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Hyman, 1956

Diagnosis: Nesion arcticum Hyman, 1956 can be recognized easily by its characteristic pattern of unpigmented patches on the dorsal body surface, by the presence of two ventral and one dorsal genital pore, and by its peculiar penis.
Habitus: Preserved specimens are up to 8 mm long (Hyman 1956). The body of preserved specimens is elongate, the hind end being obtusely pointed, and the front end broadly rounded. Auricles may be evident at the level of the eyes (Hyman 1956).
The major part of the dorsal surface shows a brownish pigmentation in a reticulate pattern. According to Hyman (1956) unpigmented areas are present around the eyes and in the form of a broad mid-dorsal stripe, extending from shortly behind the eyes towards the hind end of the body. In the larger specimens elongated, unpigmented areas may be present to the sides of the median stripe, according to Hyman (1956).
In the specimens studied I found the situation to be slightly different. The animals showed conspicuous unpigmented areas around the eyes as well as at the antero-lateral body margins. These unpigmented patches may be in contact with each other. Another conspicuous unpigmented area occurs just anterior to the root of the parynx; it is an elongated or cresent-shaped patch situated perpendicular to the body axis. Posterior to this patch there are several broad patches which may be more or less confluent, thus giving the impression of an unpigmented mid-dorsal stripe. About half-way the cresent-shaped patch and the posterior margin of the body, there is an elongated, unpigmented patch on either side of the mid-dorsal stripe. Contrary to Hyman's observations, the specimens examined did not show an unpigmented stripe between the eyes and the root of the pharynx.

Alimentary System
The pharynx is situated at about the middle of the body and measures about one-sixth of the body length. In the specimens examined, the inner circular muscle layer of the pharynx is much thicker than the outer one; in the type material Ball (1973a) found the outer circular muscle zone to be almost of the same diameter as the inner circular muscle layer. However, the type specimens appeared to have been compressed on fixation (Ball 1975a). The mouth opening is situated at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends anterior to the eyes and gives off two pairs of small and unbranched, pre-ocellar diverticula. Behind the eyes the anterior ramus gives off about six pairs of branched lateral diverticula. Each posterior intestinal trunk gives rise to about 12 either forked or unbranched, lateral diverticula. The tips of the posterior rami do not meet in the hind end of the body but several commissures are present.

Male Reproductive System
The shape of the large testes varies from rounded, oval-shaped, more or less cuboidal to irregularly-formed. The testes are situated ventrally but some large, oval-shaped, follicles may extend to almost the dorsal body surface. Testicular follicles extend from the level of the ovaries up to about half-way along the pharyngeal pocket. There may be up to 10 follicles on either side of the body. At the level of the pharyngeal pouch the vasa deferentia enlarge to form false seminal vesicles. The latter run posterior to about the level of the male atrium, then curve antero-medially and penetrate the latero-medial surface of the highly muscular penis bulb. At this particular point of entrance the vasa deferentia show a considerable constriction. Inside the penis bulb the ducts broaden again and open separately into a seminal vesicle. The distal portion of this vesicle shows a more or less pronounced S-shaped bend after which it narrows to the long ejaculatory duct. The seminal vesicle is lined with a layer of tall cells.
The penis has a horizontal disposition and consists of a strongly muscularized bulbous part and a very long, narrow papilla. The bulb as well as the papilla consist of very loose parenchymatic tissue. Because of this delicate structure, the penial complex may be difficult to reconstruct in many preparations, due to damage to the tissues as a result of sectioning or preservation. The penis papilla is usually folded back inside the male atrium. Because of its length, the papilla generally is folded several times, which obscures much of its structure. The distal portion of the papilla carries well-developed, slighltly curved conical spines, which are clearly visible in properly stained sections. The tip of the penis papilla is blunt. The musculature of the penis bulb is strongly developed and consists of interwoven fibres. This musculature extends on the papilla, but here it occurs in the form of two layers: immediately beneath the epithelium there is a thin layer of circular muscle fibres with entally to it a layer of longitudinal fibres. The seminal vesicle is surrounded by a well-developed layer of intermingled muscle fibres, as are those parts of the vasa deferentia inside the penis bulb. The ejaculatory duct is surrounded by a thick layer of circular muscles (Ball 1973a). The male atrium opens to the exterior through a separate gonopore.

