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Platyhelminthes, or Flatworms, are the simplest kind of worms. There are about 20,000 species in this group. 

Type of Skeleton A flatworm has no skeleton but has tiny bristles called cilia that help it move at least two layers of  muscles under its skin. It has three cell layers called the endoderm, the  mesoderm, and the ectoderm. They also have the beginnings of a head  region. They are a phylum of triploblastic acoelomate invertebrate animals. The word Platyhelminthes comes from the Greek words 'platy' meaning flat and 'helmins' meaning worm. Their bodies are soft, unsegmented, dorsoventrally flattened with bilateral symmetry and resemble ribbons.


(source: http://skeletalphylum.weebly.com/platyhelminthes.html)

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