Relatively large larvae, up to 9 mm long.
Antenna short and stubby, 5 segmented, with segments consecutively shorter. Ring organ in mid-basal segment. Blade shorter than flagellum; accessory blade and Lauterborn organs not visible. Style as long as segment 3.
SI bifid, with furcation apical to mid-point, remaining S setae simple. Chaetae and spinulae simple and reduced. Labral lamellae absent. Pecten epipharyngis of 3 simple, sclerotised, pointed, smooth scales; chaetulae laterales and basales all smooth. Premandible with 5 pointed, dark, apical teeth and 1 thumb-like, pale, more basal tooth; brush absent.
Mandible with apical tooth longer than combined width of 3 inner teeth. Seta subdentalis very weak. Seta interna with 7 smooth branches.
Mentum with 8 median, lighter teeth at approximately the same level and 6 pairs of lateral teeth, often barely distinguishable. Ventromental plate elongate but not very wide; beard absent.
Parapods well-developed, but short; a few claws of anterior parapods complex. Procercus low, with about 7 relatively short anal setae. Supra-anal setae about 1/2 as long as anal setae. Anal tubules apparently absent.
The last 2 larval instars of the only species, Baeoctenus bicolor, live within the gills of Anodonta mussels. Known only from Manitoba and New Brunswick, Canada.