The phylum Platyhelminthes or Plathelminthes are relatively simple, bilaterian, unsegmented and invertebrate animals.
The Platyhelminthes has a vermiform (i.e., worm like shape) and are bilaterally symmetrical. This is also called triploblastic as it is composed of three fundamental layers of cells.
This is the first species to have a third layer of cell called mesoderm along with epidermis and gastrodermis. This mesoderm produces the muscular layers, excretory system and reproductive system. Platyhelminthes, the flatworms have only gut, no body cavity and no anus. The pharyngeal opening is playing the dual role as intake the food and expels the waste.
The gut is highly branched to transport food to all parts of the body. They can respire only through diffusion. The shape of the body is necessarily flattened because of the lack of cavity. They do not have any circulatory and respiratory organs.
Platyhelminthes are commonly categorized into parasitic and non-parasitic animals. Planarians are the non-parasitic animal. And Cestoda, Trematoda and Monogenea are the three entire parasitic groups.