Stenostomum leucops O. Schmidt 1848

Stenostomum leucops (Dugès, 1828) O. Schmidt, 1848 (Fig. 2D) Fasciola composita Schrank, 1776 Derostoma leucops Dugès, 1828 Stylacium isabellinum Corda, 1838 Microstomum leucops Ørsted, 1844 Stenostomum torneense Schmidt, 1852 S. neoborecense Girard, 1893 S. tenuicaudatum Nuttycombe & Waters, 1938 S. sthenum Borkott, 1970 S. platycaudatum Borkott, 1970 S. plebejum Borkott, 1970 Description: Length variable. Two-zooid chain 1– 1.75 mm. Cylindrical body. Rounded or pointed anterior end. Posterior end of the body with intestinelacking region forming a tail. Ciliated epithelium with small rhabdites. Epidermic cilia with rigid sensitive cilia at anterior and occasionally at the posterior end. Colour in life yellowish white. Anterior brain lobes deeply dentate. A pair of type 1 light-refracting bodies associated with the posterior brain lobes. Oral pore shape variable, rounded, triangular or elongated. Simple muscular pharynx. Two types of pharyngeal glands: type a, abundant at the distal end, opening onto the posterior region of the pharynx; and type b, arranged in bunches lateral to the pharynx, opening onto the anterior end. Excretophores present. Nephridiopore subterminal, in the intestine-lacking region. Distribution: Cosmopolitan. In South America, margins of the Surinam River, near Paramaribo, Surinam (Van der Land, 1970); Paraná River, Argentina (Noreña-Janssen, 1995). Discussion: Nuttycombe & Waters (1938) consider the validity of this species questionable. This criterion has been followed by Marcus (1945b). These authors consider that the descriptions of this species are so broad and ambiguous as to make its recognition difficult. By contrast, Borkott (1970) in an exhaustive study of Stenostomum leucops subdivides this species into three new species: S. sthenum, S. platycaudatum and S. plebejum. The results of this research did not confirm the existence of these species in South America (Noreña-Janssen, 1995). Young & Kolasa (1974b) identified specimens from Africa as S. leucops leucops, on the basis of Luther’s (1960) description, ascribing the differences described by Borkott (1970) to those originated in the cultures or not visible in animals captured in natural environments. In the 1990s, Reuter (Reuter, 1988, 1991, 1994; Reuter & Palmberg, 1990; Reuter & Eriksson, 1991; Reuter & Kuusisto, 1992; Reuter, Joffe & Palmberg, 1993; Reuter et al., 1995) as well as Wikgren & Thorndyke (1990), Lindroos & Reuter (1991), Palmberg & Reuter (1992) and Grahn et al. (1995) carried out ultrastructural studies of stenostomids, mainly S. leucops. S. sthenum was not mentioned in any of them.Published as part of Noreña, Carolina, Damborenea, Cristina & Brusa, Francisco, 2005, A taxonomic revision of South American species of the genus Stenostomum O. Schmidt (Platyhelminthes: Catenulida) based on morphological characters, pp. 37-58 in Zoological Journal of the Linnean Society 144 (1) on pages 46-47, DOI: 10.1111/j.1096-3642.2005.00157.x, http://zenodo.org/record/463468