Platyhelminthes have dorso-ventrally flattened body, hence are called flatworms. These are mostly endoparasites found in animals including human beings. Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with organlevel of organisation. Hooks and suckers are present in the parasitic forms. Some of them absorb nutrients from the host directly through their body surface. Specialized cells called flame cells help in osmoregulation and excretion. Sexes are notseparate. Fertilisation is internal and development is through many larval stages. Some members like Planaria possess high regeneration capacity.