(Bequaert and Goetghebuer, 1913)
Body
Small to medium sized larvae up to 5 mm long. The anterior and posterior parapods are separate and bear rows of claws. The procercus and anal tubules are absent Thalassosm anal segments (Wiederholm, 1983).
Head
Antenna
The antenna is short with five segments. The blade is subequal to the flagellum. Lauterborn organs are small Thalassosm antenna.
Mentum
The mentum has one median tooth and 4 pairs of lateral teeth. The ventromental plates are weak. The beard is absent Thalassosm mentum.
Labrum
The pecten epipharyngis consists of three short scales, SI is serrate, the labral lamellae are broad spines Thalassosm labrum and the premandible has one apical tooth and two inner teeth (Wiederholm, 1983).
Mandible
The mandible has an apical tooth and 4 inner teeth. The seta interna has simpel and serrate branches Thalassosm mandible (Wiederholm, 1983).
Differential characteristics
The combination of procercus and anal tubules absent Thalassosm anal segments, 4 pairs of lateral teeth on mentum Thalassosm mentum, the presence of labral lamellae and the serrate SI Thalassosm labrum are distinctive for Thalassosmittia.