Diagnosis: Pentacoelum punctatum (Brandtner, 1935) can be distinguished from other bdellourids by the characteristic configuration of its intestinal branches, the small number of large testes, the common vas deferens which penetrates the penis bulb, and by the oviducts which open separately into the bursal canal.
Habitus: Preserved, mature specimens have a length of 0.475-0.9 mm and a width of 0.25-0.5 mm. According to Brandtner (1935) living specimens of P. punctatum may reach a length of 1.9-2.0 mm. Front and hind end are broadly rounded, whereas the latter may also have a somewhat truncated appearance. The relatively short pharynx is situated in the middle of the body.
The dorsal surface is densely pigmented in an irregularly reticulate pattern, giving the animals a mottled brown appearance; the eyes are set in pigment-free areas. The ventral body surface is unpigmented.
The pharynx measures between one-sixth and one-fourth of the body length. The inner circular muscle layer of the pharynx is considerably thicker than the outer circular muscle layer. The mouth opening lies at about the posterior third of the pharyngeal cavity.
The anterior ramus of the intestine extends anterior to the eyes, giving off a pair of short and unbranched, preocellar diverticula. From the anterior gut trunk arises only one large pair of post-ocellar diverticula, which curve forwards and extend anterior to the eyes, to a level corresponding to the distal tip of the anterior gut trunk. The two posterior gut trunks give rise to 5-6 lateral diverticula, and meet in the hind end of the body.
Male Reproductive System
The number of testes is small: 5-6 follicles on either side of the body and one follicle between the two posterior gut trunks, posterior to the copulatory apparatus. The last-mentioned testis follicle is a so-called inter-intestinal testis. The other follicles are situated between the intestinal diverticula. The anteriormost pair of testes lies just anterior to the ovaries, in the space which is left between the anterior gut trunk and its single pair of post-ocellar diverticula. The testicular follicles are large, and occupy the entire space between dorsal and ventral body surface.
The vasa deferentia unite behind the pharyngeal cavity to a common vas deferens, which penetrates the shallow penis bulb. The nucleate ejaculatory duct opens at the tip of the penis papilla. The conical penis papilla has a ventral or slightly oblique, ventro-caudal orientation. Penis glands, which lie outside the bulb, discharge their secretion into the ejaculatory duct.
The penis papilla is lined with a nucleate epithelium which is underlain with a subepithelial layer of circular muscles and a layer of longitudinal muscles. The penis bulb is only weakly muscularized.
Female Reproductive System
In the specimens examined the vitellaria were well developed, occupying the entire space between dorsal and ventral body surface. The vitellarian follicles extend from anterior to the ovaries and the anteriormost pair of testes into the hind end of the body.
The large, and rather flattened, elongate ovaries reach from dorsal to ventral body surface and are situated laterally to the ventral nerve cords; they lie at about half-way the distance between the brain and the root of the pharynx, but always anterior to the post-ocellar pair of diverticula that arises from the anterior gut trunk. Usually, a major part of the ovaries is situated underneath the anteriormost pair of testes.
The oviducts arise from the ventro-lateral surface of the ovaries and run laterally to the ventral nerve cords. In the hind end of the body the oviducts open separately into the bursal canal.
The last-mentioned duct is lined with a cuboidal, nucleate, and ciliated epithelium, and receives the secretion of shell glands ectally to the openings of the oviducts.
In some specimens the bursal canal seemed to end blindly, but in several animals the duct communicated with a relatively small, thin-walled expansion. According to Brandtner (1935) the latter structure constitutes the copulatory bursa. The alleged bursa is devoid of a resorptive, vacuolated epithelium. Brandtner suggested that presence or absence of the copulatory bursa is influenced by ventral expansion of the fully developed inter-intestinal testis. From the specimens examined it was indeed obvious that in case of a fully developed testis not much space is left for a bursa.
A rounded lateral bursa is situated on either side of the body, at about the level of the hind end of the pharyngeal cavity. The lateral bursae may occupy most of the space between ventral and dorsal body surface. The bursae are situated in the lateral portions of the body and are lined with a nucleate, tall and highly vacuolated epithelium, the large vacuoles generally containing sperm. At a lateral position the bursae open through a ventral pore to the exterior. Generally, this pore is rather minute and not always visible in sections. The antero-ventral wall of each bursa communicates with the oviduct via a very short and narrow passage, which is lined with a number of small, nucleate cells.
The pigment cups contain two retinal cells; an eye lens is absent.
Life Cycle: According to Brandtner (1935) the triclad disappeared from the Ryck about the end of September, whereas it was present again during spring and also in summer when the cocoons were deposited.
In Louisiana, however, P. punctatum is present throughout the year. The specimens from Louisiana appeared to be sexually mature throughout the year, although cocoon-laying has only been observed in April.
According to Brandtner (1935) P. punctatum would be clearly protandrous, but neither the type material, nor the newly collected specimens provided any indication of protandry.
Brandtner (1935) obtained his specimens from algal growth (Enteromorpha compressa) on timber planks. The various sample localities were subject to salinity fluctuations and, according to Brandtner, the habitat of P. punctatum is oligo-mesohaline brackish water.
The animals from Louisiana were also collected from a low salinity habitat, with salinity ranging between 0.1 to 6.2 ppt.
Type locality: The river Ryck near Greifswald, D.D.R. (54°06'N 13°24'E). P. punctatum is known from its type locality in northern Germany and from Lake Pontchartrain, Louisiana, U.S.A. There is an unverified record of P. punctatum from near Harlingen, The Netherlands, where the species was collected from a brackish inland water (A. S. Tulp in litt.).
Material Examined and Type Material
U.S.N.M.: series of serial sections and whole mounts (USNM 102143-102167) of specimens collected in 1975 and 1976 from Lake Pontchartrain, Louisiana, U. S. A.
TYPE: Z.M.G.: Acc. Cat. II nr. 26474 (1), whole mount on 1 slide; nr. 26474 (2), sagittal sections of one specimen on 1 slide; nr. 26474 (3), sagittal sections of one specimen on 1 slide; nr. 26474 (4), whole mount on 1 slide.