Image below courtesy of Ohio State University

Class Turbellaria are the nonparasitic, free-living flatworms. These worms have large eyespots and flaps on each side of the head to detect chemicals. These worms live in water, usually on the underside of rocks. They have a highly branched digestive tract, but the one opening, or mouth, is on the ventral surface, rather than in the anterior end. The tube extending from the mouth is used to suck in materials as the worm uses cilia to crawl on the floor of the water.

This class belongs in the phylum Platyhelminthes, consisting of all flatworms. They are more evolved than the Cnidaria because they have bilateral symmetry, with a distinct anterior, posterior, dorsal, and ventral end, with a defined head. However, flatworms lack a body cavity, a fluid filled region between the epidermis and the digestive tract. Flatworms also lack a complete digestive tract, but instead only have one opening for substances to both enter and leave the body. This is because the gastrula opening in the development of a flatworm never fully evolves into a hole in the anterior and posterior end. The class Turbellaria is one of four classes in this phylum.

Information provided by: http://www.sidwell.edu