(FIG. 6B)
Alternative species name: trigonostomummessoplanoides
Trigonostomum messoplanoides Artois et al., 2000: 104–105, fig. 1.
Known distribution: Weddell Sea, Antarctica (Artois et al., 2000).
Material examined: Holotype (LUC, no. 207).
Diagnosis: Trigonostomum species with stylet 225 Mm long, proximally bent over 270∞. Mantle with stylet 75 Mm long, flagelliform spine surrounding only the distal part of the stylet. Bursal appendage 22 Mm long, with a proximal ring and two distal tubes.
Remarks: The copulatory organ of T. messoplanoides resembles that of some species of Messoplana (Artois et al., 2000), for instance M. elegans (Luther, 1948) Den Hartog, 1966, M. helgolandica Ax, 1971, M. pacifica Karling, 1986 and M. rugata Ehlers, 1974. However, the presence of a ventral invagination clearly confirms that it is a species of Trigonostomum.