Introduction to the Flatworms.
Turbellarian Worms Gallery.
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Platyhelminthes: The Flatworms.
The phylum Platyhelminthes contains the simplest forms of the bilaterally symmetrical metazoa (the group which includes humans). There are about 20,000 species of flatworm. They are considered structurally more advanced than the Cnidarians (eg. Hydra) in having a distinct mesoderm tissue rather than the gelatinous mesoglea layer of the Cnidarians, but in common with them, lack an anus.
They are however, considered less developed than the Bryozoa and other coelomates in that they never develop a body cavity (coelom ) in which the organs are suspended.
There are three classes of flatworm:
The Turbellarian Worms.
The bodies of these worms are covered with cilia which, together with muscular contractions of their bodies and the secretion of mucus, enable them to glide smoothly over the surfaces of the pond sediments and submerged plants which are their hunting grounds.
They all possess both male and female sex organs (are hermaphroditic). They do however, mate -- each worm depositing sperm in the other's sperm receptacle.
The eggs hatch into miniature adults.
Here is a diagram of a planarian worm.