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(Claparède, 1863)

Overview
Diagnosis: Sabussowia dioica (Claparède, 1863) can be recognized easily by the fact that it shows an almost complete separation between the sexes, only male animals still showing some traces of the copulatory apparatus of the opposite sex.
Habitus: Live specimens up to 7-8 mm long and 1.5 mm wide, and for preserved animals these figures are about 4.5 mm and 0.9 mm, respectively.
The body is elongated, with rounded hind end and truncated front end. At the level of the eyes the body may show a slight constriction.
The dorsal body surface is chestnut brown, the pigment mainly being arranged in a reticulate pattern, which is most conspicuous in male animals; in males the meshes in the pattern indicate the location of the testes. In female specimens the pattern of pigmentation is much denser than in males. The eyes are set in pigment-free areas, and similar white spots occur at the anterior body margin and on either side of the head. The ventral body surface only is very weakly pigmented.

Alimentary System
The pharynx measures one-fifth to one-fourth of the body length and is inserted at about the middle of the body. The inner circular muscle layer of the pharynx is much thicker than the outer one. The mouth opening is at the hind end of the pharyngeal cavity.
The anterior ramus of the intestine extends anterior to the eyes, without giving rise to preocellar diverticula. Behind the eyes this gut branch gives off 5 pairs of lateral diverticula. The first, anteriormost pair of postocellar diverticula is curved forwards and extends anterior to the eyes. Each of the posterior gut trunks gives rise to about 15 lateral diverticula; the posterior rami do not communicate in the hind end of the body (Böhmig 1906, Tekaya 1982).

Male Animal
In principle, the testes are situated dorsally, but often the follicles occupy the entire space between dorsal and ventral body surface. There are in total about 90, closely packed testicular follicles, extending from a short distance behind the brain to the level of the gonopore.
Behind the pharyngeal pocket the vasa deferentia unite to form a broad, winding common duct. This common vas deferens narrows before penetrating the anterior section of the strong muscle coat of the penis bulb and, subsequently, opens into a spacious intrabulbar seminal vesicle. The musculature of the penis bulb consists of irregularly arranged circular fibres. Through a narrow opening the large intrabulbar seminal vesicle communicates with the ejaculatory duct, which runs through the penis papilla, to open at the pointed tip of the latter.
The ejaculatory duct is penetrated by openings of two types of penis gland, of which the glandular elements lie just outside the papilla. The first type of gland produces an erythrophilous secretion that is discharged in the proximal section of the ejaculatory duct. The second type of penis gland produces a secretion that stains with much more difficulty and which has a pale-yellow colour. This second type of secretion is discharged into the major, distal portion of the ejaculatory duct.
The penis papilla consists of a well developed cone which tapers to form a pointed tip; it is situated parallel to the body surface. Penis bulb and papilla are separated by a thick zone of concentric muscles. The papilla is lined with a flat, nucleate epithelium, which in underlain with a thin, subepithelial layer of circular muscles and an equally thin layer of longitudinal muscles. The muscle fibres of the last-mentioned layer are continuous with those of the above-mentioned zone of concentric muscles.
The male atrium is separated from the genital canal through a constriction, which is surrounded by a rather thick zone of circular muscles.
Male animals possess a small copulatory bursa which is lined with a nucleate, and sometimes vacuolated, epithelium. Through a narrow bursal canal the copulatory bursa opens into the posterior section of the male atrium, the opening being situated just in front of the muscularized narrowing of the atrium; shell glands discharge their secretion into the distal section of the bursal canal, i.e. close to the opening into the atrium (Bhmig 1906).

Female Animal
The ovaries are situated at about one-third of the distance between the eyes and the root of the pharynx, and lie dorsally to the ventral nerve cords. The germ centre is in the anterior portion of the ovaries.
The oviducts arise from the postero-lateral wall of the ovaries and immediately thereafter branch to form a posteriorly and an anteriorly running section. The posteriorly directed branches run laterally to the ventral nerve cords and open separately into the bursal canal. The anterior branches of the oviducts extend forewards for a considerable distance, although they do not reach the brain; from the point of branching these anterior sections run laterally to the nerve cords and they have a length of about 0.55 mm. The opening of the oviducts into the ovaries is closed by a large clump of closing cells ("Verschlusszellen") inside the female gonad. According to Böhmig (1906) the hind wall of the ovaries lacks the tunica propria or surrounding epithelium where they communicate with the oviducts. At that particular point occurs a proliferation of syncytial parenchymatic tissue in which vacuoles are present and which gradually passes over into normal parenchymatic tissue. This situation, as described by Böhmig, could partly be observed in specimen ZMB 4344.
The vitellaria are well developed, extending from directly behind the brain into the hind end of the body, and occupying the entire space between the dorsal and ventral body surfaces.
The female atrium is lined with rather tall, cells which are nucleate and non-cliated. The atrium is surrounded by a subepithelial layer of circular muscles and a layer of longitudinal muscles, although these layers are not always discrete. The atrium communicates with the genital canal.
A relatively long bursal canal connects the small copulatory bursa with the genital canal. The bursal canal is lined with nucleate and ciliated cells, and it receives the secretion of shell glands ectally to the separate openings of the oviducts; the canal is surrounded by a layer of circular muscle fibres. The copulatory bursa is lined with tall, vacuolated and nucleated cells.

Eyes
Each eye cup contains three retinal cells and is devoid of a lens.

Reproduction
Claparède (1863) described a female specimen with a cocoon in its atrium, which was collected in the period mid-July / end-September. According to Tekaya (1982) cocoons are probably deposited during the entire year since she encountered in all her samples always a few females with cocoons. One female may deposit in her period of cocoon-laying 1-5 cocoons at a rate of 1-2 cocoons per day (Tekaya 1982). The globular cocoons are 0.5 mm in diameter and are devoid of a pedicel; cocoons are attached to the substrate (Tekaya 1982).

Ecology
From the literature (Claparède 1863, Gamble 1893, Tekaya 1982) appears that Sabussowia dioica occurs among algae, notably Zostera and Posidonia. The species was obtained not only from driftweed on mudflats at eb, but also from places which remained flooded during low tide (Claparède 1863).

Distribution
S. dioica is known only from a few localities, viz. the island Tatihou, Normandie, France (Claparède 1863; type locality), Plymouth (Gamble 1893, Wilhelmi 1908a, Material Examined), Trieste (Böhmig 1906, Material Examined), Naples (Material Examined), Nice and Toulon (Du Plessis 1907), Gulf of Tunis (Zghal and Tekaya 1980, Tekaya 1982).

Material Examined
Z.M.B.: 4344, male specimen, Trieste, det. Böhmig, sagittal sections on 1 slide; 4344, female specimen, ibid., sagittal sections on 1 slide; 8515 + 5415, female, Plymouth, August 1908, coll. Gamble, sagittal sections on 2 slides; 8486a+b, male, ibid., sagittal sections on 2 slides.
B.M.N.H.: 1930.3.27.78, female, Plymouth, sagittal sections on 3 slides.
S.M.N.H.: male specimen, Naples, 18.05.1928, leg. S. Bock, horizontal sections on 1 slide; male specimen, Naples, 14.06.1928, leg. S. Bock, sagittal sections on 1 slide; female specimen, Naples, 18.05.1928, sagittal sections on 1 slide.

Type Material
No type specimen available.

Sabussowia dioica