Phylum Platyhelmithes - The Flat worms

Gegenbeur (1859) placed flat worms in a separate group and gave the term Platyhelmithes. The Phylum Platyhelminthes includes about 13,000 species.

General Characterstics

1. Habitat: The flatworms are mostly parasites but some flatworms are free-living.
2. Body Form: The body is dorso-ventrally flattened and is without true segments.
3. Symmetry: bilaterally symmetrical.
4. Germ layers: From the evolution point of view, they are first triploblastic animals, e.g., consisting of three germ layers: ectoderm, mesoderm and endoderm.
5. Level of Organisation: The flatworms have organ level of organisation.
6. Body wall: The body covering is soft and may or may not be ciliated. Rod shaped bodies, the rhabdites are present in epidermal cells of the living platyhelminthes. They are protective in nature and used for capturing food.
7. Body cavity: The space between the body wall and the organs are filled with a peculiar connective tissue, called the parenchyma. The parenchyma helps in transporting the food materials. Flat worms are acoelomates.
8. Digestive Tract: If present, is incomplete ( without anus ) . Digestive tract is absent in Tapeworms.
9. Skeletal, Respiratory and Circulatory system are absent. The fluid in the parenchymal network maintains the body shape. It is called hydoskeleton.
10. Excretory system : It consists of special flame cells ( Solenocytes / Protonephridia) which are meant for excretion and osmoregulation.
11. Nervous System : The nervous system is ladder like. It consists of the brain and two main longitudinal nerve cords connected at intervals by transverse commissures.
12. Reproductive System : These animals are generally hermaphrodite[i.e., male and female organs in single individual] , and the reproductive organs are well developed. The fertilization is cross and internal. In tapeworms self fertilization is found. 
13. Development : Life cycle is complicated in most flatworms with one or more larval stages. In liver fluke miracidium, sporocyst, radia, cercaria and metacercaria larvae are present.
14. Regeneration : It is well marked in some flatworms like Planaria.