(Hallez, 1906)
Overview
Diagnosis: Obrimoposthia wandeli (Hallez, 1906) can be recognized easily by the fact that (1) its copulatory bursa is situated dorsally to the male atrium, (2) the diverticulum arises from the dorsal portion of the bursal canal, (3) the bursal canal is surrounded by a thick and conspicuous muscle coat.
Habitus: Preserved specimens vary from 4-8 mm in length and from 2.5-4 mm in width (Hallez 1907, 1913, Böhmig 1908). The shape of preserved specimens varies considerably due to varying states of contraction. The least contracted specimens are oval-shaped to elongated; front and hind end are rounded, whereas there may be either a constriction at the level of the eyes or the head may be slightly attenuated. The body margins are undulated to a greater or lesser extent (Böhmig 1908, Nurse 1964, Material Examined). According to Hallez (1913), who relied on paintings made by Liouville, live specimens show a broadly rounded hind end and a strongly attenuated front end.
The coloration of O. wandeli is variable. The colour of the dorsal surface varies from black to brown-yellow and dark violaceous. Two to three mid-dorsal yellowish or whitish spots may be present; occasionally these spots may be confluent, thus forming a mid-dorsal stripe. The hind end of the body may also show a whitish spot. The number of spots actually present may vary considerably between individual animals. Whitish spots also occur at the front end, viz. one mid-frontal patch, and two spots extending from the eyes towards the antero-lateral body margins. The ventral surface is yellowish or yellowish white (Hallez 1907, 1913, Böhmig 1908, 1914, Material Examined). The specimens examined by Nurse (1964) are slightly different in that some animals showed dark dorso-lateral bands, which were confluent at the front and hind end of the body.
The eyes are situated at some distance from the anterior margin.
Alimentary System
The pharynx measures about one-fifth (Böhmig 1908) to one-fourth of the body length; it is situated in the middle of the body. The inner circular muscle layer of the pharynx is much thicker than the outer one. The mouth opening is situated at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends anterior to the eyes, without giving rise to preocellar diverticula. The anterior gut trunk gives off 4-5 pairs of unbranched or only forked, postocellar lateral diverticula. Each posterior intestinal trunk gives rise to about 15-18 lateral diverticula and 3-5 short, medially directed branches (Böhmig 1908, Material Examined). The posterior rami meet in the hind end of the body.
Male Reproductive System
The small, ventrally situated testes extend from directly behind the ovaries up to the level of the copulatory apparatus or somewhat behind the latter. According to Böhmig (1908) there are in total about 200 testicular follicles, but in the material examined I never found more than 100 follicles.
At the hind end of the pharyngeal pocket the vasa deferentia enlarge to form false seminal vesicles which run caudally for a short distance but then recurve to ascend to the penis. The ducts enter the basal portion of the penis and run side by side through the proximal portion of the penis papilla. Within the penis papilla the narrow vasa deferentia unite to a very short common duct which opens into the more or less funnel-shaped proximal portion of the ejaculatory duct; the latter opens at the tip of the penis papilla.
The penis papilla has a ventro-caudal disposition and consists of the following three parts: (1) a proximal or basal part, consisting of an extremely thick annular zone of circular muscles, (2) a middle part, containing the vasa deferentia and vesicles with the granular secretion of penis glands, (3) a distal part comprising the distal section of the penis papilla and which is traversed by the ejaculatory duct.
Most likely due to different states of contraction of individual muscles, the thick circular muscle zone at the base of the penis may either appear as being composed of densely staining dots or stripes or may have a reticulate appearance. A well developed layer of longitudinal muscles may be present at the very base of the penis papilla. These muscles are continuous with the fibres surrounding the male atrium and the proximal surface of the basal circular muscle zone. A well-defined penis bulb is absent.
The second, middle, portion of the penis is traversed by a number of septa. These septa extend radially from the vasa deferentia and divide the middle part of the penis into 15-20 elongated chambers (Böhmig 1908). These chambers, in turn, are divided into smaller units by secondary septa. The chambers accumulate the granular secretion of penis glands and open into the funnel-shaped proximal portion of the ejaculatory duct. The glands lie just outside of the penis, usually dorsally and ventrally to the male atrium.
The penis papilla is covered with a low epithelium which is underlain with a thin layer of small circular muscles fibres. This muscle layer surrounds the entire papilla. At about the distal part of the thick circular muscle zone at the base of the penis this thin circular muscle layer becomes thicker over a short distance. Hereafter it diminishes again in diameter, but extends to the base of the penis. This particular muscle layer is clearly distinguishable from the thick, circular muscles at the base of the papilla by the fact that the fibres of the former stain differently, a situation already noted by Böhmig (1908).
Longitudinal muscles enter the penis via the space left over by the vasa deferentia and the cicular muscle zone at the penis bulb, and, subsequently, radiate to the tip of the penis papilla. Vasa deferentia and ejaculatory duct are surrounded by a layer of circular muscle fibres.
