(Edwards, 1929)
Body
Medium sized to large larvae up to 8.1 mm long. The anterior parapods are separate. The two posterior parapods bear rows of claws. Procercus and two pairs of anal tubules present (Saether, 1975; Wiederholm, 1983).
Head
The head length is 0.36-0.39 mm and postmentum not contrasting with the rest of the gula (Saether, 1975).
Antenna
The antenna consists of seven segments Heterotr marcidus antenna. Segment 3 much shorter than segment 4, The blade is shorter than the flagellum (Saether, 1975d). Lauterborn organs reduced or absent (Wiederholm, 1983).
Mentum
The mentum has two median teeth and 5 pairs of lateral teeth. The ventromental plates are large. The beard is absent Heterotr grims mentum.
Labrum
The pecten epipharyngis consists of three serrated spines, SI is plumose and the premandible has two indistinct teeth (Wiederholm, 1983).
Differential characteristics
H. grimshawi can be separated from the other Heterotrissocladius species by
Paired mendial mentum teeth, pale gula and antennal blade < the flagellum.