The ligament of Mya arenaria allows the valves to open normally and permits a rocking movement of the valves about a dorso-ventral axis to take place. This movement, which makes burrowing possible, is also associated with the divarication of the posterior end of the shell to allow the withdrawal of the siphon.
A method of determining the resistance of various marine soils, ranging from clean sand to offshore mud, to the opening of the valves of M. arenaria is described. Even in the offshore mud, which is much less resistant than the sand, the opening moment of the ligament is approximately 6 times too small to cause the valves to gape.
Records have been made of the opening of the valves of M. arenaria when loaded experimentally to varying degrees. A closing pattern of activity was observed with loads applied not exceeding the opening moment of the ligament, and with greater loads an opening pattern. The opening of the valves, in the latter instance, is found to be associated with a sudden increase in the water pressure in the mantle cavity. The increase in pressure is shown to be of the correct order of magnitude to open the valves of Mya in muddy sand by the application of a steady force. The liquefaction of the sand surrounding the lamellibranch, possibly by the expulsion of water through the pedal opening, would greatly facilitate the opening of the valves.