Yup. OED says this for abactinal: "Remote from the actinal area; pertaining to that part of the surface of a radiated animal which is opposite to the mouth, e.g. the apex of a sea-urchin, or upper surface of a star-fish."
Found on page 373 of my 1988 paperback edition of Mervyn Peake's "Gormenghast", in the first paragraph of the chapter entitled "Countess Gertrude". The sentence is: "Long after the drop of lake water had fallen from the ilex leaf and the myriad reflections that had floated on its surface had become part of the abactina of what had gone on forever...". The continuing story or procession of time? Man, Peake likes to assault the reader with this stuff on damn near every page. I have never reached for a dictionary as much as I have with this book. Found this site through it, however, so that's a plus. Would love some etymology for this unusual word.