Possess several distinct organs, organ systems and true muscles. Although more advanced than cnidarians, two things point to the early evolution of platyhelminths in bilateria history. A gastrovascular cavity is present. They have an acoelomate body plan. Classes Trematoda and Monogenea.
Planarians are familiar and common freshwater forms.
Carnivorous, feed on small animals and carrion. Lack specialized organs for gas exchange or circulation. Highly ramified gastrovascular cavity distributes food throughout the body. Flame cell excretory apparatus present which functions primarily to maintain osmotic balance of the animal. Move by using cilia on the ventral dermis to glide along a film of mucus.