fig. 7A–D) have shafts that are approximately half the width of the inner marginal teeth, and are even more asymmetric with one outer rounded cusp and one or two pointed inner cusps. The most central of these pointed cusps is equal in length to the rounded cusp.

Oesophagus: The anterior oesophagus emerges from the dorso-posterior aspect of the buccal apparatus and forms a simple, narrow, somewhat dorsolaterally flattened tube (especially in males due to the bulging prostate) that bears continuations of the dorsal folds and dorsal food groove; oesophageal pouches are lacking ( Fig. 6A). In the mid-oesophagus, which starts just behind the nerve ring, the continuation of the dorsal folds and the dorsal food groove display the effects of torsion, which are abrupt rather than gradual. The mid-oesophagus is somewhat glandular and curves broadly beneath the food groove; its walls are irregularly longitudinally ridged and glandular, but it does not produce outpocketings or pouches; it deflects towards the right at the posterior part of the mantle cavity to continue posteriorly along the columella, passing below the reno-pericardial complex. The posterior oesophagus is finely longitudinally ridged and widens just before opening ventrally to the posterior gastric chamber.

Gastric chamber: The gastric chamber ( Fig. 6B) lies at the left aspect, just behind the elongate pericardium and embedded in the lobes of the digestive gland. It occupies ~0.5 whorls, and has thin, non-muscular walls. A prominent T-shaped ridge compartmentalizes the gastric chamber floor into three regions. The longitudinal limb (=glandular pad) emerges from the right, posterior gastric chamber wall and forms a ridge along the floor to the right of the oesophageal opening, which opens ventrally at the left, roughly at the level of the posterior gastric shield. No digestive gland duct opens in the vicinity of the glandular pad. At the anterior end of the longitudinal limb, the transverse limb crosses the gastric chamber floor just in front of the oesophageal aperture and bears the gastric shield to the right. A prominent sorting area is developed in the left gastric chamber floor and roof, anterior to the oesophageal opening and extends anteriorly to the intestinal groove. Two curving longitudinal folds in the gastric chamber roof border the left side of the sorting area; the inner represents an extension of the minor typhlosole, with a prominently curved anterior tip, and the outer has its anterior tip near the level of the anterior end of the gastric shield. The inner fold often diminishes posteriorly, but both folds continue into the recess that receives the oesophagus. Two additional longitudinal folds border the sorting area to the right, and extend posteriorly from the vicinity of the proximal end of the major typhlosole, i.e. the ciliated or marginal fold, and the second right fold. These folds are joined by a third fold that emerges from the anterior end of the gastric shield. All three of these folds curve posteriorly to the right into a recess that receives the oesophagus, and that deepens at the left of the longitudinal limb. The folds curve around the posterior margin of this recess and continue anteriorly to end in the roof of the gastric chamber at the height of the gastric shield. Another prominent ridge starts at the centre of the recess, opposite to the oesophageal opening and continues anteriorly in the roof with the other folds. Just to the left of the gastric shield, two additional elongated ridges run along the right side of the longitudinal limb for most of its length; they connect anteriorly to the rightmost longitudinal folds bordering the sorting area. Within the gastric chamber roof, from the prominent outer longitudinal fold to the left side of the main sorting area, a series of flattened finger-like ridges are elaborated. The ridges begin as small undulations at the anterior tip of the outer longitudinal fold, gradually elongating and broadening, spanning the roof and extending onto the gastric chamber floor into a deep trough behind the gastric shield at the right side of the glandular pad. Anteriorly the transverse limb of the T-shaped ridge separates a deep, concave, cuticularised style sac pocket in the right aspect; this pocket is interrupted ventrally to the left by the extensions of the major and minor typhlosoles and the sorting area. The transition between the gastric chamber and the style sac is demarcated by a sulcus that is bordered by two transverse ridges of which the posterior is more prominent. The sulcus and these ridges are more strongly developed at right, and in the right ventral aspect of this sulcus, two digestive gland ducts open; at the left a shallow gastric pouch is present within the sulcus.

Digestive gland: The digestive gland is a large beige organ located in the visceral hump, usually occupying the ~1.5 apical whorls, with its anterior ~0.5 whorl supporting the right aspect of the gastric chamber ( Fig. 4) and the posterior part of the pericardium. The digestive gland is drained by tubules that join the digestive gland ducts, which run along the columellar aspect of the visceral whorls; however, in males the testes overlie the columellar aspect of the digestive gland. The ducts increase in size anteriorly, and open to the gastric chamber via two openings in the sulcus between the gastric chamber and the style sac.

Style sac: The style sac is a voluminous undivided sac that connects broadly to the gastric chamber and narrows strongly distally. Its ventral and right walls are embedded in the digestive gland. It bears two ventral typhlosoles that traverse its length, bounding the intestinal groove. Anteriorly the typholosoles curve towards the left and subsequently to the right to continue into the intestine; below the oblique limb of the major typhlosole a small, non-glandular caecum is elaborated ( Fig. 6C). A crystalline style is absent.

Intestine: The intestine emerges from the anterior end of the style sac following a slight constriction, and curves ~180° around the distal end of the style sac to continue posteriorly; it overlies the pericardium and the right side of the style sac ( Fig. 8A). The typhlosoles of the style sac continue laterally into the intestine for its entire length. The intestine continues posteriorly and its posterior limit coincides with the posterior wall of the pericardium and the anterior part of the gastric chamber; there the intestine turns back on itself to widen into the much larger, thin-walled rectum.

Rectum: The rectum is thin-walled and semitransparent so that the contents are easily visible externally ( Fig. 5A, B, D, E). Posteriorly the rectum borders the right dorsal side of the pericardium, and continues anteriorly at the right side of the mantle roof in the pallial cavity. Its walls contain abundant transverse ridges, and a ventral, longitudinal groove with thickened epithelium. The rectum is surrounded by a layer of connective tissue that thickens ventrally and is penetrated by blood vessels, including a prominent vessel dorsally at the left side, which comprises the rectal sinus. At the mantle collar, the rectum becomes more muscular, and terminates in a free anal papilla. The rectum typically contains faeces that are compacted into small, oval pellets.