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(Schmidt, 1861)

Diagnosis: Procerodes plebeius (Schmidt, 1861) can be recognized by its brownish-black pigmentation, small tentacles and small eyes. With respect to anatomical features, the species is characterized by a well developed penis papilla with a thick zone of circular muscles, by the fact that the vasa deferentia only unite in the penis papilla, by the vertically oriented bursal canal, and by the presence of a common oviduct.
Habitus: Live specimens up to 7 mm long and 1-1.25 mm wide (Wilhelmi 1909, Den Hartog 1968); the length of preserved specimens is much smaller, viz. 1.25-2.25 mm. Living animals show a small but conspicuous auricle on either side of the head, but this feature may hardly be visible in preserved specimens. From the rounded posterior end the body margins run almost parallel or converge slightly towards the eyes. The largest body diameter is at the level of the pharynx. In front of the eyes the body margins diverge and give rise to the pair of auricles. The front end of the body is convex. In preserved specimens hind and front end are generally rounded.
In general, the dorsal surface shows a brownish to brownish-black coloration; the ventral surface is also pigmented, albeit less dense; in the animals from the Black Sea the dorsal body surface was devoid of pigment. Tentacles, areas in front of the eyes, and the body margins are devoid of pigment, whereas the pigmentation is less dense in the region of the pharynx and the copulatory apparatus. Ventral nerve cords and a number of testicular follicles may be visible through the body wall. The eyes are very small and spaced wide apart, lying very close to the lateral body margins.

Alimentary System
The pharynx is about one-third of the body length. Both the inner and outer circular muscle layers are well developed, the first one being considerably thicker than the last-mentioned layer. The anterior ramus of the intestine extends anterior to the eyes and gives off a pair of pre-ocellar diverticula and 3-4 pairs of post-ocellar diverticula. Each posterior intestinal ramus gives rise to about 8 lateral diverticula. The posterior rami do not meet in the hind end of the body, but it is uncertain whether commissures occur behind the copulatory apparatus. The mouth opening is situated at the hind end of the pharyngeal pocket.

Male Reproductive System
In horizontal sections the testes appears as oval-shaped follicles, situated in a regular fashion between the intestinal diverticula. The testes are situated dorsally, but their ventral portion extends beyond the mid-line of the body. There are 28-32 testicular follicles in total.
At the hind end of the pharyngeal pocket the vasa deferentia enlarge to form moderately developed false seminal vesicles, which narrow before entering the ventro-lateral portion of the penis bulb. After having penetrated the penis bulb, the vasa deferentia enlarge slightly in diameter and run, side by side, towards the proximal portion of the penis to unite only in the proximal third of the penis papilla. The ejaculatory duct is rather wide and opens at the tip of the penis papilla.
The large penis papilla may be conical or it may have a more slender appearance; it has a vertical or oblique orientation. The very tip of the penis papilla may show a more or less pronounced, caudally directed bend. The penis papilla also may be folded inside the male atrium, pointing backwards or even to the roof of the atrium.
The musculature of the penis bulb is well developed, and consists of interwoven fibres. The circular musculature of the penis papilla is strongly developed, the thick zone of muscles having a reticulate appearance in serial sections. Entally, the circular muscle layer is bounded by a single row of longitudinal muscle fibres. Male and common atrium are surrounded by single rows of circular and longitudinal muscles. The portion of the vasa deferentia inside the penis bulb, and the ejaculatory duct are surrounded by a rather thick layer of circular muscles. Penis glands discharge into the distal section of the ejaculatory duct.

Female Reproductive System
The paired, rounded ovaries are situated directly behind the brain. The oviducts arise from the postero-ventral surface of the ovaries and follow the course of the ventral nerve cords. Posterior to the male copulatory apparatus the oviducts turn medially and unite to a common oviduct which communicates with the diverticulum of the bursal canal.
The vitellaria are well developed, occupying the entire space between dorsal and ventral body surface and extending from anterior to the ovaries into the hind end of the body.
The large, rounded copulatory bursa is lined with a tall, vacuolated epithelium. From the antero-dorsal surface of the bursa arises the bursal canal which, apart from a short horizontally oriented proximal part, has an almost vertical disposition. From the short, horizontally oriented part of the bursal canal arises the vertically directed diverticulum. At its proximal end the diverticulum receives the opening of the common oviduct, while it also receives the openings of shell glands. Bursal canal and diverticulum are lined with a ciliated epithelium which is underlain with several layers of circular and longitudinal muscles. The copulatory bursa is surrounded by a very thin layer of interwoven muscle fibres.

The eyes are small. Each pigment cup contains three retinal cells; there is no lens to the eyes.

Life Cycle: Wilhelmi (1909) observed cocoon laying at the end of July. The cocoons are brown and devoid of a pedicel. Their diameter varies from 0.75-1 mm (Wilhelmi 1909, Den Hartog 1968).

P. plebeius occurs in brackish water springs or in otherwise brackish habitats.

Type locality: Cephalonia, Argostoli. The species has been reported from Cephalonia, Greece (Schmidt 1861, Wilhelmi 1909), and has been found on Crete (see Material Examined) and in the Black Sea (Material Examined). In England the species has been reported from near Plymouth (Den Hartog 1968), and it has been collected also from Lulworth Cove in Dorset (Material Examined).

Material Examined
Z.M.A.: V.Pl.605.1, Greece, Crete, Agios Nikolaos, 23.09.1972, sagittal sections on 1 slide; V.Pl.605.2, horizontal sections on 1 slide; V.Pl.606.1, Greece, Corfu, Limni Antinioti, 10.06.1977, sagittal sections on 1 slide; V.Pl.606.2, sagittal sections on 2 slides; V.Pl.607.1, Greece, Crete, Agios Nikolaos, 9.05.1979, sagittal sections on 2 slides; ZMA V.Pl.607.2, horizontal sections on 1 slide; V.Pl.607.3, sagittal sections on 2 slides; V.Pl.608.1, Greece, Crete, Agios Nikolaos, 16.12.1981, horizontal sections on 1 slide; V.Pl.608.2, sagittal sections on 1 slide; V.Pl.608.3, sagittal sections on 2 slides; V.Pl.609.1, Greece, Crete, Georgioupolis, 22.02.1982, sagittal sections on 2 slides; V.Pl.718, Lulworth Cove, Dorset, England, 16.09.1984, sagittal sections on 3 slides.
Z.M.B: 5418, Black Sea, leg. Böhmig, whole mount on 1 slide; 8393a+b, ibid., sagittal sections on 2 slides.

Type Material
No type specimen available.

Procerodes plebeius