Mollusks with two shells held together by muscles are called a. bivalves b. cephalopods c. gastropods d. cnidirapods

Explanation/

A valve is a part of a shell (a molluscs shell) that can articulate (move). Bivalves are are molluscs that have 2 of these articulated shells, called valves, are named bivalves:

Bi - 2

valves - articulated parts of a shell

Example:

Clams and mussels are both bivalves.

→ Hopefully the image below helps you get a deeper understanding of what bivalves are and look like. Image source: Pinterest.

Hope it helped,

BioTeacher101