2. (Arthropoda: Crustacea) In bivalves, muscles attached to the carapace that pull it to the body, or connect the carapace. a. In Barnacles, any transverse muscle, especially those attached to the scutum for closing the aperture.
3. (Mollusca: Bivalvia) A single posterior or an anterior and posterior muscle connecting the two valves. a. In oysters, the crescent shaped "catch" muscle that holds the valves in a set position, or the "quick" muscle, the main opening and closure muscle.