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Westblad, 1935

Overview
Diagnosis: Pentacoelum fucoideum Westblad, 1935 is characterized by (1) the poorly pigmented eye cups, (2) the common oviduct opening dorsally into the atrium, (3) one pair of large testes, and (4) the narrow and long connecting ducts between the oviducts and the lateral bursae.
Habitus: Live animals up to 1.2 mm long and 0.5 mm wide; body broadly oval-shaped, with broadly rounded hind end and rounded or obtusely pointed front end in living specimens (Westblad 1935). The body is devoid of any pigmentation and eyes are usually not visible in living animals, preserved specimens or whole mounts.

Alimentary System
The short pharynx measures about one-sixth of the body length and is situated in the posterior half of the body. The outer circular muscle layer of the pharynx is well developed and made up of strong muscle fibres. The inner circular muscle layer has about the same thickness as the outer layer, but the individual muscle fibres are much thinner. The inner longitudinal muscle layer is very thin and is also made up of very weakly developed fibres, in contrast to those of the well developed outer longitudinal muscle layer. The mouth opening is at the hind end of the pharyngeal cavity.
The anterior ramus of the intestine gives rise to about two pairs of preocellar diverticula and one pair of large postocellar lateral diverticula. The latter extend forward to beyond the level of the eyes and give rise to branched, secondary lateral diverticula and also to a short and undivided medially directed diverticulum between the ovaries and the testes. Each of the caudally running gut trunks gives off 6-7 branched lateral diverticula and several median diverticula, while the rami meet in the hind end of the body.

Male Reproductive System
The paired testes lie between the ovaries and the root of the pharynx, immediately anterior to the pair of postocellar gut diverticula. The follicles are very large and completely occupy the space between dorsal and ventral body surface. The vasa deferentia expand to form well developed false seminal vesicles on either side of the pharyngeal cavity, but the ducts narrow considerably before separately penetrating the dorso-lateral surface of the weakly developed penis bulb. Immediately after entering the bulb the ducts open, still separately, into a voluminous seminal vesicle, occupying the major part of the penis. The distal part of this vesicle tapers to form the ejaculatory duct, which opens at the tip of the cone-shaped penis papilla. The ejaculatory duct receives the secretion of penis glands, the glandular elements being situated outside the penis bulb. The penis papilla is lined with a flat, nucleate epithelium and is provided with a relatively well developed, subepithelial, layer of circular muscles and a very thin layer of longitudinal muscle fibres.

Female Reproductive System
The vitellaria extend from anterior to the ovaries into the hind end of the body, occupying the entire space between dorsal and ventral body surface.
The ovaries are situated directly behind the brain, medially to the ventral nerve cords. The gonads have the shape of a U, turned upside down. The germ center is located at the ventral end of the posterior leg of the U, whereas fully mature oocytes fill the anterior leg. The ventral surface of the anterior leg of the ovaries meets a rounded expansion at the anteriormost section of the oviducts. These expansions or tubae usually contain sperm. The oviducts run laterally to the ventral nerve cords, at a considerable distance from the latter. At the level of the penis papilla the oviducts start to curve upwards, and dorsally to the male atrium they unite to form a common oviduct that penetrates the dorsal wall of the atrium. This common oviduct receives the openings of shell glands. While they start to curve dorsally in the hind end of the body, the oviducts meet a long and narrow duct that communicates with the lateral bursae. At the point of communication with the bursae these ducts are very narrow and provided with a small sphincter; the ducts are lined with nucleate cells.
A lateral bursa is located on either side of the most posterior part of the male atrium, i.e. the bursae are situated in the very hind end of the body. The voluminous and sac-shaped bursae are lined with a vacuolated epithelium and open ventrally to the exterior via a broad stalk. The stalks are surrounded by a subepithelial layer of circular muscles and a thin layer of longitudinal muscles. The actual openings to the exterior are very minute and occur latero-caudally to the gonopore, and, consequently, far behind the neural arch formed by the ventral nerve cords that lies at the level of the penis bulb.

Eyes
The eye cups only contain very small and faintly coloured pigment granules. Each eye cup houses two retinal cells, which do not show the characteristic differentiation into rod border, middle region, and stalk; a lens is absent.

Reproduction
Life Cycle: Schütz (1966) studied the life cycle of P. fucoideum in the Nord-Ostsee Kanal. The first juvenile specimens are observed in May, in the B-mesohaline zone (10-7 o/oo S) at depths of 3-4 m and temperatures between 12-13 degrees C. Already after 3-4 weeks, i.e. beginning to mid-July and at temperatures between 14-15 degrees C, these young planarians are mature and able to produce a new generation. This second generation hatches mid-July at temperatures between 18 and 19 degrees C and starts producing cocoons already at the end of August. Probably only few animals of the third, late summer, generation are able to produce cocoons because in the beginning of October, at temperatures of about 10 degrees C, few specimens are present.
In the upper layer of water, i.e. between depths at 0-1 m, P. fucoideum produces only two generations. The first young animals are observed in the beginning of June at a temperature of about 15oC. The cocoons laid by this first generation hatch at the end of July. Animals of the second generation deposit cocoons in the beginning of September, at temperatures between 15-17 degrees C. At the end of September/beginning of October, and at a temperature of about 10 degrees C, the animals disappear from the upper water layer.
It has been found that decreasing salinity has a negative effect on the growth of the young planarians: in waters of low salinity young animals start to grow at a later date than in waters with a higher salt concentration.

Ecology
P. fucoideum is a typical brackish water species and in that biotope it has been collected from (1) Bladder Wrack Fucus vesiculosus in the Baltic Sea, near Karlskrona and Stockholm (Westblad 1935) and at various localities in the Gulf of Finland (Luther 1961), (2) algal growth on poles in the Nord-Ostsee Kanal, northern Germay (Ax 1952, Schütz 1966), and in the Caspian Sea, near Lenkoran (Beklemischev 1954, Dyganova 1983).
Schütz (1966) made a detailed study of the ecology of the animals in the Nord-Ostsee Kanal. It appeared that P. fucoideum prefers to live in the tidal zone where the water level changes quickly and is dominated by red and brown algae; the species avoids the surf zone and reaches its highest population density in B-mesohaline water, whereas it does not occur in waters with a salinity below 5 . The animals are most abundant during the explosive growth of diatoms.

Distribution
Type locality: Knösö at the Säljö sound, near Karlskrona, Sweden. The species has been collected from (1) Bladder Wrack Fucus vesiculosus in the Baltic Sea, near Karlskrona and Stockholm (Westblad 1935) and at various localities in the Gulf of Finland (Luther 1961), (2) algal growth on poles in the Nord-Ostsee Kanal, northern Germay (Ax 1952, Schütz 1966), and in the Caspian Sea, near Lenkoran (Beklemischev 1954, Dyganova 1983).

Material Examined and Type Material
S.M.N.H.: Erstavik, 28.8.1939, sagittal sections of 3 specimens, horizontal sections of 3 specimens, transverse sections of 3 animals, each series of sections on 1 slide, and 1 whole mount; Utö, 30.8.1937, sagittal sections of 3 specimens, transverse sections of 3 specimens, each series of sections on 1 slide, 2 whole mounts on separate slides, and 2 whole mounts on 1 slide.
TYPE S.M.N.H.: Säljö" Sund, 27.8.1929, sagittal sections of 3 specimens, transverse sections of 2 specimens on 4 slides, transverse sections of 2 specimens on 2 separate slides, 2 whole mounts on 1 slide; Ut, August 1934, sagittal sections on 1 slide, transverse sections on 1 slide.

Pentacoelum fucoideum