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(Böhmig, 1902)

Overview
Diagnosis: Procerodes variabilis (Böhmig, 1902) is characterized by a stout penis with a thick circular muscle zone, and by the lack of a copulatory bursa.
Habitus: Preserved specimens vary from 2.6-5 mm in length and from 1.3-2.6 mm in width (Böhmig 1906, Nurse 1964). The posterior end is broadly rounded; the body margins run parallel from the posterior end up to about the middle of the body, from where they converge towards the front end. In the neck region there is a slight constriction in front of which there is a pair of auricles. The anterior margin of the head is convex. The two eyes are small and set wide apart.
The coloration of the animals is variable. Living specimens may be (1) white due to lack of pigment, (2) dirty grey-brown on the ventral surface and brownish on the dorsal body surface, (3) pale brown with very fine mottling on the dorsal surface, while the ventral surface is paler, or (4) may show greyish, reddish or yellowish dendritic markings (Böhmig 1906, Nurse 1964).

Alimentary System
The pharynx is about one-third of the body length. The outer as well as the inner circular muscle zone of the pharynx consist of several rows of fibres; the inner circular muscle layer is considerably thicker than the outer one. The mouth is situated at the hind end of the pharyngeal pocket.
The anterior ramus of the intestine extends anterior to the eyes, giving off a pair of pre-ocellar diverticula, and 6-8 pairs of furcated, post-ocellar diverticula (Böhmig, 1906). Each posterior intestinal ramus gives off 14-17 lateral diverticula; medially directed diverticula also may be present but these do not anastomose (Böhmig, 1906).

Male Reproductive System
The numerous small and egg-shaped testes are situated dorsally between the intestinal diverticula. Two follicles may be present in one septum. The exact number of testes could not be determined. It is not uncommon for testicular follicles to be situated half-way between the ventral and dorsal body surfaces or to occur at the level of the ventral portion of the intestinal diverticula. The testes extend from the level of the ovaries into the hind end of the body.
At the hind end of the pharyngeal pocket the vasa deferentia enlarge to form false seminal vesicles, which turn dorso-medially towards the penis bulb. While ascending towards the bulb, each vas deferens enlarges somewhat in diameter, thus forming an accessory seminal vesicle. Just before entering the penis bulb, the vasa deferentia narrow considerably. In some specimens these accessory seminal vesicles are absent (Westblad 1951). Immediately after having penetrated the penis bulb, the vasa deferentia enlarge to form large seminal vesicles. Fusion of both ducts only takes place in the basal portion of the penis papilla, but not before the ducts have narrowed again. The wide, strongly ciliated ejaculatory duct opens at the tip of the penis papilla.
The penis papilla is a plump, conical structure with a blunt or pointed tip. The papilla has a ventro-caudal orientation and is lined with tall epithelial cells. The penis papilla is provided with a thick, subepithelial layer of circular muscles and a much thinner layer of longitudinal muscle fibres. Both muscle layers are continuous with those surrounding the male and common atrium. However, the layer of circular muscles around the atria is much thinner than the one on the penis papilla. The ejaculatory duct and the portion of the vasa deferentia inside the penis bulb are surrounded by a well developed layer of circular muscles. The sections of the vasa deferentia outside of the bulb are surrounded by thin layers of circular muscle fibres. The ejaculatory duct and the vasa deferentia are lined with cuboidal, nucleate cells. The distal portion of the ejaculatory duct receives the openings of penis glands. The musculature of the penis bulb is highly developed.

Female Reproductive System
The small, rounded or oval-shaped ovaries are situated directly behind the brain. The oviducts arise from the frontal surface of the ovaries, and run anteriad for a short distance. Subsequently, the oviducts curve backwards, running just dorsally to the ventral nerve cords. Behind the gonopore the oviducts turn medially and fuse to a common oviduct, which opens into the diverticulum of the female genital duct.
The vitellaria are well developed and extend from anterior to the ovaries into the hind end of the body, filling most of the space between the intestinal diverticula and the testes.
The female genital duct arises obliquely from the common atrium. The dorsal section of the duct has a larger diameter than the proximal part, and has an irregular sac-shaped outline. A copulatory bursa is absent. The female genital duct is lined with cuboidal or cylindrical, nucleate cells which may bear long and thick cilia. These cilia may be absent in some animals. Moreover, it is not certain whether these cilia correspond to proper cilia or have a different structure, a situation already noted by Böhmig (1906). The cilindrical diverticulum of the female genital duct arises from the postero-dorsal wall of the latter. This diverticulum is lined with a ciliated, nucleate epithelium and is penetrated by the openings of shell glands. The distal end of the diverticulum receives the common oviduct, the latter generally having a considerable length. The musculature of male and common atrium continues on the female genital duct; the diverticulum is only provided with a thin layer of circular muscles.

Eyes
The eye cups contain three retinal cells and lack a lens.

Ecology
P. variabilis may be found in the interidal zone under stones or in sand, or under rocks in shallow water (Bergendal 1899, Westblad 1951, 1952, Nurse 1964, Material Examined). Specimens have been dredged also from a depth of about 22 m (NMCI 1985-0233 of Material Examined). Specimen K977 (Material Examined) was collected from the skeleton of a Rockhopper Penguin.

Distribution
Type locality: Punta Arenas, Magellanes Sound, Tierra del Fuego. The species has been reported from Punta Arenas (Bergendal 1899, Böhmig 1902, 1906), Isla Navarino, Puerto Toro (Böhmig 1902, 1906), Peninsula Scott and Punta Percy (Material Examined) in Tierra del Fuego, from Port Stanley on the Falklands (Westblad 1952), Tristan da Cunha and Inaccessible (Westblad 1951), Macquarie Island (Nurse 1964, Material Examined), and Gough Island (Holdgate 1961). The species has been sampled also from Marion Island (Material Examined).

Material Examined
N.M.N.S.: NMCI 1985-0239, Peninsula Scott, Tierra del Fuego, 10.02.1970, sagittal sections on 1 slide; NMCI 1985-0237, sagittal sections on 2 slides; NMCI 1985-0238, sagittal sections on 3 slides; NMCI 1985-236, Punta Percy, Tierra del Fuego, 22.02.1970, sagittal sections on 3 slides.
M.V.: G1230, Macquarie Island, 2.01.1949, sagittal sections on 6 slides; NMV G1229, Macquarie Island, 26.01.1950, sagittal sections on 3 slides.
T.M.: K977, Macquarie Island, 2.01.1967, sagittal sections on 4 slides.
B.M.N.H.: 1960.2.24.27-36, Gough Island, serial sections of 3 specimens; 1968.9.24.1-5, Marion Island, serial sections of 3 animals.

Type Material
No type specimen available.

Procerodes variabilis