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Wilhelmi, 1909

Overview
Diagnosis: C. cerruti Wilhelmi, 1909 is characterized by (1) a well developed penis papilla, into which the vasa deferentia enter separately, with its large chitinized tip, (2) a musculo-glandular organ, and (3) a single pair of large testes in front of the pharynx.
Habitus: Living animals about 2 mm long and 0.45 mm wide. The body is elongate with a narrow, rounded front end and a broadly rounded hind end. The dorsal surface is dirty white or yellowish due to the presence of yellow-brown pigment granules. The eyes are very far removed from the anterior body margin (Wilhelmi 1909).

Alimentary System
The pharynx measures between one-seventh and one-sixth of the body length and is inserted posteriorly to the middle of the body. The mouth opening is at the hind end of the pharyngeal cavity.
The anterior ramus of the intestine extends anterior to the eyes and gives off two pairs of preocellar diverticula. Behind the eyes the same gut trunk gives rise to three pairs of lateral diverticula. From each posterior intestinal ramus arise 11-12 lateral diverticula; these posterior rami do not meet in the hind end of the body.

Male Reproductive System
There only is one pair of large, global-shaped testicular follicles situated just in front of the root of the pharynx. The vasa deferentia arise from the hind wall of the testes and expand to form false seminal vesicles at the level of the pharyngeal pouch. Behind the pharyngeal cavity the vasa deferentia curve medially and narrow to such an extent that only very thin ducts penetrate, separately, the penis. Once within the penis papilla both ducts expand again to form very large accessory seminal vesicles, which open into the proximal, more or less funnel-shaped section of the ejaculatory duct.
The penis is more or less bean-shaped and carries a long sclerotized tip. The ejaculatory duct becomes very thin while running through this sclerotized section; the proximal part of this duct receives the secretion of penis glands. The penis papilla is provided with a strong layer of circular muscles. A penis bulb is absent.

Female Reproductive System
The paired ovaries are situated just in front of the testes, at a considerable distance behind the brain, viz. at about half-way between the eyes and the root of the pharynx. The ovaries lie dorsally to the ventral nerve cords. The major portion of the ovaries is global-shaped but, according to Meixner (1928), the germ centre ("Keimlager") occurs in a short tube-shaped, ("schlauchförmigen") and mediocaudad oriented sack, as in Cercyra. Some sections suggested that the oviducts arise from the posterior surface of the ovaries but no certainty could be obtained on this topic. According to Meixner (1928: 578) the oviducts extend anteriorly to the ovaries, but he did not specify in which way this occurs.
The vitellaria are well developed, being situated between the intestinal diverticula and extending from anterior to the ovaries into the hind end of the body.
The female copulatory apparatus consists of a short bursal canal, which recieves the separate openings of the oviducts, and a very small copulatory bursa. The bursa may be absent in fully mature animals (Meixner 1928). The bursal canal communicates with the atrium and is lined with a ciliated epithelium, and it receives the secretion of shell glands ectally of the openings of the oviducts (Meixner 1928).
The bursa is situated antero-dorsally to the major section of the bursal canal and consists, according to Meixner (1928), of a flat syncytium containing a granular mass.

Musculo-glandular Organ
Between the mouth and the gonopore is situated, at the ventral body surface, an oval-shaped structure made up of interwoven muscle fibres and a granular mass. According to Meixner (1928), dorso-ventrally running muscles attach to the surface of this organ, which is considered to be a rudimentary musculo-glandular organ; it has no opening to the exterior.

Eyes
There is no lens to the eyes, and the number of retinal cells contained in each pigment cup remains unknown.

Ecology
Wilhelmi (1909) obtained his specimen from a plankton sample, but considered this to be an artifact and assumed that the species belongs to the beach fauna. This was shown to be correct since Meixner (1928) collected many specimens from brown algae and hydroids growing on "Piloten" (sheds ?) and stones.

Distribution
Cerbussowia cerruti is known from the Gulf of Naples (Wilhelmi 1909; type locality) and from Grado at the Gulf of Trieste (Meixner 1928). Recently, it has been collected also from the Gulf of Tunis (S.Tekaya in litt).

Material Examined, Type Material
Z.M.B.: 5416, Gulf of Naples, 1908, horizontal sections on 1 slide.

Cerbussowia cerruti