glochidium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Zoology The parasitic larva of certain freshwater mussels of the family Unionidae, having hooks for attaching to the gills or other external parts of a host fish.
2. n. Botany Variant of glochid.

Century Dictionary and Cyclopedia

1. n. [capitalized] In zoology, a generic name given to the young of certain fresh-water mussels, as Unio and Anodonta, which are hatched in the gills of the parent, and were at one time supposed to be parasites. Rathke
2. n. In botany, a hair-like appendage to the massulæ of heterosporous Filicineæ, by which the massulæ attach themselves to the macrospores after both have been discharged into the water.

Wiktionary

1. n. zoology The larva or young of the mussel.

GNU Webster's 1913

1. n. (Zoöl.) The larva or young of the mussel, formerly thought to be a parasite upon the parent's gills.

WordNet 3.0

1. n. a barbed spine or bristle (often tufted on cacti)