The Collaborative International Dictionary of English v.0.48:

Abactinal \Ab*ac"ti*nal\ ([a^]b*[a^]k"t[i^]*nal), a. [L. ab + E.
   actinal.] (Zool.)
   Pertaining to the surface or end opposite to the mouth in a
   radiate animal; -- opposed to actinal. "The aboral or
   abactinal area." --L. Agassiz.
   [1913 Webster]

WordNet (r) 3.0 (2006):

abactinal
    adj 1: (of radiate animals) located on the surface or end
           opposite to that on which the mouth is situated [ant:
           actinal]