While both flatworms and roundworms are soft-skinned invertebrates, their biology differs greatly enough to put them into to distinct categories. Flatworms belong to the phylum Platyhelminthes, while roundworms belong to Nematoda.
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Flatworms have flat, ribbon shaped bodies, while roundworms are round (but not segmented) and taper towards the end. Both are bilaterally symmetrical.
Roundworms are sexed, with two different genders, while flatworms may either be sexed, hermaphroditic, or use asexual reproduction.
Coelom (Body Cavity)
Flatworms are acoelomates; they have no body cavity between the digestive tract and the body wall. Roundworms are pseudocoelomates; while they have a body cavity, it is not composed of mesodermal epithelium and is thus not a true coelom.
Flatworms have a one-way digestive tract. Waste is taken in and expelled through the same hole. Roundworms have a two-way tract, with a mouth and anus.
Roundworms have nerve bundles on one end, but no optical receptors, making them less cephalized than flatworms. Flatworms have nerve bundles as well as a primitive brain and eyespots.
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