Diagnosis: The characteristic arrangement of circular and longitudinal muscles on the penis forms an adequate feature for identification of the species, as was already pointed out by Marcus (1954a: 44).
Habitus: Preserved specimens are 2.2-3.5 mm long and 0.7-1.3 mm wide. Front and hind end of the body are rounded, and auricles are absent. The dorsal surface is black, except for a pale, narrow median line; the ventral surface is pale.
The pharynx lies in the posterior half of the body and is about one-fourth of the body length. The inner circular muscle layer of the pharynx is considerably thicker than the outer layer of circular muscles. The mouth opening lies somewhat anteriorly to the posterior wall of the pharyngeal cavity.
The anterior ramus of the intestine gives rise to 6-7 pairs of postocellar lateral diverticula and to one undivided preocellar branch; each of the posterior rami gives rise to about 10 lateral diverticula, and the rami meet in the hind end of the body (Böhmig 1908, Marcus 1954a).
Male Reproductive System
The small and ventrally situated testes extend from directly behind the ovaries up to the level of the hind end of the pharyngeal cavity. There are 50-60 follicles on either side of the body (Marcus 1954a).
The vasa deferentia penetrate separately, but closely together, the dorsal surface of the penis bulb. At about the level of the base of the penis papilla the vasa deferentia join to form a short common vas deferens which opens into the funnel-shaped, proximal section of the ejaculatory duct. According to Böhmig (1908) the last-mentioned duct is lined with an infranucleate epithelium.
The vertically oriented penis consists of a strongly muscularized bulb and a more or less conical papilla. The major part of the penis bulb consists of a very broad annular zone of strong circular muscle fibres, which is confluent with the circular muscle layers around the male atrium and on the penis papilla. The individual muscle fibres in the two last-mentioned layers are considerably less in diameter than the fibres making up the annular zone. The central part of the penis bulb and the proximal section of the penis papilla are traversed by the two vasa deferentia and by bundles of strong longitudinal muscles, the latter attaching to the funnel-shaped part of the ejaculatory duct. A granular secretion of penis glands runs between the bundles of longitudinal muscles and is discharged into the funnel-shaped section of the ejaculatory duct; the gland cells lie outside the penis bulb (Böhmig 1908). The fibres of the afore-mentioned thick bundles of longitudinal muscles fan out over the penis bulb, thus forming a well-developed layer around the annular zone of circular muscles. This particular layer of longitudinal muscles is confluent with the layer of longitudinal muscles which surrounds the male atrium. The latter communicates via a narrowing with the common atrium, which is lined with a nucleate epithelium, as is the male atrium.
Female Reproductive System
The paired ovaries lie at a short distance behind the brain, dorsally to the ventral nerve cords. The oviducts arise from the ventro-lateral wall of the ovaries.
Behind the gonopore the oviducts curve medially and, subsequently, unite to form a common oviduct which opens through the rear wall of the bursal canal.
The vitellaria extend from anterior to the ovaries into the hind end of the body, while they occupy the entire space between dorsal and ventral body surface.
The sac-shaped copulatory bursa is lined with tall, nucleate and vacuolated cells. A bursal canal arises from the antero-ventral surface of the bursa and curves antero-ventrad to open into the atrium. In the specimen from Chile the bursal canal actually opens into a small female atrium or vestibulum, but in the type specimen the last-mentioned structure has a more elongate shape (cf. Böhmig 1908). Bursal canal and female atrium (or vestibulum) are lined with nucleate cells, and especially the former is surrounded by a rather thick, subepithelial layer of circular muscles and a thinner layer of longitudinal muscles.
Each pigment cup contains three retinal cells and one distinct eye lens.
The type specimen was collected from below small molluscs on a flat, sandy beach, and the specimens described by Marcus (1954a) were obtained from among barnacles and mussels in the tidal zone.
Type locality: Lapatäia Bay (54°52'S 68°32'W), Beagle Channel, Tierra del Fuego, Argentina. O. hallezi is at present known only from the Beagle Channel and Moraleda Channel.
Material Examined and Type Material
K.B.I.N.: Type specimen from the Beagle Channel, collected at 24.12.1897, sagittal sections on 3 slides (in very poor condition).
S.M.N.H.: Lund University Chile Expedition, station M71, Canal Moraleda, Cayo Blanco (44°28'20"S 73°35'W), Chile, 21.02.1949, sagittal sections on 1 slide.