Prostomium with no more than 2 pairs of antennae. The prostomium of some species is deeply imbedded within the first segment but in others projects freely beneath it. A pair of lateral palps are present (may be fused) The first segment has 2 pairs of tentacular cirri. The proboscis is eversible and carries 2 pairs of jaws. Most body segments uniramous with bottle-shaped cirri. Setae compound and/or simple.