Class Turbellaria is a paraphyletic subrank of the phylum Platyhelminthes which encompasses all free-living flatworms. Turbellaria possess the highest degree of cephalization of the platyhelminths to survive their non-parasitic behavior. This trend shows in their distinguished anterior and posterior sides, of which the anterior contains many important organs at the cephalic region.
The anterior contains a pair of eye spots, auricles, and ganglia clusters. The ganglia form a primitive brain for the Turbellaria which is connected to the rest of the body using a long pair of ventral longitudinal nerve cords. These longitudinal chords are connected to one another using several transverse nerve cords. Lateral nerve branches are also present at the junctures of the nerve chords.
The longitudinal nerve cords intersect with the gastrovascular cavity many times through the body: a large, blind-ended organ which possesses an anterior trunk, two posterior trunks, and many diverticula branching from the organ. The cavity has a single opening at the mouth of the pharynx, which is located on the ventral side of the body near its center, in the pharyngeal region.
Turbellaria possess a developed digestive system in spite of having an incomplete gut and a blind-ended cavity. The pharynx extends out from the pharyngeal region to probe and feed on its prey. Digestive enzymes in the pharynx loosen nutrients and muscle contracts to move food into the gastrovascular cavity. Columnar, vacuolated epithelial cells line the gastrodermis, phagocytosing smaller particles through extracellular digestion while enzymes break down larger nutrients outside of the cell.
Food waste is excreted via an opening at the tip of the pharynx, while liquid waste and interstitial fluids are excreted through the protonephridia. Excess fluids are drawn through currents made by the protonephridia that lead through the excretory canals and out through small excretory pores. Parenchyma composes the remainder of the body.
All members of the class Turbellaria are carnivores. Many are benthic flatworms who reside in marine, freshwater, and damp terrestrial environments. They have a variety of compositions with a ciliated ventral surface and many placements of their gut. All members are motile and possess locomotion for their entire life. All are monoecious and collectively can perform sexual reproduction, tending to cross-fertilize with other members of their species. Some can perform asexual reproduction by fission, fragmentation, or budding. Some of the most well known Turbellaria are Planarians such as those of the Planaria and Dugesia genera.