Minute to large; white, yellowish, greenish, brownish, bluish, rarely red. Head minute to large, mostly about 1.5 times as long as wide. Eye spots usually single, or if present, 2nd (dorsal) eye-spot always posterior to ventral one.
Normally with frontoclypeal apotome, occasionally with frontoclypealabral apotome and single labral sclerite, sometimes with frontal apotome, clypeus and several labral sclerites.
Strongly reduced to longer than head; 4-7 segmented; normally each segment consecutively smaller or segment 3 smaller than 4th; when 6 or 7 segmented ultimate segment usually vestigial; segment 3 never annulate. Blade shorter to longer than flagellum. Lauterborn organs absent to well developed, normally at apex of segment 2, occasionally alternate on segment 2.
S seta simple, bifid, branched, serrate, palmate or plumose; SII usually single, occasionally bifid, branched or plumose; SIII simple, rarely bifid; SIV normal. Labral lamellae present or absent; single or paired, simple, serrate or apically plumose. Chaetae absent to numerous, normally 6-9 simple or branched. Spinulae 0-6. Pecten epipharyngis of 3 usually free and subequal spines, occasionally fused at base; normally smooth, scale-like and sclerotised, occasionally weakly sclerotised and serrate. With 6-9 pairs of chaetulae laterales, usually all simple, often some or all serrate or branched; chaetulae basales usually 2 terminally notched pairs, sometimes simple or only 1 pair present. Premandible usually present, with 1-6 teeth; with or without brush.
With single apical tooth and 2-6 inner teeth, normally 3 inner teeth and darkened apical part of mola present. Pecten mandibularis absent. Seta subdentalis nearly always present. Seta interna usually present, with 4-8 often serrate or plumose branches.
Very variable, with 3-29 teeth, usually 8-12. Ventromental plate vestigial to relatively large, normally not extending beyond outer mental tooth, with or without beard.
Palp with about 9 tiny sensillae and 2 setae; sensillae distributed into 2 groups with bisensillum lying between the two. Galea with 6 sensillae; with or without pecten galearis. Laciniae low, usually with overlapping row of about 8 chaetae. Appendix seta usually present, but highly variable in form. Cardinal beard present or absent.
Parapods usually well developed, separated and with apical claws; anterior parapods occasionally fused; posterior parapods sometimes fused, reduced or absent. Procercus normally present, usually higher than wide, with 2 lateral setae and 3-7 anal setae, sometimes reduced, normally present. Anal tubules usually 2 pairs, sometimes reduced or absent (marine species), when present usually shorter than posterior parapods. Body setae usually not conspicuous, but in several genera with longer, single setae, or brushes of setae.