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Other Platyhelminthes characteristics shared by these creatures is they are bilaterally symmetrical, this means the left and right sides of their bodies are mirror images of each other. They have distinctive head and tail ends and their body contains three cell layers.
Introduction. Platyhelminthes, or flatworms, are considered by many to be ancestral to most of the metazoans that were to follow and for the first time we see the development of the final germ layer, the mesoderm, with its potential to form muscles and other structures.ut, unlike the other animals that are to follow, in flatworms the mesoderm remains as a solid mass of tissue without a coelom and the advantages that this adaptation could have provided to the group are lost. This probably explains why most of today's flatworms are parasitic rather than free living.