1. The burrowing activity of Donax vittatus, Tellina tenuis, Macoma balthica and Cardium edule consists of a cyclical series of changes, termed the digging cycle, involving protraction of the foot, closure of the siphons, adduction of the valves and retraction of the foot.
2. Detailed comparison shows marked difference in the time involved in pedal protraction and a see-saw motion of entry to the sand by Cardium and Macoma with bulging shells in contrast to a more direct downward movement of Donax and Tellina with slim shells.
3. The structure of the foot and the hydrostatic fluid-muscle mechanism is described.
4. Recordings were made of the hydrostatic pressures directly by cannulation of the mantle cavity or pericardium and indirectly from the sand adjacent to the foot.
5. While extension and probing occurs at relatively low pressures dilation of the foot and ejection of water from the mantle cavity occur simultaneously at pressures of 25 cm. of water and are caused by the adduction of the valves.
6. The foot obtains a firm anchorage by means of the dilation while the pedal retractors pull the shell downwards and so sustain the pressure in the haemocoele.
7. Water ejection from the mantle cavity loosens the adjacent sand so that penetration by the shell is facilitated.