borrowed from New Latin, from Greek glōchid-, glōchī́s "projecting point (as the end of a yoke fastening or the barb of an arrow)" + New Latin -idium-idium; so named from the hook-like appendages with which the larvae attach themselves to fish — more at gloss entry 3
Term introduced by the Norwegian zoologist Jens Rathke (1769-1855) in "Om Dam-Muslingen," Skrivter af Naturhistorie-Selskabet [Copenhagen], 4. Bind, 1. Hefte (1797), p. 166. As a vernacular equivalent of glochidium Rathke gives Krogskjæl, "hook shell."