Why two structures are missing in bivalves but are present in all other mollusks?

Bivalves lack certain structures, such as a radula and tentacles, which are present in other mollusks due to their unique feeding and lifestyle adaptations. Bivalves primarily filter-feed by siphoning water through their shells, making the radula unnecessary for scraping food. Additionally, their body plan has evolved to prioritize a two-part shell and a sedentary lifestyle, resulting in the loss of structures that are more suited for active predation or grazing seen in other mollusk classes.