Oregoniplana pantherina, SLUYS, 1989

OREGONIPLANA PANTHERINA SLUYS, 1989 View in CoL

Material examined

South Africa, Cape of Good Hope , 18°28’26”E, 34°21’25”S, 12 October 2012, coll. Edgar Vila-Farré, ZMA V. Pl. 7284.1, sagittal sections on four slides GoogleMaps ; ZMA V. Pl. 7284.2, horizontal sections on three slides ; ZMA V. Pl. 7284.3, sagittal sections on three slides ; ZMA V. Pl. 7284.4, sagittal sections on seven slides .

Description

In elongated state, live sexually mature specimens are around 5 mm long. The body is elongated with broadly rounded hind end and moderately undulating body margins. Preserved specimens are up to 3.3 mm long, as determined from histological sections. Most of the preserved specimens are ventrally curved, with certain lateral areas protruding from the body margin, thus giving them a very singular appearance ( Fig. 7B). The two eyes are placed close to the body margin at a certain distance from the frontal margin. A small indentation at the hind end of the body is conspicuous in living and preserved specimens, corresponding to the secondary gonopore. Ventral adhesive papillae are most clearly visible at the anterior and posterior body margins.

Dorsal surface of the body is dark brown, except for the tip of the head and three pairs of symmetrical patches along the body margin. Small clusters of pigment granules are densely distributed on the dorsal surface, clearly visible in animals squashed between a coverslip and a glass slide ( Fig. 7C). The ventral surface is also pigmented but paler than the dorsal one, thus making it possible to discern the large testes.

The tip of the head is white, tinged light cream along the margins of the white patch. The first two, anterior-most lateral patches are white, while the third posterior one is more brownish. The first lateral patch is bigger than the others and extends from the body margin to the eyes. The second patch is placed midway between the anterior one and the posterior patch, the latter located at the level of the pharynx. These three white patches correspond to the regions protruding laterally in preserved specimens ( Fig. 7B).

The eye cups (diameter 42–50 μm, as measured in histological sections) contain multiple retinal cells and are placed dorsally, below the body wall musculature ( Fig. 8A, B). A so-called Substanzinsel or ‘island’ (formed by a section through a vertical tract comprised of an aggregation of ganglion cells, muscles and mesenchymatic tissue) is discernible in at least one of the cerebral ganglia of specimen ZMA V.Pl. 7284.2.

A ciliated duct arises at the centre of the dorsoventral diameter of the body, very close to the dorsal surface of the brain ( Fig. 8B). From thereon, the duct runs towards the dorsal epidermis anterior to the eyes and then narrows just before opening to the exterior through the epidermis. Its diameter varies among specimens but it is usually wider near the brain. In specimens ZMA V.Pl. 7284.2 and ZMA V.Pl. 7284.4 the canal is filled with an unknown substance.

The anterior branch of the intestine extends anteriorly to the eyes and gives off at least three pre-ocellar diverticula. The cylindrical pharynx lies approximately in the middle of the body and measures about 1/6 of the body length. The outer epithelium of the pharynx is ciliated, except close to the root of the pharynx and is underlain by a subepithelial layer of longitudinal muscles, followed by layer of circular muscles. A thick subepithelial layer of circular muscles is present underneath the inner pharynx epithelium, followed by a layer of longitudinal muscle fibres. The mouth is situated below the anterior portion of the male atrium and is connected to the pharyngeal pouch through a tubeshaped canal ( Fig. 9B).

