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Sluys and De Vries, 1988

Overview
Diagnosis: Obrimoposthia aparala Sluys and De Vries, 1988 can be distinguished from the other members of its genus through the following combination of features: infranucleate lining of bursal canal, the latter receiving the openings of shell glands and unicellular glands; voluminous copulatory bursa; strong circular muscles of penis papilla concentrated in a broad annular zone at the base of the penis.
Habitus: Preserved specimens are 5.25-6.5 mm long and about 3.5 mm wide. In the preserved state the body shows broadly rounded front and hind ends; in some animals there was a faint indication of the presence of auricles. The dorsal surface is black, the pigment being arranged in a patchy pattern; the eyes are situated in small pigment-free areas. The ventral surface shows a similar, but less dense pattern of pigmentation.

Alimentary System
The pharynx is situated in the middle of the body and is between one-fourth and one-third of the body length. The inner circular muscle layer of the pharynx is much thicker than the outer circular muscle layer. The mouth opening is at the hind end of the pharyngeal pouch.
The anterior ramus of the intestine extends anterior to the eyes, and behind the latter the gut trunk gives off about 5 pairs of lateral diverticula. The posterior rami are confluent in the hind end of the body; each branch gives rise to about 14 lateral diverticula.

Male Reproductive System
The relatively small, rounded testes are situated ventrally between the lateral diverticula of the intestinal branches. The follicles extend from the level of the ovaries to a short distance behind the copulatory apparatus. There are 75 or more testicular follicles on either side of the body.

At the hind end of the pharyngeal cavity the vasa deferentia expand to form false seminal vesicles which follow an S-shaped path as they curve dorso-medially in order to penetrate, separately, the muscles at the base of the penis. During their course through the penis the vasa deferentia run side by side, fusing only at their very distal ends, thus making it difficult to establish whether they communicate with the ejaculatory duct through an extremely short common section or open separately into the ejaculatory duct.
The penis lacks a well-delimited bulb and only consists of a highly developed papilla, filling the entire male atrium. The papilla is lined with a flat, nucleate epithelium. The penis is dominated by an extremely thick zone of circular muscles, spanning the basal half of the papilla. In sagittal sections this muscle zone exhibits a reticulate pattern due to the particular shape and arrangement of the muscle fibres. This zone of circular muscles is entally bounded by a thin layer of longitudinal muscles, which is continuous with the somewhat thicker layer at the mid-basal section of the penis papilla.
The central part of the proximal section of the penis papilla is provided with radially arranged, muscular septa, which delimit about 14 elongated chambers. The latter accumulate the secretion of penis glands, which is transported towards the ejaculatory duct with which these chambers communicate. The penis glands lie outside the penis and produce a granular, erythrophilous secretion. The ejaculatory duct is lined with a low, infranucleate epithelium.
The epithelium on the distal section of the penis papilla is underlain with a thin, subepithelial layer of small circular muscles and an equally thin layer of longitudinal muscles. This thin layer of circular muscles is somewhat thicker where it meets the strong fibres of the thick circular muscle zone at the proximal part of the penis papilla.

Female Reproductive System
The paired ovaries lie at a short distance behind the brain, medially to the ventral nerve cords. The oviducts arise from the lateral wall of the ovaries and run laterally to the ventral nerve cords.
The vitellaria occur throughout the body length, from anterior to the ovaries into the hind end of the body.
Posterior to the gonopore the oviducts curve medially and, subsequently, unite to form a rather long common oviduct. This common duct meets the rear wall of the bursal canal and opens into the ventral portion of the latter. The ventral section of the bursal canal is rather narrow but dorsally to the opening of the common oviduct the canal is much wider and provided with a very tall, papillose lining. The epithelium of the entire bursal canal is infranucleate and is underlain with a relatively thick layer of circular muscles and a thinner layer of longitudinal muscles. Further, the bursal canal is surrounded by numerous unicellular glands which discharge their granular secretion into the broad section of the canal, i.e. entally to the opening of the common oviduct. The bursal canal receives the openings of shell glands ectally to the opening of the common oviduct.
The bursal canal communicates with a large, sac-shaped copulatory bursa which is lined with tall cells.

Eyes
Each pigment cup contains three retinal cells and has in front of its opening an oval-shaped lens.

Ecology
The species is known from Possession Island and Saunders Island (West Falklands), where it was collected from freshwater biotopes.
Two of the sectioned specimens from Possesion Island had in their guts large parts of their fellow planarians, which could be established quite easily since the remains showed lensed eyes similar to the ones of the predators.

Distribution
Type locality: Crozet Islands, Possession Island, river Styx at exit from Lake Perdu. The species is known from Possession Island and Saunders Island (West Falklands).

Material Examined
B.M.N.H.: 1975.9.18.10-12, Crozet Islands, Possession Island, River du Camp, 2.03.1973, sagittal sections on 6 slides; 1975.9.18.10-12, 2 preserved specimens.
S.A.M.: V3132, station 119, American Bay, freshwater pool, 1/2 mile inland, Possession Island, Crozets, 3.11.1929, sagittal sections on 4 slides.
Z.M.A.: V.Pl.827.1, Saunders Island (West Falklands), 26.12.1986, sagittal sections on 5 slides; V.Pl.827.2, sagittal sections on 5 slides; V.Pl.827.3, horizontal sections on 3 slides.

Type Material
B.M.N.H.: Holotype: 1975.9.18.1, Crozet Islands, Possession Island, river Styx at exit from Lake Perdu, 18.02.1973, sagittal sections on 5 slides.
Z.M.A.: Paratypes: V.Pl. 789.1, sagittal sections on 4 slides; V.Pl. 789.2, horizontal sections on 1 slide.

Obrimoposthia aparala