These marine worms are cylindrical, somewhat flattened, and can be ribbon-like.

Some species within the genera Hermundura and Litocorsa are known to burrow, having reduced heads and parapodia.

Pilargis berkeleyae will live in the tubes of Chaetopteridae, and Ancistrosyllis commensalis will live in Capitellidae burrows.

Pilargid worms are almost all exclusively predators, classified as carnivore omnivores.

They are similar in appearance to Hesionidae, with a peristomium often with two pairs of tentacular cirri (with 1 pair or 0 in some species), reduced or absent notopodia, and a lack of pharyngeal jaws.