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Jägerskiöld, 1896

Overview
Diagnosis: Micropharynx parasitica Jägerskiöld, 1896 is unlikely to be confused with any other marine triclad because of its ectosymbiotic occurrence on skates, the lack of eyes, and its simple female copulatory apparatus. It is more likely that Micropharynx will be confused, at first sight, with monogean trematodes, but the triclad may be distinguished from the latter by the lack of true suckers, the presence of a plicate pharynx, and the tripartite intestine.
Habitus: Preserved specimens are 5-7 mm long and 3-6 mm wide. The shape of the body varies with the state of contraction of preserved specimens, and ranges from oblong to broad and leaf-like. The front end is rounded; the hind end is truncated and rolled inwards, thus forming the zone of attachment. The animal is devoid of pigment and eyes.

Alimentary System
The pharynx is small and stubby, measuring about one-tenth of the body length; outer and inner circular muscle layers are of about the same thickness. The anterior ramus of the intestine extends far towards the anterior body margin, and gives rise to several pre-ovarial diverticula. The posterior rami show laterally as well as medially directed diverticula. The posterior gut trunks may anastomose, as is true for their medially directed diverticula.

Male Reproductive System
The numerous testes are situated ventrally, extending from the ovaries up to the root of the pharynx. The follicles have a median position and lie directly adjacent to the anterior ramus of the intestine.
At the level of the pharynx region, the vasa deferentia expand to form large false seminal vesicles which again decrease in diameter and, subsequently, unite to a common duct. Shortly after this fusion, the common vas deferens shows a twist before it enters the penis bulb. The common vas deferens expands slightly just in front of the penis bulb and narrows while it traverses the muscle layers of the bulb. Then, the duct expands again to form a kind of seminal vesicle. The latter may narrow gradually or more abruptly, after which it opens into the ejaculatory duct, which traverses the intrapenial papilla. This intrapenial papilla may be short and stubby in some specimens, but in others it is slender and pointed.
The large and conical penis has a horizontal disposition. The intrapenial papilla empties into the spacious penis lumen, the shape of which depends on the shape of the papilla. In case of a short intrapenial papilla, the penis lumen narrows gradually towards the tip of the penis, only showing a dilatation shortly before it opens at the tip of the penis. But in case of a slender intrapenial papilla, the penis lumen is very wide and shows a constriction in its distal part. After this dilatation the penis lumen expands again before opening at the tip of the penis papilla.
The intrapenial papilla is highly glandularized. The ducts of the glands follow a characteristic path before opening into the common vas deferens (see also Ball and Khan 1976: Fig.5).
The musculature of the penis bulb and the male atrium is well developed. On the anterior portion of the atrium, circular and longitudinal muscle layers generally are thicker than those surrounding other parts of the atrium. The penis papilla and the intrapenial papilla are provided with a subepithelial layer of circular muscles, which is overlain with longitudinal muscle fibres.

Female Reproductive System
The two ovaries are situated directly behind the brain; the follicles are large and usually occupy most of the space between dorsal and ventral body surface. The oviducts run at first ventro-laterally, after having arisen from the ovaries, before they make a sharp posteriorly directed bend. The oviducts run slightly laterally to the line of demarcation between testes and vitellaria. At about the level of the penis, the ducts curve dorso-medially and, subsequently, open separately into the female genital duct.
The vitellaria are highly developed; the follicles are situated laterally to the testes and extend throughout the body length in the form of two bands, one on either side of the body. The vitellaria occupy the entire space between dorsal and ventral body surface.
In some specimens the female copulatory organs may only be partly developed, or may be completely absent, while the rest of the reproductive apparatus is well developed. The common genital atrium is in line with the male atrium and opens ventrally to the exterior. Both atria are separated from each other by a muscular fold, the development of which depends on the state of contraction of the preserved animal. Extensive shell glands empty into the female genital duct, just ventrally (=ectally) to the oviducal openings.

Ecology
M. parasitica is an ectosymbiotic triclad which has been found on four species of Skate, viz. Raja clavata Linnaeus, 1758, R. batis Linnaeus, 1758, R. radiata Donovan, 1806, and R. laevis Mitchell, 1817 (record of M. parasitica on the last-mentioned species of skate from Stafford 1904). Although there is no reason for assuming that the triclad is restricted to these four host species, Micropharynx has not been found on any of 80 specimens of R. ocellata Mitchell, 1817 (Ball and Khan 1976). Ball and Khan (1976) found the rate of infestation to be low: only about 6 % of the skates examined, appeared to be infested with Micropharynx. Some skates, however, were infested with over 100 triclads.

Distribution
Type locality: Kattegat, Sweden. M. parasitica has been reported from the Kattegat, Newfoundland, off Murmansk in the Barentsz Sea, and from the east coast of Canada (last-mentioned record based on Stafford 1904). In case the distribution of M. parasitica depends only on the range of its host species, the triclad may expected to occur over a much vaster area.

Material Examined
Private collection I. R. Ball: one specimen from Newfoundland, Canada, November 1971-73, sagittal sections on 11 slides; C143, Grand Banks, Newfoundland, sagittal sections on 7 slides; C143, horizontal sections on 4 slides.
S.M.N.H.: no. 648, Atlantic Ocean (59°14'N 0°37'E), 9.06.1926, serial sections; no's 704-706, Kristineberg, Bohuslan, 27.4.1931, serial sections; no. 224, Gullmarfjorden, Bohuslan, serial sections.

Micropharynx parasitica