An order of marine platyhelminthes of the class Turbellaria, generally regarded as primitive, and lacking protonephridia, oviducts, yolk glands, a permanent digestive cavity, and strictly delimited gonads. The nervous system is epidermal in the most primitive representatives and resembles that of cnidarians, but it is generally submuscular and consists of three to six pairs of longitudinal strands without anterior concentrations recognizable as a brain. The anterior or median ventral mouth opens into a simple pharynx which lacks elaborate musculature and leads directly to the syncytial vacuolated mass of endoderm forming the spongy intestinal tissue and lacking any true lumen.