The mostly free-living, primarily carnivorous, flatworms of class Turbellaria are characterized by a soft epidermis that is ciliated, at least on the ventral surface. The movement of the cilia propels the smaller forms. Larger species glide along by muscular waves, usually over mucous beds secreted by special cells.
Turbellarians are generally divided into five groups based primarily on differences in the form of the digestive cavity, a structure that is readily observable through the transparent body wall. The most primitive turbellarians, the acoels, have no digestive cavity. The ventral mouth, and sometimes a simple pharynx, lead to an inner mass of nutritive cells. Most species measure less than 1/8 in. (3 mm) in length.
The rhabdocoels have straight, unbranched digestive cavities. Some authorities believe that the rhabdocoels gave rise to both the trematodes and cestodes because several rhabdocoel species exhibit commensal relationships, which presumably could have given rise to parasitism. The allocoels were formerly classified together with the rhabdocoels; the gut can be either saclike or branched.
The triclads, also known as planarians, are relatively large flatworms named for their three-branched gut. Most species range from 1/8 in. (3 mm) to about 1 in. (2.5 cm) in length. Planarians have more sense organs and a more complex brain than the other turbellarians. The freshwater species Dugesia tigrina has primitive eyes and tactile lobes, or auricles, on the sides of the head. The muscular pharynx can be extruded for food capture. Dugesia and many other planarians can regenerate entirely new individuals from small pieces cut from the body.
The group of turbellarians known as polyclads tend to be larger (1–2 in./2.5–5 cm) and more oval-shaped than the triclads. Their bodies are extremely flat and leaflike, and the gut is subdivided into numerous branches. Many are brightly colored and some have ruffled edges. Some species have numerous eyes scattered over the front end of the body.