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Phylum 05, Platyhelminthes
Flatworms in brooks o'er stones do slither,
Or else invade some creature's liver.
All these worms are very flat:
Platyhelminth means just that.

Unlike other bilaterians (a front and a back end, as well as an upside and downside) they have no body cavity, and no specialized circulatory and respiratory organs, which restricts them to flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion.

This entry is located in the following unit: Zoology Phyla in Poetic Rhyming + (page 1)