Dorsiventral Describing a structure that has distinct upper and lower surfaces. A dorsiventrally flattened structure has flat dorsal and ventral surfaces and very narrow lateral surfaces.

• 异面（背腹型）:指上下表面结构具有明显差异。异面平滑结构指有平滑的背、腹面，以及非常狭窄的侧表面。