扁形動物皆為
兩側對稱(英语:Symmetry (biology))动物:牠們的左半側與右半側互為鏡像;這亦表示了這些動物可以分辨出其頂部與底部 、以及其頭部及尾部。就像其他的两侧对称动物,牠們的细胞可分成三個胚層,即:內胚層、中胚層及外胚層,而刺胞動物及栉水母等輻射對稱動物(英语:Symmetry (biology))則只有兩個胚層。Beyond that, they are "defined more by what they do not have than by any particular series of specializations."。
不同於其他两侧对称动物,扁形动物並沒有體腔,
所以被稱為無體腔動物。牠們也沒有特化的循环系统及呼吸系統器官,而這些都是為扁形動物分類時的解剖特徵。牠們的身體是柔軟且沒有分節的。
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^ 5.05.1Hinde, R.T. The Cnidaria and Ctenophora. Anderson, D.T. (编). Invertebrate Zoology. Oxford University Press. 2001: 28–57. ISBN 978-0-19-551368-4.
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