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The movements of burrowing, and the muscular systems, are quite 
similar in all three genera, and Yoldia will be taken as an example. 
If a specimen of Yoldia is laid on its side in a dish of sea-water, 
the foot is likely soon to be thrust from between the valves of the 
shell, bent back beneath the valve on which the specimen is lying, the 
flaps spread apart, and the foot withdrawn. If these movements are 
made on sand or soft mud, the lower flap of the foot catches and 
gives the necessary purchase for the animal to right itself, that is to 
turn the shell on its ventral and anterior borders. 
Fig. 2. Yoldia limatula seen from the ventral side, with the foot protruded and 
the muscular flaps closed together, This is the position at the end of the thrust, in the 
movements of burrowing. 
Fig. 3. Yoldia limatula, seen from the ventral side, with the foot protruded and 
the muscular flaps spread apart to form an anchor. This is the position just before the 
retractor foot-muscles contract. 
Fig. 4. Yoldia limatula, seen from the ventral side, with the shell drawn up to 
the position of the spread flaps. This is the position after the retractor foot-muscles 
have contracted. By closing the flaps together the foot is made ready for another thrust 
into the position shown in fig, 2. 
The movements of burrowing consist of thrusting the closed foot 
far out of the shell anteriorly, Fig. 2, spreading the flaps, and with 
the flaps spread, Fig. 3, withdrawing it, Fig. 4. The spread flaps are 
closed together when they come to the margin of the shell, and the 
foot is ready for another thrust. 
If these movements take place in mud, they will cause the animal 
to change its position. The closed foot is wedge-shaped, Fig. 2, and 
easily penetrates the mud, while the spread flaps form a very perfect 
anchor, Fig. 3. Anchored in this way, when the retractor muscles of 
= gee 