A pair of very large testis follicles is situated between the ovaries and the root of the pharynx ( Fig. 7F). In specimen ZMA V.Pl. 7284.4, the maximum diameter of the testes in anterior–posterior direction is 590 μm and their height is 300 μm, the follicles thus occupying most of the dorsoventral space. The narrow vasa deferentia arise from the posteroventral surface of the testis follicles and the ducts enlarge at the level of the pharynx to form sac-shaped vesicles that contain sperm. At the level of the base of the penis papilla, the ducts curve dorsally and, subsequently, penetrate the penis bulb to open separately into an intrabulbar cavity. From the latter arises a long ejaculatory duct that runs centrally through the penis papilla, gradually tapering towards the tip of the papilla, where the duct opens to the exterior. The ejaculatory duct is lined with a low, nucleated epithelium that widens considerably after having received a coarse cyanophilic secretion at its central section ( Fig. 9B). Abundant erythrophilic glands, located in the parenchyma just outside the penis bulb, open at the distal end of the ejaculatory duct and at two central areas of the duct ( Fig. 9B). The ejaculatory duct is surrounded by a thin layer of circular muscles, which becomes thicker at its central section ( Fig. 9B).

The penis papilla is a very slender, elongated cone that is inserted at the posterodorsal wall of the male atrium. From this root, the papilla first runs sharply in the anterior direction and then exhibits an equally sharp knee-shaped bend in the caudal direction beneath its own root. This results in the fact that the tip of the penis papilla again faces the caudal end of the body. The papilla is covered with a low, nucleated epithelium that is underlain by a thin, subepithelial layer of circular muscles, followed by a layer of longitudinal muscles. However, the posterior section of the papilla is provided with a well-developed coat of muscles, consisting of two layers of obliquely oriented bundles of fibres with a cross-wise orientation ( Fig. 9C). Longitudinal muscle fibres form the small and weak penis bulb.

A thin and long flap of tissue originates from the dorsal atrial wall immediately posterior to the root of the penis papilla. This flap projects into the male atrium as far as the knee-shaped bend in the penis papilla, thereby following the same course as the basal portion of the papilla.

The atrium is divided into two parts. The proximal portion is narrow and corresponds to the male atrium, which houses the penis papilla and is lined with a low epithelium, underlain by a thin subepithelial layer of circular muscle fibres, followed by a layer of longitudinal fibres. The epithelium of the penis papilla frequently touches the wall of the atrium. The distal part of the atrium is tubular and is lined with a nucleated and tall epithelium, underlain by a thin, subepithelial layer of circular muscle fibres, followed by a layer of longitudinal fibres. It communicates with a gonoduct that opens to the exterior through the primary gonopore. The ventral epidermis around the primary gonopore contains numerous small, erythrophilic granules, being more abundant in the cells posterior to the gonopore.

From the level of the gonoduct the tubular atrium proceeds backwards as a female genital duct, opening to the exterior by means of a secondary, caudal gonopore at the very tail end of the body ( Fig. 9A, B). At about halfway along its length, the female genital duct receives the separate openings of the oviducts at a slightly laterally enlarged section of the female duct, as well as the openings of erythrophilic shell glands that discharge anteriorly to the oviducal openings. The portion of the female genital duct running between the oviducal openings and the caudal gonopore is highly ciliated ( Fig. 9B, D).

A very short duct arises from the dorsal side of the most posterior portion of the female genital duct and then opens into a small and poorly defined sac-shaped vesicle that resembles a reduced copulatory bursa. In the holotype this presumed bursa is probably connected to the intestine.

The pear-shaped ovaries are situated at a short distance behind the brain, occupying a little less than half of the dorsoventral diameter of the body ( Fig. 10A). The ventral portion of each of the ovaries has a thinner wall and gives off a duct that communicates with a large sac-shaped body. This structure, called a stromatic sac by Sluys (1989), lies laterally to the ovaries and partially envelopes them, occupying most of the dorsoventral diameter of the body ( Fig. 10B). This stromatic sac is made up of nucleated cells, forming an ill-defined tissue surrounded by circular muscle fibres. The enlarged proximal section of the oviduct arises from the lateral wall of this structure, after which it immediately receives several ducts coming from vitellaria around the duct ( Fig. 11A, B). The vitellarian follicles are well developed and extend from in front of the ovaries to the level of the copulatory apparatus, usually occupying the entire dorsoventral space. The oviducts run laterally to the ventral nerve cords and at the level of the gonopore curve towards the centre of the body to open, separately, into the female genital duct.

Habitat

Specimens were collected from the under-surface of stones in the intertidal zone of a beach with both stones and sand.