In 1735 Linnaeus in his book systema nature" started the phylum vermes". He kept all invertebrates except insects in this phylum.

In 1879-1893 Pagenstecher" and "Brun" gave an exhaustive history of "vermes".

In 1766 Pallas recognised the "cestode" nature of bladder worms. The knowledge of tapeworms was first statedjsy "Goeze". Rudolphi gave the names Nematoda, Acanthocephala, Trematoda, Cestoda and Cystica.

In 1859Gegenbaur proposed the name "Platyhelminthes".

Gegenbaur's Platyhelminthes composed of 3 groups. 1) Turbellaria, 2) Trematoda and 3) Cestoda.

This group includes flat-worms. They show bilateral symmetry. They are triploblastic worms. It is acoelomate, metazoan group. They are advanced than coelenterates, because of the development of organ system.

Platyhelminthes are acoelomate bilatrians. They do not show anus. The body is dorsi-ventrally flat. They are worm-like. Most of the flat-worms are of moderate size. Some tape worms are as long as 10 to 15 meters in length. Some flat-worms are microscopic. They are generally colorless. Some polyclads and planarians are coloured.

Many of the flat-worms have flame cell as excretory organs. They do not show coelome. The spaces between the organ systems being filled with special connective tissue called parenchyma. They do not show blood vascular system. They are not metamerically segmented.

The ovary of these animals is divided into two parts 1. Germarium 2. Vitellarium. Germarium produces fertile eggs. Vitellarium produces cells containing food and shell forming substance.

In this phylum many organisms are parasites. They exhibit a very high degree of morphological adaptation to parasitism. Most of them are hermaphrodites.

  Phylum Platyhelminthes classification

This phylum includes 15,000 species.

It is divided into 3 classes.

Class Turbellaria includes ciliated flat-worms. They are generally free living.

Class Trematoda includes non-ciliated flat-worms .They are usually parasitic. They are called flukes.

Class Cestoda includes endo parasitic flat-worms. They are called Tape-worms.