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(Oersted) Bock, 1925

Overview
Diagnosis: Foviella affinis (Oersted) Bock, 1925 can be recognized by its triangular head, the large and curved penis, and by the absence of a copulatory bursa.
Habitus: Live specimens measure up to 11 x 2 mm but are usually smaller (Van Beneden 1860, Ball and Reynoldson 1981). Head triangular or, when not moving, rounded. The body is constricted slightly posteriorly to the eyes and reaches its maximum breadth behind the pharynx.
The colour of the dorsal surface is yellow to yellow-brown. Pigment free areas occur around the eyes and along the anterior body margin. The ventral surface is white. Living animals are usually not transparent, only the branches of the intestine showing through the pigment (Bock 1925, Ball and Reynoldson 1981). The two eyes are closer to each other than each of them is to the lateral body margin.

Alimentary System
The pharynx is about one-third or more of the body length; the inner circular muscle layer is much thicker than the outer one. The anterior ramus of the intestine extends anterior to the eyes. The two posterior rami do not anastomose in the hind end of the body.

Male Reproductive System
Testes numerous, situated between the diverticula of the intestine and distributed throughout the body length. The follicles are mainly situated ventrally, but often they are quite large, almost occupying the the entire space between the ventral and dorsal body surfaces.
Near the pharynx region the vasa deferentia enlarge considerably to form false seminal vesicles, which narrow again before entering the penis bulb. The vasa deferentia enter the bulb separately and shortly thereafter each duct enlarges to form a highly muscularized seminal vesicle. Hereafter the ducts narrow gradually and fuse in the penis papilla to form the ejaculatory duct. The latter increases slightly in diameter before opening at the tip of the penis papilla.
The musculature of the penis bulb is weakly developed. The penis is large and its middle portion shows a pronounced, caudally directed bend. Sometimes the penis papilla is curved even sofar that the tip faces the dorsal body surface. The subepithelial circular muscle layer of the penis is strongly developed, in contrast to the longitudinal muscle layer which lies interiorly of it.

Female Reproductive System
The ovaries are situated directly behind the brain, medially to the ventral nerve cords. The oviducts open separately into the female genital duct. The vitellaria are extensive and well-developed; they extend from the level of the ovaries into the hind end of the body and reach from dorsal to ventral body surface.
A copulatory bursa is absent; there is only a female genital duct which is penetrated by the openings of shell glands. The wall of the female genital canal is relatively thick and its lumen narrow. The duct is surrounded by a thin layer of circular muscles.

Eyes

Each pigment cup contains three retinal cells and a large, rounded lens.

Reproduction
Karyology: The diploid complement consists of 16 metacentric chromosomes, of which some are almost submetacentric. Only one chromosome pair is distinctly larger than the rest; the remaining chromosomes gradually decrease in size. With respect to meiosis, the male line apparently behaves normal, but oogenesis has not been studied (Ball 1976).

Ecology
F. affinis occurs under stones in the intertidal zone (Ball and Reynoldson 1981) and on submerged plants, e.g. Knotted Wrack (Ascophyllum nodosum), at a depth of several metres (Bock 1925), or in detritus at depths up to 120 m (Steinböck 1928).

Distribution
Type locality: Kallebodstrand, Copenhagen (Oersted 1843) and Gullmarfjorde, Sweden (Bock 1925). The species has been recorded from Newfoundland, Nova Scotia, New Brunswick, and Maine (cf. Hyman 1944, Tomkiewicz and Ball 1973, Ball 1975a). In the north-west Atlantic region F. affinis has also been reported from western Greenland (Steinböck 1928). In Europe the species has been recorded from resund, Skagerrak, Belgium and, with respect to Dalyell's (1853) Planaria hebes, from Scotland.

Material Examined
Private collection I. R. Ball: C65, Harris' Point, Maine, USA, 27.08.1973, sagittal sections on 1 slide; C65, sagittal sections on 1 slide; C7, Queensport, Nova Scotia, 22.07.1962, sagittal sections on 3 slides; C61, Gleason Point, Maine, USA, 30.08.1973, sagittal sections on 1 slide; C61, sagittal sections on 1 slide; C61, transverse sections on 2 slides; C48.2, Nova Scotia, Canada, 8.08.1973, sagittal sections on 2 slides; C48.1, sagittal sections on 4 slides; C56a, Dog Island, Eastport, Maine, USA, 30.08.1973, sagittal sections on 4 slides.

Foviella affinis