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Westblad, 1952

Overview
Diagnosis: Centrovarioplana tenuis Westblad, 1952. The peculiar orientation of the penis papilla and the presence of a musculo-glandular organ directly behind the pharyngeal cavity differentiate C. tenuis from every other marine triclad known to date.
Habitus: Preserved specimens up to 3.5 mm long and 0.7 mm wide. The body is elongate, with rounded front and hind end.

Alimentary System
The pharynx is situated in the middle of the body and measures only between one-twelveth and one-nineth of the body length. Both outer and inner circular muscle layer of the pharynx are well developed, the layers being of about equal thickness. The mouth opening lies at the hind end of the pharyngeal cavity.
The anterior ramus of the intestine extends anteriorly to the eyes, and gives rise to about 10 pairs of postocellar lateral diverticula. It could not be established whether or not the caudal gut trunks communicate in the hind end of the body.

Male Reproductive System
The anteriormost testicular follicles are situated at some distance behind the brain, from where the testes extend into the hind end of the body. The testes are principally situated ventrally, but often the follicles extend to almost the dorsal body surface.
The vasa deferentia expand to voluminous false seminal vesicles which unite to form a much narrower common vas deferens, close to the copulatory apparatus. This common vas deferens is surrounded by a layer of circular muscles and opens into a large seminal sac. This voluminous sac is lined with tall, glandular cells.
The conical penis papilla has a very peculiar orientation because it projects into the lumen of the seminal sac, thus pointing to the dorsal body surface. The papilla houses a broad ejaculatory duct which communicates at the tip of the penis with the seminal sac and opens ventrally into the atrium.
The ejaculatory duct and the dorsal part of the atrium are lined with an infranucleate epithelium which is pierced by numerous, thin muscle fibres. These muscle fibres run more or less perpendicular to the ejaculatory duct and delimit channels through which a granular secretion is discharged into the lumen of the duct. The fibres constituting these musuclar septa originate from bundles of muscle fibres that run through the penis papilla.

Female Reproductive System
The vitellaria are well developed, extending from a short distance behind the brain into the hind end of the body and occupying the entire space between the dorsal and ventral body surfaces.
The small, rounded ovaries are situated ventrally, on either side of the pharyngeal cavity; they lie dorsally to the ventral nerve cords. The oviducts arise from the ventro-lateral wall of the ovaries and run dorso-laterally to the ventral nerve cords. Each oviduct consists of a forward running branch and a caudally directed branch. It was impossible to determine how far the anterior oviducal branch extended forwards. The caudal branches unite to form a short common oviduct which opens into the atrium through the rear wall of the latter. This common oviduct receives the secretion of shell glands.
A granular secretion is discharged into the ventral part of the atrium, the openings of the glands being concentrated in two regions.

Musculo-glandular Organ
A well defined, large and tubular indentation of the ventral body wall is present directly posterior to the mouth opening. The cuboidal cells lining this indentation are nucleate and provided with cilia, but they lack rhabdites. This blindly ending indentation comes into close contact with a rounded or elongate mass of parenchymatic cells. The entire organ is poorly defined, only being distinguishable from the surrounding parenchyma by its more dense structure and by muscle fibres traversing the organ. Moreover, a fine, granular secretion(?) is dispersed throughout the organ.
It is remarkable that this histologically poorly defined structure is clearly visible in preserved specimens as a well defined body situated directly behind the pharyngeal cavity.

Eyes
The eye cups contain three retinal cells; eye lenses could not be discerned.

Ecology
The species was collected (1) from Pot Bay, South Georgia from clay and seaweed at a depth of 22 m, and (2) at Port Albemarle, Falkland Islands from sandy bottom with seaweed at a depth of 40 m.

Distribution
Type locality: As type locality been designated by Sluys (1989) station 33 of the Swedish South Polar Expedition, viz. Pot Bay, South Georgia (54°22'S 36°28'W), being the site where most of the type material has been collected.
The species was collected (1) from Pot Bay, South Georgia from clay and seaweed at a depth of 22 m, and (2) at Port Albemarle, Falkland Islands from sandy bottom with seaweed at a depth of 40 m.

Material Examined and Type Material

S.M.N.H.: station 33, Swedish South Polar Expedition, South Georgia: transverse sections of the front end of a specimen and sagittal sections of the hind end of the same animal, each series mounted on a separate slide; sagittal sections on 1 slide; transverse sections on 5 slides; sagittal and transverse sections of 2 immature animals, each on 1 slide; station 55, Falkland Islands: transverse sections of the front end and sagittal sections of the hind end of a specimen, each series on 2 slides.

Centrovarioplana tenuis