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The Platyhelminthes are a phylum of 20,000 species of relatively simple animals, called flatworms. The body is ribbon-shaped, soft-bodied and has flattened dorsoventrally, from top to bottom. However,it possess no respiratory or circulatory system, and there are no body cavities except the gut, which is absent in some highly reduced forms. Usually the digestive tract has one opening, but in particularly long worms or those with highly branched guts, there may be one or more anuses. In acoel flatworms, now thought to be unrelated to the Platyhelminthes, the gut is absent or non-permanent. Since most flatworms have only one opening to their digestive tract (their gastrovascular cavity), they can't feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts can. Muscular contraction in the upper end of the gut cause a strong sucking force allowing flatworms to ingest their food and tear it into small bits. The gut is branched and extends throughout the body, functioning in both digestion and transport of food.

Flatworms used to be considered basal among the protostomes. On the basis of molecular evidence it is now thought that they belong in the Lophotrochozoa, except for the orders Acoela and Nemertodermatida, which are considered basal bilaterians and together form the phylum Acoelomorpha. Within the true flatworms, the following three classes, grouped together based on some characteristics of the skin, probably form a monophyletic group:

The remaining orders included in the phylum, grouped together for convenience as the class Turbellaria, are the following:

Most of these groups include free-living forms. The flukes and tapeworms, though, are parasitic, and a few cause extreme damage to people and other animals. All in all there are about 15000 modern species.


Crawley, John L., and Kent M. Van De Graff.(editors);A Photographic Atlas for the Zoology Laboratory; Colorado, Morton Publishing Company;ISBN:0-89582-613-5(fourth edition,2002)