For 10
 points, name this class of mollusks with two shells, which includes clams and oysters.
Some members of this class remain stationary with the help of byssal threads. The species Crassostrea
 gigas in this class introduced the parasite MSX to a closely related species. When this class of animals first
 evolved, they outcompeted and almost entirely replaced a phylum of similar-looking animals called
 brachiopods. These animals often feed by using elongated siphons to filter water. These animals have an
 open circulatory system with hemolymph, and like other members of their phylum they move using a single
 fleshy foot. Unique parts of their anatomy include a hinge, and they are bilaterally symmetrical. For 10
 points, name this class of mollusks with two shells, which includes clams and oysters.
Some members of this class remain stationary with the help of byssal threads.
{bivalves} [or {bivalva}; or {pelecypoda}; prompt on “mollusks” or “mollusca”]