Cilia are the structures on the ventral side of the planaria that propel it forward.
Planaria is a type of flatworm that belongs to the class Turbellaria. Some species of planaria are terrestrial, while some are aquatic. Planarians have a soft and ciliated body. Cilia are hair like organelles situated on the ventral dermis of the planarians. Cilia function for locomotion by propelling the planarians forward, and allowing them to move smoothly on a layer of mucus.