Platy helminthes are also known as flat worms. It was the Aristotle who first mentioned it. They were placed in the same group by Gegenbaur. Most of them are parasite and rest of them are free living in fresh water, land etc. They have bilateral symmetry and dorso ventral flattened body. They have the organ system of level of organization. They have soft body covering and incomplete digestive tract. It has flame cells for excretion and ammonia as their primary waste product. They have the higher power of regeneration and have one or more larval stages. They arise from three germ layers such as ectoderm, mesoderm and endoderm. They are known as triploblastic animals and they were the first organism of this type. They have no skeletal, respiratory and circulatory system. The space between the body wall and the organs is filled with the connective tissues and is known as parenchyma. It helps in transport of the food and maintains the shape of body. The flat worms do not possess coelom. This is known as acoelomates. Their nervous system is ladder like and has brain and longitudinal nerve cords. They have fully developed reproductive system. They have the internal mode of fertilization and the anatomy of the organism encourages cross fertilization. Transverse binary fission is the preferred method of asexual reproduction. It occurs in some flat worms.
The platy helminthes are better than the cnidarians as they are triploblastic, have organ system level of organization, with well developed nervous and excretory system and formation of copulatory organs.
