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Ball, 1973

Overview
Diagnosis: Sabussowia wilhelmii Ball, 1973 can be recognized by its rounded head, truncate posterior end, extra-bulbar seminal vesicle, and sac-shaped bursa.
Habitus: Live animals about 4.5 mm long and 0.8 mm wide. Head rounded and slightly constricted behind the eyes. The body margins run parallel to the truncated posterior end.
Dorsal surface of the body pale olive-brown, ventral surface paler. Pigment concentrations occur at the anterior margin and in the form of two longitudinal stripes which extend backwards from behind the eyes. The two eyes are set in unpigmented areas (Ball 1973b).

Alimentary System
The inner circular muscle layer of the pharynx is much more developed than the outer one. The mouth opening lies at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends anterior to the brain and gives rise to two or three pairs of pre-ocellar diverticula. The two posterior intestinal trunks do not meet in the hind end of the body (Ball 1973b).

Male Reproductive System
The numerous testes are situated dorsally and are distributed throughout the body length.
At the level of the pharynx the vasa deferentia enlarge to form false seminal vesicles. The ducts narrow again while they ascend to the penis bulb and, subsequently, unite to an extra-bulbar vesicle. The distal section of this vesicle narrows to a duct which penetrates the penis bulb. Inside the bulb this duct shows a dilatation and thus forms a seminal vesicle, which empties into the ejaculatory duct.
The penis is large and its hemispherical bulb is slightly directed towards the front end of the body. The musculature of the bulb is weakly developed. The penis papilla is conical, tapering to an almost vertically oriented point. The proximal portion of the papilla is highly muscularized. Beneath the lining epithelium of the papilla there is a relatively thick, subepithelial layer of circular muscles which is overlain with a thin layer of longitudinal fibres. The diameter of the circular muscle layer diminishes towards the tip of the penis papilla, muscle fibres even being absent on the very tip. The ejaculatory duct is surrounded by a thin layer of circular muscles. The male atrium is narrow; its dorsal portion is surrounded by a well-developed, subepithelial circular muscle layer and a much thinner layer of longitudinal muscles.

Female Reproductive System
The two large ovaries are situated directly behind the brain. The oviducts arise from the ventro-lateral surface of the ovaries and give rise to a short anterior branch. The posterior branches of the oviducts enter the bursal canal separately.
The vitellaria are well developed and situated dorsally as well as ventrally; they extend from anterior to the ovaries into the hind end of the body.
The copulatory bursa is of a sacciform or an irregular shape. The lining epithelium of the bursa is only weakly differentiated from the surrounding tissues, whereas it is not surrounded by any muscles. The bursa is connected with the atrium through a more or less horizontally running, rather wide, bursal canal. The lining epithelium of the canal bears long cilia and is underlain with single layers of circular and longitudinal muscles, respectively. Extensive shell glands open into the bursal canal, ectally to the openings of the oviducts.

Eyes
The eyes are devoid of a lens.

Ecology
S. wilhelmii has been collected from algal fronds in brackish water.

Distribution
Type locality: Pisquid River, just west of Pisquid, Queen's County, Prince Edward Island, Canada. The species is known only from its type locality.

Material Examined and Type Material
N.M.N.S.: Holotype: NMCP 1900-2833, sagittal sections on 2 slides.
Private collection I. R .Ball: paratype C6, Pisquid River, Prince Edward Island, Canada, 22.07.1969, sagittal sections on 2 slides.

Sabussowia wilhelmii