What are diductor muscles in articulate lamp shells?

In articulate lamp shells (brachiopods), the diductor muscles are responsible for opening the shell valves. These muscles extend from near the center of the ventral valve to a process located under the dorsal beak, often attaching to a ridge or boss known as the cardinal process.1

When the diductor muscles contract, they pull the dorsal beak forward, causing the shell to rotate on the line joining the hinge teeth, thereby opening the valves. This action is the functional opposite of the adductor muscles, which contract to close the valves.1