This group probably evolved from a planuloid type of common ancestor along with the radiate animals. The members of the class Turbellaria are found in various habitats and are usually free living. While they have a head, the mouth is located on the ventral surface near the center of the body. They are carnivorous, feeding with a proboscis. The food is detected by chemoreceptors.
Planarians move by gliding over a slime track that is secreted by the marginal adhesive glands.
The epidermal cilia beat to move the animal along the slime track. The turbellaria have simple life cycles with direct development in most cases. The members of the class Trematoda are complex endoparasites that utilize two different hosts during their life cycle. Atypical life cycle would include the adult, shelled zygote, miracidium, sporocyst, redia, cercaria, and metacercaria. Some of the most important members of this class are the schistosomes and the liver flukes.