Unusually, the male gonopore opens on the dorsal surface of the animal, above the pharynx, while the female reproductive system lacks any of the usual ducts and related structures found in other flatworms.

[2] Members of the symbiotic genus Paracatenula lack a digestive tract, and instead harbor intracellular chemoautotrophic bacterial symbionts that are assumed to provide their nutrition.

Most of them live in freshwater, being usually very abundant in mires, ponds, streams and moist terrestrial habitats.

[6] Others, such as those of the genus Paracatenula use chemoautotrophic bacterial symbionts that live inside their cells as a food source.

All characters common to both clades, such as the internal fertilization and the simple gut with a single opening, are found in other groups as well.