"Their enlarged gils are covered with cilia (fine hairs) and they have the additional functions of filtering microscopic food particles from the water as it flows over them."
"Bibalves life on the bottom of the sea or lakes and are sedentary or not moving around."valva, "section of a folding or revolving door"; literally, "that which turns"; related to volvere "to roll".
The meaning was extended in about 1659 to "a mechanical device that works like a valve"; and in 1661 in zoology to "halves of a hinged shell".
An oyster is a bivalve because it has a two-valved shell.