An Introduction to Platyhelminthes

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 recognized 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 1859 Gegenbaur proposed the name "Platyhelminthes". Gegenbaur's Platyhelminthes composed of 3 groups. 1) Turbellaria, 2) Trematoda, and 3) Cestoda.
This group includes flatworms. They show bilateral symmetry. They are triploblastic worms. It is an acoelomate, metazoan group. They are more advanced than coelenterates, because of the development of organ system.
Platyhelminthes are acoelomate bilaterians. They do not show the anus. The body is dorsi-ventrally flat. They are worm-like. Most of the flatworms are of moderate size. Some tapeworms are as long as 10 to 15 meters in length. Some flatworms are microscopic. They are generally colorless. Some polyclads and planarians are colored.
Many flatworms have a flame cell as excretory organs. They do not show coelom. The spaces between the organ systems are filled with special connective tissue called parenchyma. They do not show a 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.
Classification of Phylum Platyhelminthes
This phylum includes 15,000 species. It is divided into 3 classes.
- Class Turbellaria includes ciliated flatworms. They are generally free living.
- Class Trematoda includes non-ciliated flatworms. They are usually parasitic. They are called flukes.
- Class Cestoda includes endo-parasitic flatworms. They are called Tape-worms.
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