As an aside, oysters are not bivalves, they are brachiopods. Brachiopods do have a nervous system - some even have eyes.
What’s the difference and how do you tell a brachiopod from a bivalve? It’s the plane of symmetry. In bivalves the plane of symmetry is where the shells (also known as valves) join. So bivalves have two identical shells. Whelks and razor shells are bivalves. Brachiopods also have two shells, but the shells are normally quite different. The oyster for example has one big concave shell and one small flat one on top. The big shell has a hole at the apex (just next to the hinge) and a root-like anchor grows from it to bind the brachiopod to the matrix on which it lives. Brachiopods have an axis of symmetry from this root/foot that vertically separates each shell into two mirrored parts.
As an aside, oysters are not bivalves, they are brachiopods. Brachiopods do have a nervous system - some even have eyes.
What’s the difference and how do you tell a brachiopod from a bivalve? It’s the plane of symmetry. In bivalves the plane of symmetry is where the shells (also known as valves) join. So bivalves have two identical shells. Whelks and razor shells are bivalves. Brachiopods also have two shells, but the shells are normally quite different. The oyster for example has one big concave shell and one small flat one on top. The big shell has a hole at the apex (just next to the hinge) and a root-like anchor grows from it to bind the brachiopod to the matrix on which it lives. Brachiopods have an axis of symmetry from this root/foot that vertically separates each shell into two mirrored parts.
It’s always been my understanding that oysters are bivalves.