Material examined

Wood’s Hole, Massachusetts (USNM 890939).

External anatomy and mantle cavity

Pedal gland opening to anterior margin of propodium. Operculum present. Mantle margin smooth. Hypobranchial gland well developed.

Reproductive System

Renal oviduct functioning as seminal receptacle, storing orientated sperm. Gonopericardial canal absent. Oviduct opening to glandular gonoduct at base of mantle cavity. Pallial oviduct closed along entire length. Anterior bursa present.

Proximal vas deferens convoluted, forming seminal vesicle. Prostate open to mantle cavity. Open seminal groove extending from prostate to tip of penis lying behind right cephalic tentacle.

Alimentary system

Foregut. Radula taenioglossate. Paired cartilages supporting odontophore. Subradular membrane almost completely covering odontophore. Weakly glandular, narrow papilla, extending forward ventral to radular ribbon (Figs 4E, 5F, sro). Dorsal surface of papilla lined with thin layer of cuticle. Buccal cavity lined with cuticle. Glandular mid-ventral fold (Fig. 8F, vf) beginning in buccal cavity, extending posteriorly to buccal pouches, then diminishing. Mid-ventral fold not continuing into mid-oesophagus. Buccal pouches opening to posterior buccal cavity and anterior oesophagus. Anterior ducts of buccal pouches lined with highly glandular, acidophilic epithelium, expanding dorsally into buccal pouches. Remainder of pouch epithelium strongly basophilic. Apertures of pouches bounded by prominent ventro-lateral folds (Fig. 9H, vlf). Oesophageal gland present within mid-oesophagus, forming transverse septae of glandular tissue partially occluding oesophageal lumen.

Midgut. Oesophagus (Fig. 15B, e) opening near proximal end of minor typhlosole (t2); mucus-secreting pouch at base of minor typhlosole absent. Three ducts of digestive gland (dgd) and surrounding ciliary sorting area (sa) lying at proximal end of major typhlosole (t2). Ciliary currents in sorting area flowing anteriorly toward intestinal groove. Gastric shield (gs) ventrolateral on right. Gastric chamber cuticularized. Ciliated fold (cf) bearing short, dense cilia emerging near minor typhlosole, running posteriorly to left of oesophageal aperture. Ciliated fold bordering grooved tract bearing long cilia. Grooved tract emerging from under lip of ciliated ridge near oesophageal aperture, paralleling ridge posteriorly. Grooved tract curving around posterior periphery of gastric chamber, extending short distance anteriorly along right wall. Currents within ciliary tract flowing posteriorly. Low, ciliated ridge (cs) extending posteriorly along left side of gastric shield. Currents over ciliated ridge flowing posteriorly and towards right side. Crystalline style absent. Raised tract of cilia on major typhlosole absent. Currents flowing clockwise within style sac when viewed from behind.

Hindgut. Intestine emerging from style sac, curving posteriorly around rear wall of kidney before turning anteriorly.

Reno-pericardial system

Afferent renal vessel entering kidney floor anteriorly, extending dorsally and ramifying to supply excretory tissue. Nephridial gland present. Branch from afferent renal vessel supplying nephridial gland absent.

Nervous system and sensory structure

Nervous system epiathroid, left dialyneurous. Nerve ring lying just behind base of buccal mass. Tentacular nerve bifid. Metapodial commissure present. Visceral ganglia paired. Statocysts with single statoliths present postero-dorsally on pedal ganglia, slightly asymmetrical. Small eyes present at lateral bases of cephalic tentacles. Osphradium comprising long, thin, ciliated ridge along base of ctenidium.

Remarks

The anatomy of this species has been well established in several published descriptions, most notably a thorough account by Fretter & Graham (1962). Previous studies include those of Ankel (1936, 1937), who provided information on foregut anatomy and the nervous sytsem, and Johansson (1939) who also provided details on foregut anatomy. Lacking from these accounts are descriptions of the unique, papilla-like subradular organ, and the ventral folding of the anterior oesophagus.

Johansson (1939) and Graham (1949) described midgut anatomy for this species. The present description agrees with these on most major points. However, there are several discrepancies. Both Johansson and Graham described only two ciliary tracts leading posteriorly into the gastric chamber; one bounding the oesophagus to the left and a second passing to the left of the gastric shield. In addition, Graham found a mucus-secreting pouch at the base of the minor typhlosole; ciliary currents were described as flowing anteriorly to the left of the gastric shield.

The metapodial commissure has been reported as present (Fretter & Graham, 1962; Marcus & Marcus, 1963) or absent (Simroth, 1896 –1907; Ankel, 1936).

Discussion

The basic layout of the reproductive system and the foregut of littorinids is largely congruent with Littorina littorea. Variable features of reproductive anatomy include the presence or absence of a gonopericardial canal, the looping patterns of the gladular oviduct, an open or closed prostate and seminal groove, and the morphology of the penis and accessory glands (e.g. Linke, 1933; Johansson, 1939; Fretter & Graham, 1962; Marcus & Marcus, 1963; Bingham, 1972; Reid, 1986).

All littorinids described possess an elongate, cuticularized gastric chamber, with at least one ciliary tract (e.g. Marcus & Marcus, 1963; Morton, 1975); length of the gastric chamber is variable and is not correlated with habitat (Reid, 1986). The number of ciliary tracts and digestive gland ducts and the size of the sorting area are also variable (e.g. Marcus & Marcus, 1963; Reid, 1986). Despite inconsistencies between the present and previous descriptions as summarized above, the present results are congruent with investigations of Tectarius muricatus (pers. obs.) and L. (Austrolittorina) unifasciata antipoda (as Melarhaphe oliveri) (Morton, 1975). All three species possess three ciliary tracts; one extending from the oesophageal aperture and curving around the base of the gastric chamber with two flanking tracts. Similar to Graham (1949), Marcus & Marcus (1963) documented a mucus-secreting pouch at the proximal end of the style sac in L. angulifera. Littorina littorea is intriguing because it lacks a ciliary tract on the major typhlosole; the tract is present in another non-style-bearing littorinid, T. muricatus (pers. obs.).

Although much of the detailed information regarding the littorinid nervous system concerns Littorina littorea (Bouvier, 1887; Johansson, 1939; Fretter & Graham, 1962), additional studies have found no significant differences in organization in other species (Johansson, 1939; Marcus & Marcus, 1963). One minor exception is the metapodial commissure, which has been reported as present (Johansson, 1939; Fretter & Graham, 1962; Marcus & Marcus, 1963) or absent (Simroth, 1896 –1907; Ankel, 1936).