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Sluys and Ball, 1989

Overview
Diagnosis: Vatapa tumidosa Sluys and Ball, 1989 can be recognized by its slender appearance, whereas the species distinguishes itself from other marine triclads by the presence of a large musculo-parenchymatic organ and by the fact that the common oviduct opens into the very proximal section of the bursal canal.
Habitus: Preserved specimens are elongate and up to about 3.5 mm long and 0.45-0.5 mm wide. The front end is broadly rounded, and the hind end may either be rounded or obtusely pointed. Along the anterior margin of the body a narrow pale brown pigmented zone could be observed in some of the specimens. The remaining parts of the animals appeared unpigmented. According to the collector of the specimens, live animals show "Brown tip on pointed head, black eyes, a crystalline (guanine) triangle on back of head extending as a thin line down mid-dorsum".

Alimentary System
The slender pharynx lies in the posterior portion of the body and is between one-sixth and one-fourth of the body length. The inner circular muscle layer of the pharynx is about twice as thick as the outer zone of circular muscles. The anterior ramus of the intestine extends anterior to the eyes but does not give rise to pre-ocellar diverticula. Behind the eyes the long anterior ramus gives off about 15 pairs of lateral diverticula. The two posterior intestinal rami do not meet in the hind end of the body.

Male Reproductive System
The large, oval-shaped testes occupy the entire space between dorsal and ventral body surface. There are about 15 follicles on either side of the body, extending from a short distance behind the brain up to half-way along the pharyngeal pocket. The testicular follicles are situated between the bases of the intestinal diverticula, one follicle in each septum. At about the hind end of the pharyngeal pocket, the vasa deferentia enlarge to form false seminal vesicles. Behind the pharynx the vasa deferentia curve dorso-medially and, subsequently, unite to form a very short and narrow duct which opens into a spacious seminal vesicle. The latter narrows to a broad ejaculatory duct which via an extremely narrow distal portion opens at the tip of the penis papilla; this narrow section of the ejaculatory duct usually does not show on sagittal and horizontal sections, probably because it is closed due to constriction. Ejaculatory duct and seminal vesicle are lined with a cuboidal, nucleate epithelium and are surrounded by a circular muscle layer. A coarse, erythrophilous secretion is discharged into the seminal vesicle and into the proximal portion of the ejaculatory duct. A fine granular and cyanophilous secretion is discharged into the distal part of the ejaculatory duct.
The penis papilla is a short, blunt cone with a slight inclination towards the dorsal body surface. The papilla is covered with a cuboidal, nucleate epithelium that is underlain by a row of circular muscle fibres; presence of longitudinal fibres could not be established. A penis bulb appeared to be absent, at least no musculature could be discerned forming part of such a structure.

Female Reproductive System
The paired ovaries are situated at one-sixth to one-fifth of the distance between the brain and the root of the pharynx. The ovaries lie just medially to the ventral nerve cords.
The vitellaria are well developed and extensive. They occupy the entire space between dorsal and ventral body surface and extend from anterior to the ovaries into the hind end of the body.
The oviducts arise from the anterior or antero-lateral surface of the ovaries as narrow, non-muscularized ducts. Subsequently, the oviducts make a latero-caudally directed bend, and in doing so cross the ventral nerve cords. Immediately laterally to the ovaries each duct expands to form a much broader duct of which the well-developed lumen is lined with large cuboidal cells. Sperm is present in this broad, proximal section of the oviducts. At the level of the hind end of the ovaries the broad sections taper until they have reached the diameter of the major, posteriorly running section of the oviducts. The oviducts run laterally to the ventral nerve cords. Behind the gonopore the oviducts turn medially and unite, in the ventral portion of the body, to a common oviduct. This common duct opens into the proximal section of the bursal canal, i.e. very close to the point where the canal arises from the copulatory bursa.
After having arisen from the bursa, the bursal canal at first runs obliquely towards the dorsal body surface but, subsequently, curves anteriad, following its course parallel to the body surface. The distal section of the bursal canal opens dorsally into the male atrium. The bursal canal is lined with a nucleate epithelium; cilia could not be discerned. Shell glands discharge into the canal from slightly ectally to the opening of the common oviduct up to half-way along the length, or even more, of the bursal canal. Male and female atrium, as well as the distal portion of the bursal canal are surrounded by a layer of circular and longitudinal muscles, respectively. The remaining part of the bursal canal is at least surrounded by a layer of circular muscles; a longitudinal muscle layer could not be discerned, but it may have been obscured by the abundant secretion of the shell glands.
Paratype IZ 3522/10 had in its bursal canal a rod-like structure with pointed ends, which stained densely red, thus indicating a sclerotized nature. The structure was broken into several pieces. The location of this rod suggests that it may form part of a spermatophore.
The copulatory bursa is a small, irregularly shaped structure that is lined with a cuboidal, nucleate epithelium; it is not surrounded by muscles.

