|
A large phylum of animals, called flatworms (Platyhelminthes is Greek for flatworm). Flatworms are among the simplest of the bilaterally symmetric animals. Tissues are grouped into organs, and there is a complete gut (except in acoel flatworms), but there are no respiratory or circulatory systems, and the animals lack a coelom. |
|
The Platyhelminthes are a phylum 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 two openings, but in acoel flatworms it only has one. |
|
As flatworms are primarily defined by a lack of characteristics with respect to the other triploblastic animals, it is quite possible that they form a polyphyletic group. Three classes of flatworms are generally considered to be closely related, however: |
|
Flatworms are considered basal among the protostomes. As the phylum are defined mainly by a lack of characteristics, it is quite possible that it is polyphyletic. However, the following three classes of flatworms, 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? * Acoela? * 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. |
|
* Monogonatea? * Cestoda? (flukes) * Trematoda? (tapeworms) |
|
The last two are parasitic groups, some of which are extremely damaging to people and other animals. |
Flatworms are considered basal among the protostomes. As the phylum are defined mainly by a lack of characteristics, it is quite possible that it is polyphyletic. However, the following three classes of flatworms, 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.
![[Home]](wikipedia.png)