1. Burrowing of the bivalve Mercenaria mercenaria has been re-examined using modern recording techniques. Burrowing activity consists of a series of movements, repeated cyclically termed the ‘digging cycle’, involving extension of the foot, closure of the siphons, adduction of the shell valves and retraction of the foot, Final position in the sand is only reached after many digging cycles, constituting a digging period.
2. Closure of the siphons and apposition of the mantle margins temporarily seals the mantle cavity to form a pressure chamber through which forces produced by adduction can be transmitted to act elsewhere. Adduction causes dilation of the distal regions of the foot, aiding pedal anchorage, and ejection of water from the mantle cavity loosens the sand ventrally and laterally.
3. Downward movement is by the passive dropping of the heavy shell into the fluid cavity formed at adduction, and by active movement caused by contraction of the retractor muscles pulling the shell downwards on to the anchored foot. The strength of pedal retraction is 5-6 g. in an animal of 21 g. wt. in water.
4. With the shell completely covered by sand the opening moment of the ligament is shown to be too small to effect complete opening of the valves, and under these circumstances pressures generated by withdrawal of the foot and siphons in a secondary phase of siphonal movements supplement the action of the ligament.
5. Burrowing movements in Mercenaria are compared with those of Tellina, Donax, Macoma, Cardium and Ensis.