They have one opening opening according to Biology flatworm notes I found.
By Khoa
Reptiles have 2 body openings. Mouth and an anus.
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Nematodes have a pseudocoelom (a body cavity that is not completely lined by mesoderm), whereas flatworms have an acoelomate body plan (lack a body cavity). This means that nematodes have a body cavity, while flatworms do not have a true body cavity.
they are flat
flatworms are the only triploblastic organisms that lack a coelom
Flatworms
No, a flatworm does not have a body coelom.