Platyhelminthes

Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates (having no body cavity), and have no specialized circulatory and respiratory organs, which leads to their distinct flat shape. The digestive cavity has only one opening for both ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously.

Characteristics

Platyhelminthes are bilaterally symmetrical animals: the left and right sides of their bodies mirror each other. This also implies they have distinct top and bottom surfaces and distinct head and tail ends. Like other bilaterians, their bodies are divided into three main layers of cells that originated from the gastrula during embryonic development.

Classification

The traditional classification of the Platyhelminthes involves division into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda. Modern genetic analyses, however, have revised this scheme, and have led to the understanding that the "Turbellaria" as traditionally defined is paraphyletic, i.e., it consists of several unrelated groups.

Reproduction

Platyhelminthes are hermaphrodites, and excrete their eggs along with their feces. Eggs are often eaten by other animals and humans, which cause them to enter the food chain. Some flatworm species are parasites.

See also

• List of Platyhelminthes families
• Parasitic flatworms

External links

• Introduction to the Platyhelminthes University of California Museum of Paleontology
• Tree of Life: Platyhelminthes

Platyhelminthes Resources
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