Female Reproductive System
The small, rounded or somewhat irregularly shaped ovaries are situated immediately behind the brain. They lie medially to the ventral nerve cords and close to each other. The oviducts leave from the antero-lateral surface of the ovaries. The proximal end of the oviducts is enlarged to a small, muscularized tuba which communicates with the ovaries. There may be a slight constriction between tubae and ovaries. The tubae narrow to the oviducts which run laterally to the ventral nerve cords. The ducts run caudally to about the level of the posterior surface of the copulatory bursa, then recurve and turn medially and, shortly thereafter, unite to a common oviduct. This common oviduct opens into the ventral part of the bursal canal, close to the opening of the latter into the bursa.
The vitellaria are well developed and extend from anterior to the brain into the hind end of the body, occupying the entire space between the dorsal and ventral body surfaces.
The copulatory bursa is a rather small, somewhat triangularly shaped, vesicle which communicates with the exterior through a dorsal pore and a bursal canal with a separate ventral opening. The bursa is lined with a layer of large, vacuolated cells and is surrounded by a very thin layer of muscle fibres.
The bursal canal arises from the anterior surface of the bursa. The canal may (1) first turn dorsally and, subsequently, make a sharp bend towards the ventral body surface, or (2) run almost parallel to the body surface for a short distance and then curve towards the ventral surface. The bursal canal has a rather large diameter and may open into a sort of shallow vestibulum which, in turn, communicates with the exterior. This vestibulum, however, was not present in every specimen examined. The musculature of the bursal canal is well-developed, but differs considerably among individual specimens. In the specimens N.M.N.S. Acc.61-123 and B.M.N.H. 1976.5.13.26-42 there is directly underneath the lining epithelium of the bursal canal a very thick layer of circular muscles, which is overlain by a thin layer of longitudinally running fibres. This situation is also present in the type material (cf. Hyman 1956, Ball 1973a). On the basis of the material examined, I was unable to confirm Ball's (1973a) observation that immediately below the lining epithelium of the bursal canal there is a thin layer of intermingled muscle fibres. In the specimens MG 52.1 and MG 52.2 the circular musculature of the bursal canal is much less developed, being only slightly thicker than the longitudinal layer.
Glands discharge into the distal part of the bursal canal, close to its opening to the exterior. The structure of the secretion is very much like that of shell glands in other species of marine triclads. The bursal canal of the specimens MG 52.1 and MG 52.2 is lined with a layer of large, nulceate cells, whereas those of Acc.61-123 are much flatter.

The pigment cups house a large, rounded lens and only two retinal cells.

The species has been collected from the intertidal zone (Hyman 1956, Material Examined).

Type locality: St. Matthew's Island, Bering Sea Alaska. According to the original species description the type locality would be St. Andrew's Island, Bering Sea, Alaska, but this appeared to be incorrect since there is no island with that name in the Bering Sea. From David M. Hickok of the Arctic Environmental Information and Data Center in Anchorage, who contacted the collector of Hyman's material, viz. Dr. Robert Rausch, I learned that the type specimens actually came from St. Matthew's Island. Hyman (1957) made the same error in the description of another turbellarian but in this case mention was made of the incorrect type locality in Rausch and Rausch (1968).
N. arcticum has been found on St. Matthew's Island (Hyman 1956), Amchitka Island (Material Examined) and Kenai Peninsula (Material Examined) in Alaska.

Material Examined
N.M.N.S.: Acc.61-123, Day Cove, below Anchor Cove, Kenai Peninsula, Alaska, 10.07.1961, sagittal sections on four slides.
Private collection I. R. Ball: MG 52.1, Amchitka Island, St. Makarius Bay, Aleutians, Alaska, 7.10.1971, sagittal sections on 2 slides; MG 52.2, sagittal sections on 2 slides; MG 52.3, transverse sections of hind end on 2 slides and horizontal sections of front end on 1 slide; three preserved specimens specimens from sample locality MG 52.
B.M.N.H.: 1976.5.13.26-42, Amchitka Island, Aleutians, sagittal sections on 2 slides.

Type Material
A.M.N.H.: Holotype: no. 466, sagittal sections of the posterior end on 2 slides (3.1 and 3.2). Paratypes: no. 581, sagittal sections on 2 slides (1.1 and 1.2), no. 581, sagittal sections on 2 slides (2.1 and 2.2), no. 581, transverse sections of the posterior end on 2 slides (4.1 and 4.2), no. 467, whole mount.

Nesion arcticum