Platyhelminthes

The Platyhelminthes are a phylum of 10,000 species of relatively simple
animals, called flatworms. The body is triploblastic and divided into
distinct organs, but there is 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.

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:

   * Monogenea
   * Trematoda (flukes, probably paraphyletic to Cestoda)
   * Cestoda (tapeworms)

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

   * Nemertinodermatida (now in phylum Acoelomorpha)
   * Acoela (now in phylum Acoelomorpha)
   * Catenulida
   * Macrostomida
   * Lecithoepitheliata
   * Rhabdocoela
   * Prolecithophora
   * Proseriata
   * Tricladida
   * Polycladida

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.

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