1. A comparison is made of the fluid dynamics of a shallow, yet actively, burrowing bivalve, Margaritifera, with the sessile, deeply buried Mya arenaria. 2. In both adduction produces high pressures (up to ioo cm.) in the mantle and the pericardial cavities which are utilized in Margaritifera for locomotory purposes, in Mya principally for siphonal extension. 3. With siphonal and pedal apertures closed the mantle cavity of Mya is virtually watertight and acts, together with the blood, as the fluid of an antagonistic muscle system, whereby adduction causes siphonal extension and siphonal retraction produces an increase in gape of the valves. The close interaction between these two muscle systems is illustrated by pressure recordings of Mya in the normal buried position. 4. Siphonal movements are shown to be associated with divarication of the valves and accompanying pressure changes. 5. Consideration is given to the haemodynamics of Mya and by contrast with the high pressures involved in locomotion or siphonal movement, maximum pressures of only 2.5 cm. were recorded from the heart, producing a sluggish circulation. The higher pressures derived from the body musculature make an important contribution to movements of the blood.