What cell type must enter a female clams body in order for glochidia to form

Sex cells or reproductive cells

Explaination:

Freshwater clams are hermaphrodites and produce both eggs and sperm.

So the freshwater clams reproduce by releasing their reproductive cells (sex cells or gametes) into the water column. The sex cells then enters the female clams body and undergo internal fertilization and produces glochidia. The glochidia then released in the surrounding water.

Hence, the sex cells are responsible for the reproduction of female clams.