In biology, relating to the surface or part of an organism (especially a starfish) that is opposite from where the mouth is located.
From 'a-' (without, away from) plus 'actinal' (relating to the radius or ray-like structure). 'Actinal' comes from 'actinus' (a ray), so 'abactinal' literally means 'away from the rays.' The term is used in marine biology since the late 1800s.
Starfish have a mouth on their 'actinal' side (where they grab food) and a completely different 'abactinal' side on the back—knowing this terminology helped scientists understand that these creatures had organized body plans just like we do, only flipped on a different axis.
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