Having symmetry on only one side or half of an object, rather than being fully symmetrical on both sides.
From Greek hemi- (half) + symmetrical (from syn- meaning together + metron meaning measure). The term evolved in the 19th century to describe botanical and zoological specimens that showed partial rather than complete bilateral symmetry.
Many sea creatures like starfish aren't perfectly symmetrical—they're hemisymmetrical, meaning if you drew a line down the middle, one side wouldn't be a mirror image of the other, which helps scientists understand how animals evolved from asymmetrical ancestors.
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