(Bergendal, 1899)
Overview
Diagnosis: Obrimoposthia ohlini (Bergendal, 1899) is characterized by the following features: black-brown dorsal pigmentation, sharply bent penis papilla, highly developed circular muscle layer of the penis, large copulatory bursa, common oviduct opening into the bursal canal, and by the fact that alternating rows of circular and longitudinal muscles surround the dorsal part of the bursal canal.
Habitus: Preserved specimens up to 11 mm long and 2.5-4 mm wide (Bergendal 1899, Böhmig 1906, Hyman 1958, Nurse 1964). Living animals have a plump rear end from which the body margins taper gradually towards the neck region. In front of the neck the body widens slightly and shows a small auricle on either side of the head. The anterior body margin is truncated, with a small concave middle portion (Böhmig 1906, Material Examined). The small eyes are not far from the anterior margin of the body and are spaced wide apart. The dorsal surface is black-brown and shows two whitish or yellowish, broad and irregularly formed longitudinal streaks on either side of the mid-dorsal region; the auricles are devoid of pigment. The streaks are composed of irregularly formed, pigment-free patches and run from the neck region towards the hind end of the body. At about the root of the pharynx there is a pigment-free streak connecting both longitudinally running stripes. The ventral body surface is pale cinnamon or greyish.
Alimentary System
The pharynx is about one-fourth of the body length (Böhmig 1906, Material Examined); the inner and outer zones of circular muscles consist of several rows of fibres, but the former is much more developed than the latter.
The anterior ramus of the intestine extends anterior the eyes, without giving rise to preocellar diverticula. Posterior to the eye cups the anterior trunk gives off 5-6 pairs of lateral diverticula (Böhmig 1906). Each posterior ramus gives rise to about 8 lateral branches; behind the copulatory apparatus both trunks may show a commissure (Böhmig 1906). The lateral, intestinal diverticula do not branch very much (Böhmig 1906). The mouth opening is situated at the hind end of the pharyngeal pocket.
Male Reproductive System
The numerous small, oval-shaped or elongated, testes are situated ventrally. The number of testicular follicles ranges from 100 to 150 on either side of the body (Böhmig 1906). The follicles occur immediately behind the ovaries, and extend to somewhat anteriorly to the gonopore.
At the hind end of the pharyngeal pocket the vasa deferentia expand to form false seminal vesicles. The latter run backwards to almost the genital pore and then make a sharp antero-medially directed bend. The vasa deferentia narrow considerably while ascending towards the penis bulb. Each duct enters the antero-ventral surface of the bulb and, subsequently, runs dorsad up to about the middle of the penis bulb, after which they make a rather sharp posteriorly directed bend. The vasa deferentia may be difficult to follow where they traverse the muscle layer of the penis bulb. After the posteriorly directed bend within the penis bulb, each vas deferens expands to form an acessory seminal vesicle. This vesicle may either be small and rounded or may be elongated, in the latter case only gradually narrowing towards the point where fusion of the vasa deferentia takes place. Shortly after having entered the penis bulb, the vasa deferentia come to lie side by side but they only fuse shortly before the tip of the intrapenial papilla. The short and narrow duct that arises after the fusion (common vas deferens), opens at the tip of the intrapenial papilla. This blunt papilla opens into a funnel-shaped ejaculatory duct which in turn opens to the tip of the penis papilla. The ejaculatory duct is provided with long cilia.
The penis consists of a broad, rounded basal portion and a comparatively small papilla which is covered with a flat epithelium. The bulbar part of the penis has a ventro-caudal orientation, and the papilla usually shows a sharp, ventrally directed bend. The bulbar portion of the penis shows a number of septa or lamellae which converge towards the intrapenial papilla. These septa are provided with muscle fibres and delimit "channels" through which a rich granular secretion is discharged into the funnel-shaped section of the ejaculatory duct.
With respect to the histology of the penis bulb, a basal and a differently staining, proximal portion can be distinguished. Both portions are densely granulated. The basal portion has the same structure and coloration as the tissue directly around the penis bulb and stays in contact with the latter through openings in the muscle layer of the bulb. The differently staining proximal section of the bulb most likely results from the accumulation of the granular secretion of penis glands which discharge into the proximal portion of the ejaculatory duct.
The parenchyma of intrapenial and penis papilla is characterized by a large number of conspicuously staining, nuclei. Several nuclei are present also along side the lamellae. The circular muscle layer of the penis is extremely well-developed. The width of the postero-dorsal muscle layer diminishes gradually towards the tip of the penis papilla, contrary to the antero-ventral layer. The latter decreasing rather abruptly in diameter distally to the sharp bend. This circular muscle layer envelops the entire penis, from tip to basal part, but becomes more or less interwoven on the last-mentioned portion.
Male and common atrium are surrounded by well developed layers of circular and longitudinal muscles. Vasa deferentia and common vas deferens are surrounded by a layer of circular muscles. The ejaculatory duct is devoid of any surrounding musculature.
