Many are roughly the size of ciliate protozoans, with which the small ones are easily confused. The body is cylindrical in the small species, dorsoventrally flattened in the large ones. The epidermis is ciliated. There is a mouth located somewhere on the ventral surface that opens into a blind gut, or gastrovascular cavity. There is no anus.
Excretion is via protonephridia.
Food is delivered to the tissues by the gastrovascular cavity. Oxygen diffuses across the body surface to the tissues, none of which are more than 1 mm from it. The nervous system consists of a brain and longitudinal nerve cords. There may be light-sensitive ocelli in various positions on the body. Turbellarians are hermaphrodite with internal fertilization.