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the foot contract, the shell is pulled to the position of the flaps. As 
repeated thrusts and retractions follow each other with great rapidity, 
an animal can bury itself very promptly. 
After the animal has righted itself, two thrusts and retractions 
generally suffice to carry it beneath the surface of the mud. To what 
depth animals may burrow has not been determined, but when placed 
on seven or eight inches of mud in a glass dish, they arrive at the 
bottom in a very few seconds, and frequently burrow along on the 
glass for some distance. 
As the animal normally lives with not over one-third of the 
posterior end of the shell above the surface of the mud, this 
must be an effective means of escape, as, in such a position, a single 
thrust and retraction of the foot will carry it beneath the surface of 
the mud, and once beneath the surface it would be very hard to 
follow it in its movements. 
What special enemies these creatures have has not been. deter- 
mined, but it seems to be well known to observing fishermen and 
hunters, that flounders and cormorants feed upon them. With regard 
to these enemies this method of escape must be very effective. 
Both Nucula and Solenomya burrow in a similar manner. The 
movements of Solenomya are quite as rapid as those of Yoldia. The 
movements of Nucula are quite sluggish. Both of these forms nor- 
mally live entirely covered by mud or sand. Solenomya lives in rather 
hard mud, frequently very sandy mud, and, I think, keeps its burrow 
more or less open. Nucula delphinodonta, at least, lives in soft mud 
and wanders around in it by slow movements of the foot. 
Sometimes Yoldia thrusts its foot from between the ventral borders 
of the shell valves near their posterior ends and, after spreading the 
flaps, makes an anterior sweep with it. This movement executed in 
mud would help the animal change its direction and might back it up 
towards the surface again. Nucula performs this movement with less 
energy, but the fused margins of the mantle of Solenomya interfere 
with its perfect performance. 
Another combination of movements common with Yoldia, and fre- 
quently performed by Sol- 
enomya, are the movements 
of leaping. 
Fig. 5. Yoldia limatula ready 
to leap. Seen from above. 
These movements are generally observed after specimens have 
been executing burrowing movements for sometime, in smooth bottomed 
