Source : Webster's Revised Unabridged Dictionary (1913)

Abactinal \Ab*ac"ti*nal\ ([a^]b*[a^]k"t[i^]*nal), a. [L. ab + E.
   actinal.] (Zo["o]l.)
   Pertaining to the surface or end opposite to the mouth in a
   radiate animal; -- opposed to {actinal}. ``The aboral or
   abactinal area.'' --L. Agassiz.

Source : WordNet®

abactinal
     adj : (of radiate animals) located on the surface or end opposite
           to that on which the mouth is situated [ant: {actinal}]