The Collaborative International Dictionary of English v.0.48:

Acicula \A*cic"u*la\, n.; pl. Acicul[ae]. [L., a small needle,
   dimin. of acus needle.] (Nat. Hist.)
   One of the needlelike or bristlelike spines or prickles of
   some animals and plants; also, a needlelike crystal.
   [1913 Webster]

WordNet (r) 3.0 (2006):

acicula
    n 1: a needlelike part or structure of a plant or animal or
         crystal; as a spine or bristle or crystal