(Palombi, 1938)
Overview
Diagnosis: Palombiella stephensoni (Palombi, 1938) is characterized by (1) a common oviduct which meets the postero-ventral wall of the bursal canal, (2) oviducts which arise from the antero-ventral surface of the ovaries, and (3) one lens in each eye cup.
Habitus: The size of preserved specimens varies from about 2.5 x 1 mm up to 3.5 x 1mm, but according to Palombi (1938) the animals may be as large as 4 mm in length and 2 mm in width. The body is lanceolate; the hind end is rounded, while the front end may be rounded or, according to Palombi, obtusely pointed. In front of the eyes the body shows a more or less pronounced constriction. The largest diameter of the body is at about the hind end of the pharyngeal pocket. The eyes are situated close together.
In preserved specimens the dorsal surface of the body is black, except for the body margins, a broad band along the anterior margin of the body, and broad, unpigmented patches at the level of the eyes. On the ventral side, the body shows no pigmentation. Living specimens may show a more variable pattern of coloration. According to Palombi (1938) and Nurse (1955) the tip of the front end is orange-coloured. The coloration of the dorsal surface varies from black, dark-grey, light-grey to red-orange, whereas also irregular pale cream patches may be present (Nurse 1955, Wineera 1969).
Alimentary System
The pharynx is between one-sixth and one-fourth of the body length (Palombi 1938, Westblad 1951, Material Examined). Both the outer and the inner circular muscle layer of the pharynx are well developed, but the inner layer is much thicker than 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, in the form of a club-shaped branch, anterior to the eyes; there are no pre-ocellar diverticula. Behind the eyes, the anterior ramus gives off 6, usually forked, lateral diverticula (Westblad 1951, Material Examined). According to Palombi (1938), however, there is one pair of pre-ocellar diverticula, whereas behind the eyes the anterior ramus would give rise to only 4 lateral diverticula. Each of the posterior rami gives off 9-12 forked or simply branched, lateral diverticula. There are no commissures between the two branches, whereas they do not meet in the hind end of the body.
Male Reproductive System
About 12-20 testicular follicles are situated on either side of the body (Palombi 1938, Westblad 1951, Wineera 1969, Material Examined); Nurse (1955) mentioned a total of 36-38 follicles for the specimens which she examined. The oval-shaped follicles are situated ventrally but their dorsal surface may extend dorsally to well beyond the mid-line of the body. The follicles occur from directly behind the ovaries to the level of the male copulatory apparatus. The testes lie between the bases of the intestinal diverticula; in front of the pharynx the follicles may occur in pairs between the diverticula, but laterally and posteriorly of the pharynx there usually is only one follicle in each septum.
Already well in front of the root of the pharynx, the vasa deferentia enlarge to form false seminal vesicles. Behind the pharynx the vasa deferentia curve dorsally and penetrate, separately but closely together, the mid-anterior surface of the penis bulb. Inside the bulb the two ducts run side by side towards the tip of the intrapenial papilla. Fusion of the vasa deferentia only takes place short before the tip of the blunt intrapenial papilla. Especially those parts of the vasa deferentia inside the penis bulb and the papilla and those just outside of the penis bulb, are surrounded by strong circular muscle fibres.
The intrapenial papilla actually is the central column of a number of radiating septa, delimiting six chambers in the proximal part of the ejaculatory duct.
The penis bulb is rather shallow and only weakly muscularized. The penis papilla is an elongate, conical structure, almost oriented parallel to the body axis. The lining epithelium of the penis papilla is underlain with a well developed layer of circular muscles, consisting of several rows of fibres. Entally to this layer lies a thin layer of longitudinal muscle fibres. Both muscle layers are continuous with those surrounding the atrium. Excessive penis glands discharge into the ejaculatory duct and the chambers delimited by the septa. The glandular elements lie outside the penis bulb and their dark orange staining secretion dominates in the major portion of the bulb and the penis papilla.
Female Reproductive System
The oval-shaped ovaries are situated at a short distance behind the brain, slightly medially to the ventral nerve cords. The oviducts arise from the antero-ventral surface of the ovaries; the ducts are connected with the ovaries via a muscularized narrowing. In running backwards, the oviducts stay laterally to the ventral nerve cords. At the level of the gonopore, the ducts curve medially, communicate with the female receptacles by means of a short and narrow duct and, subsequently, unite to a common oviduct. The latter meets the postero-ventral wall of the bursal canal.
