1. A tubelike spine that existed in a few fossil bryozoaris: In her book about petrified remains, Jill was amazed that some acanthopores of invertebrate animals in the phyla Entoprocia and Ectoprocta had tubular spines or thorns.
2. One of a group of outlets or holes in a nonexistent coral of the family Chætitidæ: Acanthopores of the extinct corals appeared on the surface in little tubercles.