The Collaborative International Dictionary of English v.0.48:

Acontia \A*con"ti*a\, n. pl. [NL., from Gr. ? a little dart.]
   (Zool.)
   Threadlike defensive organs, composed largely of nettling
   cells (cnid[ae]), thrown out of the mouth or special pores of
   certain Actini[ae] when irritated.
   [1913 Webster]