Biology / Unit 10 Bio 2251 short lesson, about 20 seconds
Phylum Platyhelminthes: Flatworms
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The idea
Flatworms are acoelomate, triploblastic, bilaterally symmetrical worms lacking circulatory or respiratory systems, relying on their flattened shape for gas exchange.
In practice
A planarian's flattened body keeps every internal cell close enough to its surface for oxygen to diffuse in, without respiratory organs.
Where people slip
Some flatworm lineages, such as tapeworms, are parasitic, while others, such as planarians, live freely — the phylum spans both.
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