The most recognizable organisms of flatworms are flatworms. This animal is one of a kind because it has a head. It differs from Cnidarians because it has three body layers or in other words it is triploblastic. It has an endoderm, a mesoderm and an ectoderm. This animal has four classes: Turbellaria, Monogenea, Trematoda and Cestoda. Returning to the main idea of ​​flatworms, the head directs the animal in its environment and controls sensory structures. It has an upper dorsal and a lower ventral part on the body structure. Some characteristics identified with flatworms are: bilateral symmetry, gas exchange by diffusion, organs for digestion, reproduction, excretion and cephalization. The digestive tract of free-living flatworms is branched. This animal uses diffusion to exchange oxygen into carbon dioxide across the body surface. Some interesting facts about flatworms are: They have no anus and are free-swimming animals. Another unique feature they have on the top of their head is the projections on both sides. These are called pinnae, which are used to find food chemicals in the water. This is considered an advantage for their survival. Their mesodermal layer develops into organs, and the main cavity of the mesoderm is called the coelomic cavity because it separates the body walls from the body movements.