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Marcus and Marcus, 1959

Overview
Diagnosis: Procerodes dahli Marcus and Marcus, 1959. With respect to its external appearance, P. dahli can be distinguished from other marine triclads, excepting P. sameha , by its densely pigmented body surface and by the unpigmented auricles. Concerning anatomical features, P. dahli is characterized by the spherical dilatation of the diverticulum, and by an intrabulbar common vas deferens.
Habitus: Preserved specimens vary in length from 1.5 mm to 3.0 mm and in width from 0.8-1.75 mm. The posterior end of the body is broadly rounded, the body margins running more or less parallel. The front end is provided with a pair of distinct, unpigmented auricles between which runs the slightly convex anterior body margin.
The dorsal body surface is densely black-brown pigmented, the ventral surface shows a less dense pigmentation. According to Marcus and Marcus (1959), concentrations of pigment may occur in the form of a dark patch along the anterior body margin, and as two dark lines running backwards from the eyes on either side of the pharynx. Two thin white lines may be seen running backwards from behind the eyes (the "paramedian stripes" of Marcus and Marcus). Although faintly visible, these lines extend into the hind end of the body and, contrary to the opinion of Marcus and Marcus (1959), do not represent unpigmented stripes but correspond with the dorsal nerve cords. The eyes are set wide apart in two unpigmented areas which are confluent with the unpigmented auricles.

Alimentary System
In preserved specimens the pharynx is between one-third and one-half of the body length. The size of the outer circular muscle layer of the pharynx is variable. It may be slightly less than half as thick as the inner circular muscle layer or almost as thick as the last-mentioned layer; sometimes the outer layer has even a larger diameter than the inner layer. The mouth opening lies at the hind end of the pharyngeal cavity.
The anterior ramus of the intestine extends anterior to the eyes, giving off one pair of preocellar diverticula. Each posterior gut trunk gives off about 12 lateral diverticula; the posterior branches do not meet in the hind end of the body.

Male Reproductive System
The testes are situated dorsally, but their ventral portion may extend well beyond the midline of the body. There are about 12-16 follicles on either side of the body, being arranged between the tips of the intestinal diverticula and extending from directly behind the ovaries into the hind end of the body.
Posterior to the pharyngeal cavity the vasa deferentia enlarge to form false seminal vesicles which diminish considerably in diameter where they separately penetrate the dorsal surface of the penis bulb. Immediately after having penetrated the bulb, the vasa deferentia unite to form a short common vas deferens which is lined with small, cuboidal and nucleate cells. This common vas deferens opens into a wide ejaculatory duct which is lined with relatively large, vacuolated and also nucleate cells. The proximal section of the ejaculatory duct may be widened to a seminal vesicle. The ejaculatory duct tapers gradually to open at the tip of the penis papilla and receives the secretion of penis glands over its entire length. Common vas deferens and ejaculatory duct are surrounded by a layer of circular muscle fibres.
In the contracted state, the penis is short and blunt, but in a more erectile state it is conical and pointed. The penis papilla is provided with a nucleate lining and a thin, subepithelial layer of circular muscles and a layer of longitudinal muscles. The penis bulb is moderately muscularized. The male atrium is lined with tall, nucleate cells and communicates with the gonopore through a marked constriction.

Female Reproductive System
The paired, global-shaped ovaries are small and situated directly behind the brain. The oviducts arise from the ventro-lateral wall of the ovaries and follow their course backwards just dorsally to the ventral nerve cords. The vitellaria are situated medially to the testes and occupy the entire space between dorsal and ventral body surface; the follicles extend from anterior to the ovaries into the hind end of the body. Behind the gonopore the oviducts curve medially and unite to form a short common oviduct, which meets the expanded, more or less spherical distal portion of the diverticulum of the bursal canal. This diverticulum is penetrated by the openings of shell glands.
The bursal canal has an oblique disposition and gives rise, close to the bursa, to the vertically oriented diverticulum. The bursal canal is surrounded by a subepithelial layer of circular muscles and a layer of longitudinal muscles. Common oviduct and diverticulum are surrounded by a layer of circular muscles, the proximal section of the diverticulum being also provided with a layer of longitudinal muscle fibres. The copulatory bursa may be either a large sac-shaped structure, extending from dorsal to ventral body surface, or a small and rounded body. It is lined with tall and vacuolated cells and may contain sperm.

Eyes
The eye cups contain three retinal cells and are devoid of a lens.

Ecology
The species has been collected from under stones in the eulittoral zone (Marcus and Marcus 1959).

Distribution
Type locality: Azores, Sao Miguel, Ribeira das Três Voltas, near Ribeira Cha. The species has been reported from Sao Miguel in the Azores (Marcus and Marcus 1959) and has been found on Madeira, Mallorca and in Brazil (Material Examined).

Material Examined
Private collection I .R. Ball: N57.1, Madeira, Faial, sagittal sections on 1 slide; N57.2, sagittal sections on 1 slide; 4 preserved specimens from the same locality.
Z.M.A.: V.Pl.650.1, Mallorca, Cale Estallench, 10.05.1983, sagittal sections on 3 slides; V.Pl.775-1, Haicabe, Brazil, 09.1925, sagittal sections on 2 slides; V.Pl.775-2, horizontal sections on 1 slide; V.Pl.775-3, transverse sections on 2 slides.
M.N.H.N.: V77-A0726-6, 09.1925, Haicabe, Brazil, sagittal sections on 2 slides; A0726-2, sagittal sections on 2 slides; A0726-3, transverse sections on 3 slides; A0726-5, horizontal sections on 1 slide; A0726-7, sagittal sections on 2 slides; A0726-8, horizontal sections on 1 slide; A0726-9, horizontal sections on 1 slide.

Type Material
The type series was deposited in the collections of the Zoological Institute of the Lund University, Sweden, but appears to be no longer available.

Procerodes dahli