(FIGS 5B, 8A, 9B, 10B; TABLE 1)
Alternative species name: trigonostomum-armatum
Kylosphaera armata Jensen, 1878: 7, 12, 14, 17, 45– 47, t. 3, figs 14–22.
Hyporhynchus armatus Graff, 1882: 337; Gamble, 1893: 466–467; Attems, 1897: 228, t. 2, fig. 26.
Trigonostomum armatum Gamble, 1900: 813; Southern, 1912: 3, 9; Graff, 1913: 305–307, fig. 265; Southern, 1915: 34; Meixner, 1924b: 89, 94, 96, 99–100, 102; 1925: 256; Steinböck, 1931: 13, 23; Southern, 1936: 45, 57; Steinböck, 1938: 13, 22; Ax, 1952: 90– 91, fig. 1; Westblad, 1954: 9.
Known distribution: Norway (Jensen, 1878; Graff, 1882; Westblad, 1954), English Channel (Gamble, 1893), North Sea (Attems, 1897; Meixner, 1924b, 1925), Ireland (Gamble, 1893, 1900; Southern, 1912, 1915, 1936), Faeroe Islands (Steinböck, 1931), Iceland (Steinböck, 1938), Baltic Sea (Ax, 1952).
New localities: Norway, Bergen, Karlsög, on algae, 9 July 1953, Westblad (coll. SMNH). Norway, Trondheim, Munkholmen, in sand, 45–50 m deep, 22 July 1955. Westblad (coll. SMNH). Norway, Leröy-Buröy, in fine-grained sand, 5–7 m deep, 1 August 1968, Karling (coll. SMNH; type locality). United Kingdom, Plymouth, Salcombe, Saltstone, on algae, 11 July 1949, Westblad (coll. SMNH). Sweden, Gullmaren, Gåsövik, sheltered bay on brown algae, 19 August 2001. France, Wimereux, Langue du Chien, on algae, 20 October 1999. Curaçao (Dutch Antilles), Playa Canoa, on green algae, 10 December 1998. Curaçao (Dutch Antilles), Dam di Cabicuchi, ‘Spaanse water’, on Turbinaria -like algae, 14 and 30 December 1998 and 5 January 1999. Australia, New South Wales, Arrawarra, south of the marine station, on Sargassum sp., 1 November 1997. New Caledonia, Nouméa, Nouville, on algae in a lagoon, 10 August 2003. South Georgia, Cumberland Bay, May Creek, on seaweed, 9 May 1902 (coll. SMNH).
Material examined: Several individuals studied alive. Neotype (SMNH, no. 46427) from Norway. Whole mounts from France (1), Australia (1), New Caledonia (1), Curaçao (10) and South Georgia (SMNH, no. 46448). Serially sectioned specimens from Sweden (3), Curaçao (1), Norway (SMNH, no. 46425–6) and Plymouth (SMNH, nos. 46419–20).
Diagnosis: Trigonostomum species with coiled copulatory organ, with one whole spire. Stylet 158–434 Mm long, enveloped by the mantle over its entire length. Mantle distally split into two spiny plates with terminal hook. Bursal appendage 50–149 Mm long, with two tubules, proximally curved over 270∞ and with straight distal part.