Böhmig (1908) observed remains of cilia on the lining epithelium of the ejaculatory, but I have not been able to confirm this observation. The same investigator also mentioned that he was unable to find nuclei in the lining epithelium of the ejaculatory duct and that he was inclined to consider the nuclei to be insunk. I have not been able to find nuclei in the lining epithelium of the ejaculatory duct, but neither did I observe insunk nuclei in the surrounding parenchyma. The male atrium is surrounded by well developed layers of circular and longitudinal muscles, which extend on the penis bulb.
Female Reproductive System
The rounded or oval-shaped ovaries are situated dorsally to the ventral nerve cords, immediately behind the brain. The oviducts arise from the ventro-lateral surface of the ovaries, and follow their course backwards laterally to the ventral nerve cords. Behind the gonopore the oviducts unite to a common duct which opens into the diverticulum that arises from the postero-dorsal wall of the bursal canal. According to Böhmig (1908) oviducts and common oviduct are surrounded by circular and longitudinal muscles but I could discern only the first-mentioned type of musculature.
The vitellaria are well developed; they occur from the level of the ovaries into the hind end of the body and occupy the entire space between dorsal and ventral body surfaces.
The copulatory bursa is a sac-shaped or rounded vesicle which lies dorsally to the posterior part of the male atrium. The bursa is lined with tall, vacuolated cells. The bursal canal arises from the posterior surface of the bursa. The proximal part of the bursal canal runs more or less parallel to the body surface or has a ventro-caudally oriented disposition. After a sharp, right-hand (Böhmig 1908), bend the bursal canal turns towards the ventral body surface and, subsequently, opens into the rear wall of the male atrium, very close to the point where the latter meets the gonopore. This distal part of the bursal canal shows a curved course before it communicates with the atrium (Fig. 142).
The diverticulum of the bursal canal may either be a large structure with a lumen even wider than that of the bursal canal, or a very short and rather narrow duct. Bursal canal and diverticulum are lined with an infranucleate, ciliated epithelium and are surrounded by unicellular glands which discharge into the lumen of the ducts. Shell glands, which stain differently from the above-mentioned unicellular glands, surround the distal portion of the bursal canal and discharge their secretion into the latter. Bursal canal and diverticulum are surrounded by a thick, subepithelial layer of circular muscles and a thinner layer of longitudinal muscles. According to Böhmig (1908) the bursa is surrounded by circular and longitudinal muscle fibres. Although in some cases I found the bursa to be provided with a thin muscle layer, I was unable to discern the presence of two separate layers.
Eyes
Each eye cup houses three retinal cells and a well developed, oval-shaped lens.
Reproduction
Karyology: The diploid complement consists of 12 chromosomes (Böhmig 1907).
Life Cycle: Hallez (1906, 1907) reported that the cocoon measures 1 mm in diameter and that it is provided with a short pedicel. One specimen of sample ZMB 6105 had a cocoon in its atrium which measured 1.5 mm in diameter and which indeed had a very small pedicel.
Ecology
O. wandeli has been collected from under stones and from sand in the intertidal zone. The species has been found attached to the thallus of aquatic plants and in one case it has been dredged from a depth of 20 m (Hallez 1907).
Distribution
Type locality: Strait of Gerlache, Palmer Archipelago, Antarctic Peninsula. O. wandeli is known from the following localities: various places on the Antarctic Peninsula (Hallez 1906, 1907, 1913, Böhmig, 1908, Hyman 1955); King George Island, South Shetlands (Hallez 1913); South Georgia (Westblad 1952); Kerguelen (Böhmig 1914, Hyman 1958); Macquarie Island (Hyman 1958, Nurse 1964); Possession Island (Sluys and De Vries, 1988); Elephant Island (61 10'S 55 14'W), Aspland Island 61 30'S 55 49'W), O'Brien Island (61 32'S 55 52'W) (B.M.N.H.).
Material Examined
K.B.I.N.: Böhmig's (1908) material from Two Hummoks Islands, Strait of Gerlache, 26.01.1898, consisting of one set of sagittal sections on 8 slides and one set of transverse sections on 13 slides.
S.A.M.: V3129, station 175, Kerguelen, 13.11.1929, whole mount; V3130, station 154, Kerguelen, 13.11.1929, sagittal sections on 4 slides.
M.V.: G1225 B, Macquarie Island, 30.11.1948, sagittal sections on 3 slides.
Z.M.A.: V.Pl.754, Ile Wandel (=Booth Island, 65°05'S 64°00' W), 1910, sagittal sections on 6 slides.
M.N.H.N.: V8-137-2, Booth Island, 1910, sagittal sections on 6 slides; V8-137-3, sagittal sections on 6 slides; V8-137-4, horizontal sections on 3 slides; AJ 785-1, Crozet Islands, Possession Island, sagittal sections on 4 slides.
Z.M.B.: 6105-1, Kerguelen, 7.02.1903, sagittal sections on 3 slides; 6105-2, ibid., sagittal sections on 4 slides; 6105-3, Kerguelen, 21.02.1902, sagittal sections on 4 slides.
Type Material
According to Wilhelmi (1911) the material of the First French Antarctic Expedition, containing among others 127 specimens of O. wandeli, was deposited in the Musum d'Histoire Naturelle in Paris. However, it appeared to be impossible to trace this material.