In zoology, one of the two main radii in starfish or sea urchins that are aligned with the madreporite (a sieve-like structure).
From Latin 'bivium,' literally 'two-road place,' where 'bi-' means two and 'via' means way. Marine biologists adopted this anatomical term to describe the geometric symmetry of echinoderm body plans.
Starfish have a hidden bilateral symmetry that reveals itself in their internal anatomy—the bivium represents a direct line to where water enters their internal hydraulic system, showing how 'ancient' these creatures are in evolutionary terms.
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