The simplest animals that are bilaterally symmetrical and triploblastic are the Platyhelminthes, the flatworms. Flatworm species are divided into three classes: Turbellaria, Trematoda and Cestoda. All flatworms, according to Cool Galapagos, are hermaphroditic, soft-bodied worms with rudimentary nervous systems, no respiratory or circulatory systems and simple digestive tracts with single openings.