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Worms: Platyhelminthes.
Introduction to the Flatworms.
Turbellarian Worms Gallery.

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Worms: Introduction Annelids Nematodes Flatworms Parasitic

  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 (coelomPronounced  seelom. ) in which the organs are suspended.

There are three classes of flatworm:

1. Turbellarians. 2. Cestodes. 3. Trematodes.
  1. Turbellaria.
    The class includes all of the free-living flatworms, most of which are marine. Only freshwater varieties are shown in these galleries. Flatworms can also be found in moist soil, especially in temperate climates.
    The freshwater turbellarians are further divided onto two groups: the Microturbellaria and the Planarians. (See below).

  2. Cestoda.
    The tapeworms. All members of this class are parasites.

  3. Trematoda.
    The flukes. All are either internal or external parasites of (mostly) vertebrates.
  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.


Castrella. This microturbellarian flatworm of the genus Castrella is gliding amongst strands of filamentous algae. This individual is about a millimetre long. It scavenges mainly algae and plant detritus through a mouth on its ventral side, which is connected to a large muscular pharynx. Only one of the two usually prominent eyespots is visible in this picture.
Darkfield, x100.