Musculo-parenchymatic Organ and Common Atrium
In essence, the musculo-parenchymatic organ consists of a large and irregularly shaped indentation of the hind wall of the common atrium. The cells lining this indentation are nucleate, as are the cells lining the atrium. The lining epithelium of this indentation or fold, is underlain by a layer of circular muscles with entally to it a layer of longitudinal muscles. The entire indentation is surrounded by a very broad zone of loose parenchymatic tissue which is interspersed with numerous nuclei. The parenchymatous zone is bounded by a thick muscle layer of which the composing fibres spread out over the parenchymatic body but mainly have a longitudinal orientation.
In all specimens the hind wall of the common atrium is folded in such a way that it forms a sort of cone or papilla projecting into the atrium. The epithelium lining the lumen of this fold or outbulging, actually constitutes the anteriormost portion of the lining of the above-mentioned indentation of the atrium. The common atrium into which the fold projects, communicates with the male and female atrium by means of a narrow opening. The above holds true for all specimens examined, but the specific details of the shape of the musculo-parenchymatic organ and the common atrium differ considerably between the specimens examined.
In paratype IZ 3522/5 the fold consists of a small and blunt cone which projects into a spacious common atrium. The indentation, i.e. the lumen of the musculo-parenchymatic organ, forms a broad duct, running parallel to the body axis, of which the posterior section shows a sac-shaped widening. The entire musculo-parenchymatic organ appears as a large, oval-shaped body. In the paratypes IZ 3522/8 and IZ 3522/9, the folds projecting into the common atrium are also rather small but of a more irregular shape. In these specimens the common atrium is much narrower, whereas the entire musculo-parenchymatic organ, including its lumen, is irregularly shaped as compared with the situation found in paratype IZ 3522/5. In the holotype the common atrium is much larger than in the paratypes discussed above, and almost its entire lumen is occupied by a large, blunt cone forming an elaborate fold of the hind wall of the atrium. The lumen of the musculo-parenchymatic organ coincides with the broad lumen of the cone, in contrast with the situation in the paratypes mentioned above. In these paratypes the lumen extends backwards beyond the level of the fold. The parenchymatous zone posterior to the hind wall of the common atrium, consists of a dorsally oriented bulb-like structure. The most extreme condition of the musculo-parenchymatic organ is shown by paratype IZ 3522/2. In this specimen the fold appears as a very large, baseball-shaped body, filling most of the space of a, consequently, very large common atrium. As in the holotype, the lumen of the musculo-parenchymatic organ is confined to the fold. The remaining part of the parenchymatous zone, i.e. posterior to the hind wall of the atrium, consists of a bulbous body.
The various appearances of the musculo-parenchymatic organ are probably caused by different states of contraction and extension of the papilla arising from the hind wall of the common atrium.

Eyes
The pigment cups contain a globular lens. The serial sections did not allow determination of the number of retinal cells.

Ecology
The type material was collected from coarse, broken shell at a depth of 24 m.

Distribution
Type locality: Goat Island (36°16'S 174°47'37"E), New Zealand. The species is known only from its type locality.

Material Examined, Type Material
C.M.: Holotype: IZ 3522/1, Goat Island, New Zealand, 7.12.1982, sagittal sections on 1 slide.
Paratypes: IZ 3522/2, sagittal sections on 1 slide; IZ 3522/3, horizontal sections on 1 slide; IZ 3522/4, horizontal sections on 1 slide; IZ 3522/5, sagittal sections on 1 slide; IZ 3522/6, transverse sections on 2 slides (asexual animal); IZ 3522/7, transverse sections on 3 slides; IZ 3522/8, sagittal sections on 1 slide; IZ 3522/9, sagittal sections on 1 slide; IZ 3522/10, sagittal sections on 1 slide.

Vatapa tumidosa