Female Reproductive System
The small, oval-shaped ovaries are situated directly behind the brain, on the medial portion of the ventral nerve cords. The oviducts arise from the lateral surface of the ovaries and run latero-dorsally to the ventral nerve cords. Behind the gonopore the ducts turn medio-dorsally and fuse to a common oviduct. Shortly after the fusion, the common oviduct makes a sharp anteriorly directed bend and, subsequently, opens into the bursal canal.
The main body of vitellaria is situated dorsally, but between the intestinal diverticula the follicles taper towards the ventral body surface. The vitellaria extend from the level of the ovaries into the hind end of the body.
The very large copulatory bursa is lined with large, granulated cells. The bursal canal arises from the antero-dorsal surface of the bursa and makes a posteriorly directed, knee-shaped bend before running towards the ventral portion of the common atrium. At about half-way its length the bursal canal receives the opening of the common oviduct. The distal part of the bursal canal (i.e. ectally to the opening of the common oviduct) is provided with long cilia. Entally to the opening of the common oviduct the diameter of the bursal canal decreases considerably. Shell glands discharge into the ectal portion of the bursal canal. The ventral part of the bursal canal is surrounded by well developed layers of circular and longitudinal muscles, respectively. The dorsal portion of the canal, however, is surrounded by alternating rows of circular and longitudinal muscles. The bursa is surrounded by a layer of interwoven muscle fibres.
Eyes
The pigment cups contain three retinal cells and a large, rounded or more or less bean-shaped lens.
Reproduction
Life Cycle: The brownish cocoons are spherical, 1.3-1.7 mm in diameter and have a short pedicel, which is about 0.2 mm in width (Böhmig 1906, 1914).
Ecology
O. ohlini occurs under stones in the intertidal zone and has also been dredged from 70-75 feet (21.4-22.9 m) (Material Examined). Furthermore, it has been collected from under stones among debris in a freshwater stream on Gough Island (Holdgate 1961), and on Kerguelen the species has been collected also from freshwater (Beauchamp 1940). The habitat from which the species was collected on Marion Island is not known with certainty but, according to the B.M.N.H. catalogue, it may have been a waterfall.
Distribution
Type locality: Punta Arenas, Tierra del Fuego. The species has been reported from Tierra del Fuego and its archipelago (Punta Arenas, Cape Horn, Magellanes Inlet, Juan Island, Ushuaia, Ile Navarin (Bergendal 1899, Hallez 1906, 1907, Böhmig 1908, 1914, Hyman 1955)), South Orkney Islands (Gemmill and Leiper 1907), Falklands (Westblad 1952), Gough Island (Holdgate 1961), Kerguelen (Monod and Dollfus 1932, Böhmig 1914, Beauchamp 1940, Hyman 1958), Macquarie Island (Nurse 1964).
Material Examined
N.M.N.S.: NMCI-1985-0243, Bahia Orange, Isla Hoste, Tierra del Fuego, 14.02.1970, transverse sections on 4 slides; NMCI-1985-0242, transverse sections on 4 slides; NMCI-1985-0235, sagittal sections on 5 slides; NMCI-1985-0234, sagittal sections on 4 slides; NMCI-1985-0232, sagittal sections on 3 slides; NMCI-1985-0231, transverse sections on 7 slides; NMCI-1985-0241, Punta Wulaia, Isla Navarino, Tierra del Fuego, 3.02.1970, sagittal sections on 5 slides; NMCI-1985-240, sagittal sections on 2 slides; NMCI-1985-0233, Seno Grandi, Isla Bertrand, Tierra del Fuego, 11.02.1970, sagittal sections on 3 slides.
S.A.M.: V3133, station 173, Kerguelen, 2 whole mounts on 1 slide; V3134, no. 1833, Kerguelen, sagittal sections of one specimen on 3 slides (cf. Hyman 1958: 281).
M.V.: G1226, Buckles Bay, Macquarie Island, 12.02.1950, sagittal sections on 4 slides; G1227, Buckles Bay, Macquarie Island, 29.04.1949, sagittal sections on 4 slides; G1228, Aerial Cove, Macquarie Island, 26.01.1950, sagittal sections on 6 slides.
A.M.: W5565-1, Macquarie Island, 22.10.1968, sagittal sections on 6 slides.
B.M.N.H.: 1960.2.24.16, A1-2, Glen Stream (alt. 150-200 ft), Gough Island, sagittal sections on 2 slides; B1-3, sagittal sections on 3 slides; C1-2, sagittal sections on 2 slides; D1-2, sagittal sections on 2 slides; E1-2, sagittal sections on 2 slides; A1-7, sagittal sections on 7 slides; 1968.9.24.7-10, Marion Island, Waterfall?, serial sections.
M.N.H.N.: V7-A0721-1, Kerguelen, 5.11.1939, sagittal sections on 11 slides; V7-A0721-2, ibid., sagittal sections on 5 slides.