Made up of two Greek words – Platys =flat &helmins =worms 1. Body form: They have dorsoventrally flattened body so often called as flat worms. 2. Habit and Habitat: Most of them are parasite whereas few are free living as well (in fresh water, sea or damp places on the land) 3. Cellularity: Body consists of multiple cells or possesses organ-system grade of body organization. 4. Symmetry: Body is bilaterally symmetrical. 5. Coeloem: They are a coelomates with solid body. The space between body wall and internal organs is filled up with special mesodermal tissue, paranchyma. 6. Nutrition: They exhibit parasitic and holozoic modes of nutrition. Parasitic adaptation features: Parasitic forms have hooks, suckers and other devices for attachments with host. 7. Body layers: Body is triploblastic having ectoderm, endoderm and mesoderm. 8. Segmentation: They are soft bodied, unsegmented worms. Segmented if present, is not true. 9. Color: Majority of them are colorless, some derive color from the ingested food, while some free living are brown, grey, black or brilliantly colored. 10. Cephalization: They have a clearly differentiated head situated anteriorly and delicate posterior end. 11. Locomotion: Locomotory organelles are absent. 12. Skeleton: The exo and endoskeletons are absent and hard parts are formed of Sceleroproteins. 13. Digestion: Alimentary canal is incomplete. It is totally absent in parasitic form. 14. Respiration: Gaseous exchange occurs through the general body surface. Anaerobic respiration occurs in parasites. 15. Circulation: Blood circulatory organs are absent. 16. Excretion: Excretion takes place through peculiar fame cells or protonephridia connected with excretory ducts that lead to the exterior. 17. Nervous system: Nervous system is primitive and a ladder like. It consist of a pair of ganglia with longitudinal nerve cords connected by transverse nerves. 18. Sense Organs: Sense organs are usually absent r reduced in parasitic forms. Some of them have sense organs. 19. Reproduction: Most of them are monoecious (bisexual). Reproduction occurs sexually by gametic fusion and sexually by fission and regeneration. 20. Fertilization: Fertilization is internal, may be cross or self. 21. Development: Development may be direct or indirect. Usually indirect in endoparasites with a complicated lifestyle involving one or many larval stages and hosts.
Further classifies as
a) Turbellaria: Intestine is either absent or simple and sac like or branched. They are mostly free living forms found in fresh water streams and ponds, seawater or moist places. Distinct head is present at the anterior end. Body is unsegmented and leaf – like round or oval covered with a partly ciliated or cellular epidermis. Cuticle is absent. They have remarkable regeneration cellular epidermis. Cuticle is absent. They have remarkable regeneration power. They have simple life history. E.g. Planoria, etc. b) Trematoda: They are either ectoparasites or endoparasites. They are commonly called as fluke. Digestive tract is complete consisting of mouth, pharynx and intestine. Mouth is surrounded by oral suckers. Body is elongated and unsegmented. Presence of one or two suckers without or hooks or spines. They are hermaphrodite. Cross – fertilization occurs. The development many be direct or indirect. E.g. Fasciola hepatica c) Cestoda: They are exclusively endoparasites living in small intestine of vertebrates. Body is flattened or ribbon like, usually divided into few to many proylottids. They are hermaphrodite. Anterior end of body has a small head or scolex with suckers and hooks. Life cycle is complicated with hooked embryo and two or more hooks. E.g. Taeniasoliumetc.