Definition and Introduction
Bilateral and protostominal organ grade eumetazoans without a body cavity ( acoelomates) platyhelminthes means flatworms (Gr. Platys= flat; helmins = worms. ) their body is dorsoventrally flattened. About 10,000 species known.
Aristotle mentioned tapeworms. But scientific studies of flatworms began only in the 18 th century. It was Gegenbaur (1859) who placed these in a separate group and suggested the name of the phylum.
1. Mostly parasitc , cheif invertebrates , including man. Some pathogenic, i.e. disease producing .
2. Differentiation of head ( cephalisation ) with a brain and , in some primitive eye spots, at anterior begins in eumetazoans with platyhelminths .
3. In many forms body is divided into segments, but this segmentation is not true metamerism.
4. Sukers, hooks spines, etc. Often developed for attachment to host tissue .
5. Soft and delicate, dorso ventrally flattened bilateral body is triploblastic, both triploblastic organization and bilateral symmetry also begin in eumetazoans with these animals .
6. Similary , development of distinct organs and organ system also begins in eumetazoans with platyhelminths . Distinct digestive, excretory , nervous and anus. Reproductive systems occur, but no skeletal, vascular and respiratory system ,No anus some have secondarily lost the digestive system .
7. No body cavity ,Space between body wall internal organs occupied by s loose mesodermal tissue called parenchyma.
8. Units of excretory system are peculiarly specialized protonephridial tubules with terminal flame cells or bulbs.
9. As an adaptation to parasitic mode of life , the reproductive system