Phylum: Platyhelminthes
Phylum: Platyhelminthes
Phylum: Platyhelminthes
Made up of two Greek words ? Platys =flat &helmins =worms
- Body form:
They have dorsoventrally flattened body so often called as flat worms. - 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) - Cellularity:
Body consists of multiple cells or possesses organ-system grade of body organization. - Symmetry:
Body is bilaterally symmetrical. - Coeloem:
They are a coelomates with solid body. The space between body wall and internal organs is filled up with special mesodermal tissue, paranchyma. - Nutrition: They exhibit parasitic and holozoic modes of nutrition.
- Parasitic adaptation features: Parasitic forms have hooks, suckers and other devices for attachments with host.
- Body layers:
Body is triploblastic having ectoderm, endoderm and mesoderm. - Segmentation:
They are soft bodied, unsegmented worms. Segmented if present, is not true. - Color:
Majority of them are colorless, some derive color from the ingested food, while some free living are brown, grey, black or brilliantly colored. - Cephalization:
They have a clearly differentiated head situated anteriorly and delicate posterior end. - Locomotion:
Locomotory organelles are absent. - Skeleton:
The exo and endoskeletons are absent and hard parts are formed of Sceleroproteins. - Digestion:
Alimentary canal is incomplete. It is totally absent in parasitic form. - Respiration:
Gaseous exchange occurs through the general body surface. Anaerobic respiration occurs in parasites. - Circulation:
Blood circulatory organs are absent. - Excretion:
Excretion takes place through peculiar fame cells or protonephridia connected with excretory ducts that lead to the exterior. - 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. - Sense Organs:
Sense organs are usually absent r reduced in parasitic forms. Some of them have sense organs. - Reproduction:
Most of them are monoecious (bisexual). Reproduction occurs sexually by gametic fusion and sexually by fission and regeneration. - Fertilization:
Fertilization is internal, may be cross or self. - Development:
Development may be direct or indirect. Usually indirect in endoparasites with a complicated lifestyle involving one or many larval stages and hosts.
On the basis of digestive tract and habitat, Platyhelminthes are further classified 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.