What's the difference between bivium and trivium?

Bivium

Definition:

• (n.) One side of an echinoderm, including a pair of ambulacra, in distinction from the opposite side (trivium), which includes three ambulacra.

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Trivium

Definition:

• (n.) The three " liberal" arts, grammar, logic, and rhetoric; -- being a triple way, as it were, to eloquence.
• (n.) The three anterior ambulacra of echinoderms, collectively.

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