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Sluys and Ball, 1989

Overview
Diagnosis: Synsiphonium angustus Sluys and Ball, 1989 can be distinguished from all other bdellourid triclads known to date by (1) the long common spermiduct branch which only bifurcates behind the pharynx, (2) the absence of lateral bursae (3) a muscularized constriction in the posterior spermiduct branches, just before they open to the exterior.
Habitus: The preserved specimen was about 1.5 mm long and 0.75 mm wide. According to the field label, the living animal was lanceolate with a pointed front end. In the preserved animal the hind end was broadly rounded, whereas there was still some indication of the pointed anterior margin.
The dorsal surface was covered with irregularly shaped pigment spots, which in life, according to the field label, are greenish. The ventral surface of the preserved specimen showed a faint, reticulate pattern of pigmentation.

Alimentary System
The pharynx is about one-fifth of the body length, its root being situated at about the middle of the body. The inner circular muscle layer of the pharynx is about twice as thick as the outer zone of circular muscles.
Even in the preserved animal cleared in clove oil, not much could be seen of the intestinal branches. From the sections, however, it was apparant that the anterior ramus extends anterior to the eyes and that the two posterior branches do not communicate.

Male Reproductive System
The rather large, rounded or oval-shaped testes extend from directly behind the ovaries up to the level of the copulatory apparatus. The follicles are situated ventrally but their dorsal portion may extend well beyond the mid-line of the body. There are about 18 testicular follicles on either side of the body. Most of the testes are arranged in two rows, one on either side of the body. But in front of the pharynx, on either side of the body, there are about four follicles which lie medially to the main rows of testes.
At about the level of the root of the pharynx, the vasa deferentia enlarge to form false seminal vesicles, which curve medially at the level of the penis bulb and narrow considerably before penetrating the ventro-lateral surface of the bulb. After having penetrated the musculature of the bulb, the vasa deferentia gradually expand in diameter. In the broad proximal portion of the penis papilla the vasa deferentia run side by side, but they only unite half-way in the papilla to form a very short and narrow duct which opens into the proximal section of the ejaculatory duct via a stubby papilla-like projection. The proximal, funnel-shaped part of the ejaculatory duct is lined with a tall epithelium which is provided with long cilia and stains bright red because of the secretion of penis glands. The ejaculatory duct communicates with the exterior at the tip of the penis papilla, although the actual opening could not be observed.
The penis consists of a rather shallow bulb and a massive, cone-shaped papilla. The penis projects into a narrow atrium which is surrounded by thin layers of circular and longitudinal muscles, respectively. The penis bulb and the basal portion of the papilla are surrounded by strong, interwoven muscle fibres. A large portion of the parenchyma around the vasa deferentia in the penis papilla, is traversed by numerous irregularly or more or less circularly running muscle fibres. The fibres are somewhat thinner than those on the penis bulb. The sections of the vasa deferentia inside the penis papilla are surrounded by a strong layer of circular muscle fibres. The epithelium of the papilla is underlain with a well developed layer of circular muscles.

Female Reproductive System
The vitellaria are poorly developed, having a patchy distribution between the intestinal branches.
The paired, rounded ovaries are situated directly behind the brain, medially to the ventral nerve cords. A broad oviduct arises from the ventro-lateral surface of the ovaries and, subsequently, runs backwards for a short distance, just above the ventral nerve cords. Apart from these short sections no traces of oviducts could be found in the rest of the body.
A spermiduct branch runs from a short distance behind the ovaries to posteriorly to the pharyngeal cavity. At a short distance behind the ovaries the spermiduct branch bifurcates, giving off somewhat narrower branches to each of the ovaries. These two anterior branches of the spermiduct communicate with the medio-ventral surface of the ovaries and are lined with a tall, nucleate epithelium, which stains bright orange due to some sort of secretion. The cells lining the most anterior sections of these branches do not contain the orange secretion, and bear cilia.
The single spermiduct branch which results after fusion of the two short branches from the ovaries, is at first very broad, but in running backwards it gradually tapers. The lumen of the duct is poorly defined, often not distinguishable at all; the major portion of the duct is filled with nucleate cells containing a blue-staining, granular material. Along its entire course the duct contains strongly stained orange particles. I was able to follow this common part of the spermiduct up to the root of the pharynx, only to discern it again somewhat anterior to the hind end of the pharyngeal pocket. From thereon the duct enlarges in diameter to form a large oval-shaped vesicle behind the pharyngeal pocket. This vesicle does not have any lumen, but only a few vacuoles, and is filled with the above-mentioned cells containing cyanophilous, granular material. On either side of this expansion a duct arises from its ventro-lateral surface. Each duct, or posterior spermiduct branch, runs backwards for some distance and then shows a pronounced constriction after which it widens again and, subsequently, opens ventrally to the exterior. The openings lie about half-way the distance between the mouth opening and the gonopore and are situated just laterally to the ventral nerve cords. Over their entire length these posterior spermiduct branches are lined with a cuboidal, nucleate epithelium which, however, becomes very thin at the constriction. The entire constriction is surrounded by a thick layer of circular muscles.
The remaining part of the female reproductive system consists of a modest section which communicates with the male atrium. The whole structure consists of a very small, rounded copulatory bursa from the ventral surface of which arises a short and narrow duct. The latter opens into the rounded, expanded portion of a broad duct that communicates with the atrium. This last-mentioned broad duct, the bursal canal, is lined with a nucleate epithelium bearing well developed cilia; shell glands open into its most distal portion.
The bursa is lined with a few large cells with nuclei at their bases. The bursa has only a very small lumen and, as far as could be observed, does not communicate with the intestine, although its histology is very much like that of the intestine.

Eyes
The pigment cups contain an unknown number of retinal cells. Only one of the two eyes showed a large, oval-shaped lens.

Ecology
The type specimen was collected from coarse stones and pebbles at a depth of 30 ft (9.14 m). The species is only known from the type locality.

Distribution
Type locality: Kaikoura, New Zealand.

Material Examined, Type Material
N.M.N.Z.: Holotype, ZW 1207, St. Kilde Rocks, Kaikoura, Harhon, New Zealand, 29.01.1983, sagittal sections on 1 slide.

Synsiphonium angustus