(FIGS 1, 6A)
Alternative species name: trigonostomum-venenosum
Orcus venenosus Uljanin, 1870: 19, t. 2, fig. 5.
Hyporhynchus venenosus Graff, 1875: 419; 1882: 341; Pereyaslawzewa, 1893: 265, 266, t. 4, fig. 28; Attems, 1897: 227, t. 2, figs 24, 25.
Hyporcus venenosus Graff, 1905: 110, t. 3, figs 9–11; Southern, 1912: 3, 8–9; Graff, 1913: 299–301, fig. 260; Southern, 1915: 34; Meixner, 1924b: 91, 92; Southern, 1936: 45, 57; Meixner, 1926: 577.
Trigonostomum venenosum Meixner, 1924b: 89, 92, 94, 96, 99, 102; 1925: 256; Steinböck, 1931: 12, 23; 1938: 12–13, 22; Ax, 1959: 98, figs 78, 79; Mack-Fira & Cristea-Nastasesco, 1971: 225, 227, figs 5, 6; Mack-Fira, 1974: 249, 265, 273, 281–282, 284.
Known distribution: Iceland (Steinböck, 1938), Ireland (Southern, 1912, 1936), Faeroe Islands (Steinböck, 1931), North Sea (Attems, 1897; Meixner, 1924b, 1925), Mediterranean Sea (Graff, 1882; Meixner, 1926), Adriatic Sea (Meixner, 1926), Black Sea (Uljanin, 1870; Pereyaslawzewa, 1893; Graff, 1905; Ax, 1959; Mack-Fira & Cristea-Nastasesco, 1971; Mack-Fira, 1974).
New localities: Sweden, Gullmarsfjord, Gåsövik, among algae, August 1945, Westblad (coll. SMNH). Norway, Bergen, Bay south of Tyssöy, stones, gravel, algae and fine shell-sand, 5–8 m depth, 29 July 1968, Karling (coll. SMNH). Norway, Bergen, sound between Leröy and Buröy; sand and mud, 5 m depth; 1 August 1968, Karling (coll. SMNH). France, Corsica, Port de la station Stareso, on algae, 6– 4 m deep, 9 May 1982 and 12 March 1983, Martens; Bay of Calvi, 11 April 1984 and 26 March 1985. Italy, Sardinia, Porticello, about 0.2–1.5 m deep, on Vaucheria -like algae on rocks, 14 August 1994. France, Banyuls, Ile Gros, on green algae near the jetty behind the station, 22–23 June 2000. Kerguelen, Port Couvreux, bay at the right, green algae, 25 November 1992.
Material examined: Several specimens studied alive. Neotype (SMNH, no. 47499). Whole mounts from Sardinia (2), Corsica (4), Banyuls (3), Kerguelen (6), Norway (SMNH, nos. 47496–8) and Romania (SMNH, no. 47500–1). Two serially sectioned specimens (SMNH, nos. 47493–4) from Sweden.
Diagnosis: Trigonostomum species with copulatory organ 74–130 Mm long. Stylet 117–194 Mm long, proximally bent over 270∞. Mantle with one pointed plate, surrounds only the distal part of the stylet. Bursal appendage ± 54 Mm long, with a proximal ring and two distal tubes.
Remarks and additional data: The length of the stylet shows large variation between the populations of Banyuls (117–124 Mm; n = 2), Sardinia (148– 156 Mm; n = 2), Corsica (165–179 Mm; n = 2) and Kerguelen (157–194 Mm; n = 3). The stylet resembles that of some species of Messoplana (e.g. M. elegans Luther, 1948; M. helgolandica Ax, 1971; M. pacifica Karling, 1986; M. rugata Ehlers, 1974).