Phylum Platyhelminthes: Flatworms
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Flatworms are acoelomate, triploblastic, bilaterally symmetrical worms lacking circulatory or respiratory systems, relying on their flattened shape for gas exchange.
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A planarian's flattened body keeps every internal cell close enough to its surface for oxygen to diffuse in, without respiratory organs.
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Some flatworm lineages, such as tapeworms, are parasitic, while others, such as planarians, live freely — the phylum spans both.