Platyhelminthes

Platyhelminthes include flatworms. They  are bilaterally­ symmetrical. They are acoelomate i.e, lack a coelom .they ate triploblastic  ,that is, have three germ layers.most of species are parasitic.some are free living.they show cephalization of nervous system and  their nervous system consists of a head ganglion . Excretory system consists of specialized flame cells.  respiratory or circulatory systems are absent.exchange of gases occur  by diffudion through body surface.Non parasitic forms have  incomplete gut; even this is lack­ing in many parasitic forms.Movement in some  species occur  with help of body muscle. Others move along slime trails by the beating of epidermal cilia. Sense organs like ocelli (photoreceptors), chemoreceptors,statocysts ,rheoreceptors are present in some forms. Gut if present, is incomplete (without anus).tapeworms lack gut.The nervous system is ladder-like. It consists of the brain (head ganglion) and two main longitudinal nerve cords connected at intervals by transverse commissures.These animals are generally hermaphrodite but mostly cross fertilization occur. fertilization is internal. self fertilization occur in tapeworms.in some flat worms like Planaria asexual reproduction occur  by binary fission.Life cycle consists of one or more larval stages. In liver fluke (Fasciola hepatica)the larval stages are - miracidium, sporocyst, redia, cercaria and metacercaria . larval stages in tapeworm are onchosphere, hexacanth and cysticercus larvae. Flatworms show organ level  system of organisation.