Diagnosis: Stummeria marginata (Hallez, 1906) may be distinguished from other marine triclads by its strongly muscularized penis bulb, by the peculiar construction of the penis papilla, the characteristic appearance of copulatory bursa and bursal canal, and by the presence of numerous retinal cells in each eye cup.
Habitus: Preserved specimens are 8-12 mm long and about 4 mm wide (Hallez 1906, 1907; Westblad 1952; Material Examined). The body margins run from the broadly rounded hind end almost parallel towards the front end. In front of the eyes the body is constricted and forms a short "snout", of which the anterior margin is rounded.
The dorsal surface is black-brown or sepia coloured, with an unpigmented streak running from front to hind end (Hallez 1907, Material Examined) This streak has its largest diameter at about the middle of the body, whereas it tapers towards front and hind end, without being confluent with the unpigmented body margins. The eyes are set in small unpigmented areas. The ventral surface is unpigmented (Hallez 1906, 1907).
The relatively short pharynx measures about one-seventh of the body length. In the specimens examined, the distal portion of the pharynx is turned towards the ventral body surface and situated more or less underneath the anterior portion of the male copulatory apparatus. The mouth opening is at the hind end of the pharyngeal cavity. The inner circular muscle layer of the pharynx is rather thin and only consists of a few rows of circular muscles. The diameter of the outer circular muscle layer exceeds slightly that of the inner layer, whereas the thickness of both circular muscle zones diminishes strongly towards the tip of the pharynx.
The anterior ramus of the intestine extends anterior to the eyes and gives off one or two pairs of pre-ocellar diverticula. According to Hallez (1907), the posterior rami of the intestine do not anastomose.
Male Reproductive System
The testes are few: about 12 follicles on either side of the body. The rather large, oval-shaped follicles are situated ventrally and extend from behind the ovaries up to the level of the gonopore.
In front of the pharynx the vasa deferentia enlarge to form false seminal vesicles which, still anterior to the pharynx, taper to form a narrow duct. This long duct runs posteriorly, increasing slightly in diameter at about the level of the gonopore, and at the same time starts turning medially. In following its course medially, the duct narrows again and, subsequently, makes a sharp, anteriorly directed bend, after which its diameter increases again considerably. Hereafter, the vasa deferentia taper to very narrow ducts which are aligned against the ventral surface of the penis bulb. The vasa deferentia penetrate separately the antero-lateral wall of a large, intrabulbar seminal vesicle.
The penis has a horizontal disposition and consists of a large, rounded, bulbar part and a considerably smaller papilla. The proximal portion of the bulb houses the above-mentioned seminal vesicle which opens, through a sort of intrapenial papilla, into the spacious and irregularly shaped lumen of the penis bulb. The wall of this seminal vesicle, as well as that of the short papilla, consists of a clearly nucleated epithelium. The lumen of the penis bulb narrows distally and empties into a vesicle which lies at the base of the penis papilla. This particular vesicle opens into the non-ciliated ejaculatory duct. The wall of the last-mentioned vesicle and the ejaculatory duct are lined with nucleate cells. The penis papilla ends in a short sclerotized point, around which there is a small preputial fold.
The penis bulb is surrounded by a thick and conspicuous layer of longitudinal muscles of which the corresponding nuclei lay interspersed between the fibre mass. Entally to this longitudinal muscle layer there is a much thinner layer of circular muscle fibres. Both muscle layers extend, albeit much thinner, on the proximal portion of the penis bulb and are continuous with corresponding layers around the ejaculatory duct and the seminal vesicle at the base of the penis papilla. A strong sphincter muscle surrounds the ejaculatory duct; it consists of interwoven circular and longitudinal fibres. The large seminal vesicle in the proximal (anterior) part of the penis bulb is surrounded by a thin layer of longitudinal muscle fibres.
The tissue of the penis bulb consists of two histologically different sections, viz. a densely granulated proximal part and an also densely granulated, but more strongly staining, distal portion. The proximal portion of the tissue is continuous, through openings in the anterior surface of the muscle coat around the penis bulb, with a more or less similarly structured tissue just outside the bulb. The tissue inside the penis bulb appears to be organized in broad stretches of granulated protoplasm, and it is interspersed with numerous, singular muscle fibres. The distinctly cellular tissue in the distal portion of the penis bulb consists of irregularly shaped, vacuolated cells. The same type of tissue is present around the tip of the intrapenial papilla. In the specimens examined the last-mentioned type of tissue stains much more brightly red than the surrounding tissue.
Penis glands are situated outside the penis bulb and discharge into the penis via the openings in the penis bulb. The preparations examined showed that the secretion accumulates around the tip of the intrapenial papilla and from there is discharged into the lumen of the penis bulb. Thereafter, the secretion aggregates in the distal portion of the penis bulb. Another type of secretion is discharged into the ejaculatory duct.
The tip of the penis papilla projects into a distinct male atrium, which is delimited from the common atrium by means of a ventral fold. Male, female, and common atrium are densely ciliated.
Female Reproductive System
The small, globular ovaries are situated at one-fourth to one-third of the distance between the brain and the root of the pharynx; they lie dorsally to the ventral nerve cords. The oviducts arise from the antero-medial surface of the ovaries and run forwards for a short distance (100-350 um), before turning towards the hind end of the body; the ducts run latero-dorsally to the ventral nerve cords. Behind the gonopore, the oviducts turn medially and open separately into the proximal portion of the bursal canal.
The vitellaria are well developed and extensive; they extend from a short distance anterior to the ovaries into the hind end of the body and occupy the entire space between the dorsal and ventral body surfaces.
The copulatory bursa is lined with a layer of small, rectangular and nucleated cells. The bursal canal arises from the postero-ventral surface of the bursa. The short, proximal section of the bursal canal has a vertical orientation but then the canal makes a sharp anteriorly directed bend, after which it is oriented parallel to the body surfaces. The canal opens into the common atrium through the posterior wall of the latter. The curved part of the bursal canal receives the separate openings of the oviducts.
The bursal canal receives the secretion of shell glands from its opening into the atrium up to the oviducal openings.
The bursal canal is surrounded by a thick layer of circular muscles which is overlain with a thin layer of longitudinal fibres. The copulatory bursa is surrounded by a thin layer of interwoven muscle fibres.
The large eye cups contain many small retinal cells and a large, oval-or egg-shaped lens.
According to Hallez (1906, 1907) the cocoons are unstalked and have a diameter of 1.5 mm.
S.marginata has been collected from between stones on the beach (Hallez 1906, 1907) and from sand and algae at a depth of 12-15 m (Westblad 1952).
Type locality: Booth Island (= Ile Wandel; 65°05'S 64°00'W), Antarctic Peninsula. The species has been reported from Booth Island, Antarctic Peninsula (Hallez 1906, 1907), and it is known from South Georgia (Westblad 1952; Material Examined).
S.M.N.H.: Swedish Antarctic Expedition, South Georgia, station 28, 24.05.1902, sagittal sections of one specimen on 3 slides.
B.M.N.H.: 19126.96.36.199, South Georgia, Grytviken, 11.04.1927, sagittal sections on 13 slides.
No type specimen available.