The Collaborative International Dictionary of English v.0.48:

Glochidium \Glo*chid"i*um\, n.; pl. Glochidia. [NL., fr. Gr. ?
   the point of an arrow.] (Zool.)
   The larva or young of the mussel, formerly thought to be a
   parasite upon the parent's gills.
   [1913 Webster]

WordNet (r) 3.0 (2006):

glochidium
    n 1: a barbed spine or bristle (often tufted on cacti) [syn:
         glochidium, glochid]