The following species are recognized in the genus Haplopharynx:
Subterminal mouth leads to the simple pharynx, which subsequently continues as an intestine with terminal anal pore. A short retractable proboscis is present anteriorly to the pharynx. Variable number of glands producing rhabdites open at the surface of proboscis. Nervous system built similarly as in Macrostomida. Haplopharynx are hermaphroditic, with separate male and female gonopores. Male copulatory organ is equipped with hard, sclerotised stylet and set of needles.
Copulation lead to the internal fertilization. Egg is entolecithal and show spiral cleavage pattern.