General Characteristics of Platyhelminthes
- Platyhelminthes comprises the flatworms (Gr. platys = flat; helminth = worm).
- Flatworms are triploblastic (all worms are triploblastic), bilaterally symmetrical, acoelomate.
- Body dorsoventrally flattened, leaf like or tapelike.
- Mostly parasites and few free-livings.
- First animals to have organ system organization and cephalization (differentiation of heads).
- Segmentation is absent i.e. body not metamerically segmented.
- Digestive, excretory, nervous and reproductive systems are present.
- Alimentary canal is incomplete or absent.
- Skeletal, respiratory and circulatory systems are absent.
- Anus is absent like coelenterates, with blind-sac body plan.
- Excretory organs are protonephridia or solenocytes or flame cells.
- Flame cells arise from mesoderm ; unicellular, uninucleate, ovoid.
- Main excretory products are ammonia, fatty acids and CO2.
- Respiration and nutrition through general body surface.
- Anaerobic respiration in parasites like Taenia.
- Nervous system with large, anteriorly situated ganglia and nerve cords running along the body.
- Sense organs are better developed in free living forms like Planaria.
- These are monoecious or hermaphrodite. Reproductive organs are well-developed. Eggs possess yolk and covered by shell.
- Development is direct in free-living and indirect in parasitic forms.
- Fertilization is internal.
- Spiral cleavage ; life cycle completed with many larval stages.
- Larva are free swimming.
- Ootype is surrounded by Mehli’s glands / shell glands. The secretion of this gland lubricates the passage.
Classification
3 classes on the basis of mode of life, presence or absence of digestive system and cilia in epidermis.
| Turbellaria (L;turbella = a string) | Trematoda (Gr; trema = hole, eidos = form) | Cestoda (Gr; kestos = girdle, eidos = form) |
| Mostly free living; ectocommensal or parasitic, body unsegmented | Commonly called Flukes. Ecto and endo parasites, body unsegmented. | Commonly called tapeworms. Endoparasites, body strobilated (pseudometamerism) |
| Epidermis cellular or syncytial ; covered with cilia. | Cilia and epidermis absent. | Cilia and epidermis absent. |
| Hooks and suckers absent. | Suckers present. | Hooks and suckers present. |
| Digestive system mostly present. | Digestive system well developed. | Digestive system completely absent. |
| Development usually direct, with spiral cleavage. | Development direct in ectoparasites and usually indirect in endoparasites. | Development is indirect. |
| Examples : Planaria, Bipalium, Convoluta, Mesostoma | Examples: Fasciola hepatica (liverfluke),Schistosoma (Blood/lymphatic fluke), Paragorimus westermani (Lung fluke) | Examples: Taenia solium (pork tapeworm), Taenia saginata (beef tapeworm), Echinococcus granulosus (dog tapeworm), Diphyllobothrium latum (fish tapeworm), Hymenolepis nana (Dwarf tapeworm) |
