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Marcus, 1948

Overview
Diagnosis: Vatapa gabriellae Marcus, 1948 can be recognized by its slender, posteriorly situated pharynx, the few testes, and by ovaries which are situated at a considerable distance behind the brain. The species is further characterized by a stubby penis papilla and a musculo-parenchymatic organ, which communicates with the atrium.
Habitus: Preserved specimens are about 3 mm long and 0.5 mm wide. The animals are elongate and slender, with their body margins running parallel. The front end is obtusely pointed, or rounded; the hind end is pointed. The dorsal surface may be reddish with a light yellow median stripe, or may be white. In the last-mentioned case, the yellow stripe appears after preservation. The red pigment disappears after preservation, whereas the yellow pigment remains present because it lies in the parenchyma (Marcus 1948). The small eyes lie close together, at some distance from the anterior margin.

Alimentary System
The slender pharynx is situated in the posterior third of the body; it is about one-fifth to between one-seventh and one-sixth of the body length. Outer and inner surface of the pharynx are densely ciliated. The outer circular muscle layer of the pharynx is well developed, and the inner circular muscle layer is about three times as thick as the first-mentioned zone of muscles. The mouth opening is situated at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends anterior to the eyes as a short, unbranched diverticle. The anterior ramus gives off about 20-25 forked or branched lateral diverticula. Each posterior ramus gives rise to about 15 diverticula. Behind the copulatory apparatus the posterior intestinal trunks come to lie close together, but do not communicate (Marcus 1948).

Male Reproductive System
The small, rounded and pre-pharyngeal testes are few in number: about 3-6 on either side of the body. The follicles are situated between the bases of the intestinal diverticula, i.e. medially to the vitellaria, and extend from some distance behind the ovaries to somewhat anterior to the root of the pharynx. The testicular follicles, which are elongate-sac-shaped in sagittal sections, are situated ventrally, although they may extend for a considerable distance towards the dorsal surface.
At the hind end of the pharyngeal pocket the vasa deferentia enlarge to form false seminal vesicles. The latter open behind the pharynx, at the level of the penis papilla, into a small and rounded seminal vesicle from which arises a narrow ejaculatory duct that opens at the tip of the penis papilla. The papilla is a stubby structure with a blunt, truncated tip; it is situated in a narrow male atrium. The penis papilla is covered with a nucleate epithelium which is underlain with a single layer of circular muscles. The parenchyma of the papilla is obscured by a fine, granular and eosinophilous secretion from gland cells surrounding the proximal portion of the male atrium. These gland cells discharge at the tip of the papilla and also into the ejaculatory duct. A penis bulb appears to be absent.

Female Reproductive System
The small, rounded ovaries lie at some distance behind the brain, viz. at one-sixth to one-fifth of the distance between the brain and the root of the pharynx. The oviducts are very narrow and could not be discerned in the major part of the body. But immediately laterally to the ovaries the ducts enlarge to form well-discernable, elongate tubae. The latter are lined with rather large cells, whereas their lumen contains sperm. The tubae are connected with the ventro-lateral surface of the ovaries through a short, and probably muscularized constriction.
The vitellaria are extensive and well developed. They extend from anterior to the ovaries up to the level of the posterior part of the copulatory apparatus. The vitellaria lie between the intestinal diverticula and extend from dorsal to ventral body surface.
At the level of the gonopore, the oviducts turn dorso-medially and open separately into the bursal canal. The semi-circular or oval-shaped copulatory bursa is lined with large, nucleate cells; sperm was found to be present in its lumen. The bursal canal, which arises from the anterior surface of the bursa, is lined with nucleate, cuboidal cells bearing well-developed cila. The canal is arched and meets the posterior wall of the atrium. The distal part of the bursal canal is much narrower and is lined with a much flatter type of epithelium, as compared with the proximal portion. This distal part lacks cilia and is penetrated by the openings of extensive shell glands which lie around the bursal canal. According to Marcus (1948) the oviducts open immediately anterior to the ciliated, proximal part of the bursal canal. This was indeed suggested by one of the series of sections which I examined (the one labelled "hinten"), but in the other series of serial sections the oviducts appeared to open into the broad, proximal portion of the bursal canal. In one preparation some circular muscles could be discerned around this proximal section of the bursal canal. It was impossible to make any detailed observations on the musculature of other parts of the female copulatory apparatus.

Musculo-parenchymatic Organ
The postero-ventral wall of the common atrium shows a large, spacious and more or less dorsally curved indentation or fold. The nucleate epithelium of this indentation is underlain with a thin layer of circular muscle fibres. The entire fold is surrounded by a thick parenchymatous zone which is bounded by a layer of longitudinal muscle fibres. Nuclei lie dispersed in the parenchymatous zone.

Eyes
The eye cups contain a spherical lens. The number of retinal cells could not be determined.

Ecology
Vatapa gabriellae has been collected from between algae and Bryozoa.

Distribution
Type locality: Guaruj, near Santos, Brazil. The species is known only from its type locality and another locality in the bay of Santos, Brasil.

Material Examined, Type Material
Uncatalogued slides from Dept. Zoologica Sao Paulo: 1 slide with whole mount of V.gabriellae and Dinizia divae; 1 slide with 4 whole mounts; sagittal sections on 2 slides, on the labels of which is written "vorn" and "hinten", respectively; 1 slide with sagittal sections.
No holotype has been designated by Marcus, but a lectotype may be selected from, preferably, one of the two series of sagittal sections listed above, since the material examined involves the type series.

Vatapa gabriellae