The distal portion of the bursal canal opens into the female atrium; the proximal, posterior section of the canal is curved towards the dorsal body surface and communicates with the copulatory bursa. Bursal canal and common oviduct are provided with long cilia. The female atrium receives the openings of extensive shell glands. The major portion of these glands lies around the atrium and between the posterior wall of the pharyngeal pocket and the penis bulb. The small, irregularly shaped copulatory bursa is lined with tall, nucleate and vacuolated cells.
The bursal canal appeared to be surrounded by only circular muscle fibres. No musculature could be discerned around the copulatory bursa. The common oviduct is surrounded by a layer of circular muscles.
A lateral bursa is situated on either side of the bursal canal, close to the mid-line of the body. The lateral bursae occupy the entire space between dorsal and ventral body surfaces and curve slightly towards the front end of the animal. The lateral bursae are lined with tall, vacuolated cells are are surrounded by a layer of well developed circular muscle fibres; sperm may be present in their lumen. The bursae open ventrally to the exterior, their openings being situated postero-laterally to the gonopore. Since the gonopore lies just in front of the neural arch resulting from the fusion of both ventral nerve cords, this implies that the openings of the lateral bursae occur just postero-laterally of this neural arch. The short connecting ducts between the oviducts and the lateral bursae open into the antero-ventral portion of the latter. These small ducts are surrounded by a well-developed layer of circular muscles.
The extensive vitellaria occupy the entire space between ventral and dorsal body surface and extend from anterior to the ovaries into the hind end of the body.
Nervous System and Eyes
In the posterior end of the body the two ventral nerve cords unite to a neural arch, from which branches arise which extend into the hind end of the body (Westblad 1951, Material Examined). This neural arch lies just posterior to the gonopore. The eyes consist of a single-celled pigment cup with three retinal cells and an oval-shaped or rounded lens.
Reproduction
Life Cycle: One specimen from Seal Rocks, New Zealand had a brown cocoon in its atrium. The capsule was rounded and had a diameter of about 0.75 mm; a pedicel was absent.
Ecology
P. stephensoni has been collected in the intertidal zone from under stones and from algae (Palombi 1938, Westblad 1951, Nurse 1955). Concerning Westblad's specimens, sampling depth varied from 2-40 m. With respect to the material examined, specimen W19268-1 was collected from the bottom of a 6 m deep tide pool, and the specimens from the samples ZW 1203 and ZW 1206 were obtained from holdfasts of the bull kelp Durvillea.
Distribution
Type locality: Oudekraal, South Africa. P. stephensoni has been reported from South Africa (Palombi 1938), Tristan da Cunha, Nightingale and Inaccessible (Westblad 1951). The species has been collected from various localities in New Zealand, viz. Banks Peninsula (Nurse 1955), Wellington (Wineera 1969), Snares Island, Kaikoura, Otago Peninsula and it has been found also on Gough Island (Material Examined).
Material Examined
S.M.N.H.: Paratypes: one set of transverse sections on 4 slides, transverse sections of the front end of one animal on 2 slides, and sagittal sections of the hind end of the same specimen on 1 slide.
A.M.: W19268-1, East end of Seal Point, Snares Island, New Zealand, 1.02.1975, sagittal sections on 3 slides.
N.M.N.Z.: ZW 1203-1, Seal Rocks, Kaikoura, New Zealand, 4.02. 1983, sagittal sections on 2 slides; ZW 1203-2, horizontal sections on 1 slide; ZW 1203-3, transverse sections on 2 slides; ZW 1204 + 1205, Otago Peninsula, New Zealand, 13.03.1983, 2 immature, preserved specimens; ZW 1206, Seal Reef, Kaikoura, New Zealand, 4.02.1983, sagittal sections on 1 slide.
B.M.N. H.: 1960.2.24.17-26, Gough Island, Dell Rocks and Glen Mouth, one set of sagittal sections on 4 slides; one set of sagittal sections on 3 slides; four sets of sagittal sections, each on 1 slide; three sets of sagittal sections, each on 2 slides; one set of horizontal sections on 2 slides.
Type Material
The present location of the holotype is unknown, but two paratypes are available from the S.M.N.H., viz. one set of transverse sections on 4 slides, transverse sections of the front end of one animal on 2 slides, and sagittal sections of the hind end of the same specimen on 1 slide.