Members of phylum Platyhelminthes have a dorso-ventrally flattened body, hence they are called flatworms. They are mostly endoparasites found in animals including human beings. Flatworms are bilaterally symmetrical, triploblastic and acoelomate, with an organ level of organisation. Parasitic forms possess hooks and suckers, and some absorb nutrients from the host directly through their body surface. Specialised cells called flame cells help in osmoregulation and excretion. Sexes are not separate, so the animals are hermaphrodite. Fertilisation is internal, and development occurs through many larval stages. Some members, like Planaria, possess a high capacity for regeneration. Examples include Taenia (tapeworm) and Fasciola (liver fluke). In the broad classification, Platyhelminthes are the first bilaterally symmetrical, triploblastic animals and are acoelomate, which means they have no body cavity between the body wall and the gut. The digestive system is incomplete, with a single opening, and there are no circulatory or respiratory systems. These points distinguish flatworms from roundworms, which are pseudocoelomate and have a complete alimentary canal. The parasitic adaptations, such as hooks, suckers and direct absorption of nutrients, are frequently asked in the examination.